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  • 1.
    Andersson, Erik
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Björklund, Glenn
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Swedish Olympic Committee, Stockholm, Sweden.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster, University of Southern Denmark, Odense, Denmark.
    Energy system contributions and determinants of performance in sprint cross-country skiing2017In: Scandinavian Journal of Medicine and Science in Sports, ISSN 0905-7188, E-ISSN 1600-0838, Vol. 27, no 4, p. 385-398Article in journal (Refereed)
    Abstract [en]

    To improve current understanding of energy contributions and determinants of sprint-skiing performance, 11 well-trained male cross-country skiers were tested in the laboratory for VO2max , submaximal gross efficiency (GE), maximal roller skiing velocity, and sprint time-trial (STT) performance. The STT was repeated four times on a 1300-m simulated sprint course including three flat (1°) double poling (DP) sections interspersed with two uphill (7°) diagonal stride (DS) sections. Treadmill velocity and VO2 were monitored continuously during the four STTs and data were averaged. Supramaximal GE during the STT was predicted from the submaximal relationships for GE against velocity and incline, allowing computation of metabolic rate and O2 deficit. The skiers completed the STT in 232 ± 10 s (distributed as 55 ± 3% DP and 45 ± 3% DS) with a mean power output of 324 ± 26 W. The anaerobic energy contribution was 18 ± 5%, with an accumulated O2 deficit of 45 ± 13 mL/kg. Block-wise multiple regression revealed that VO2 , O2 deficit, and GE explained 30%, 15%, and 53% of the variance in STT time, respectively (all P < 0.05). This novel GE-based method of estimating the O2 deficit in simulated sprint-skiing has demonstrated an anaerobic energy contribution of 18%, with GE being the strongest predictor of performance.

  • 2.
    Andersson, Erik
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Björklund, Glenn
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Energy contributions and pacing strategies of elite XC skiers during sprint skiing2016Conference paper (Refereed)
    Abstract [en]

    INTRODUCTION: At present, knowledge regarding energy contributions and pacing strategies during successive sprint time-trials (STTs) in cross-country (XC) skiing is limited and, therefore, the current study was designed to examine these parameters. The results shown have recently been published elsewhere (Andersson et al., 2016).METHODS: Ten well-trained male XC skiers performed four self-paced 1300-m STTs on a treadmill, separated by 45 min of recovery. The simulated STT course was divided into three flat (1°) sections (S1, S3 and S5) involving the double poling (DP) sub-technique interspersed with two uphill (7°) sections (S2 and S4) involving the diagonal stride (DS) sub-technique. Treadmill velocity and VO2 were monitored continuously and technique-specific gross efficiency (based on submaximal pre-tests) was used to estimate anaerobic energy production.RESULTS & DISCUSSION: The average STT performance time was 229 ± 9 s and the aerobic energy contribution was 82 ± 5%. A positive pacing strategy was used during all STTs, with 3-9% more time spent on the second half of the course (P < 0.05). In addition, the pacing strategy was regulated to the terrain, with substantially higher (~30%) metabolic rates, due to primarily higher anaerobic energy production, for uphill compared with flat skiing (P < 0.05). The individually fastest STT was more aggressively paced compared to the slowest STT (P < 0.05), which resulted in a higher O2 deficit rate (13 ± 4 versus 11 ± 4 mL/kg/min, P < 0.05), while the VO2 was similar (both 52 ± 3 mL/kg/min). These findings emphasise the importance of a fast start. The within-athlete coefficient of variation (CV) in performance time, VO2 and O2 deficit were 1.3 ± 0.4%, 1.4 ± 0.9% and 11.2 ± 4.9%, respectively, with the CV in O2 deficit explaining 69% of the CV in performance. The pacing strategies were highly consistent, with an average CV in speed of 3.4%.CONCLUSION: The fastest STT was characterized by more aggressive pacing and a greater anaerobic energy production. Although the individual performance time during the four STTs was highly consistent, the small within-athlete variability in performance was related to variations in anaerobic energy production.

  • 3.
    Andersson, Erik
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Björklund, Glenn
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Metabolic responses and pacing strategies during successive sprint skiing time trials2016In: Medicine & Science in Sports & Exercise, ISSN 0195-9131, E-ISSN 1530-0315, Vol. 48, no 12, p. 2544-2554Article in journal (Refereed)
    Abstract [en]

    PURPOSE: To examine the metabolic responses and pacing strategies during the performance of successive sprint time trials (STTs) in cross-country skiing. METHODS: Ten well-trained male cross-country skiers performed four self-paced 1300-m STTs on a treadmill, each separated by 45 min of recovery. The simulated STT course was divided into three flat (1°) sections (S1, S3 and S5) involving the double poling sub-technique interspersed with two uphill (7°) sections (S2 and S4) involving the diagonal stride sub-technique. Treadmill velocity and V˙O2 were monitored continuously and gross efficiency was used to estimate the anaerobic energy supply. RESULTS: The individual trial-to-trial variability in STT performance time was 1.3%, where variations in O2 deficit and V˙O2 explained 69% (P < 0.05) and 11% (P > 0.05) of the variation in performance. The first and last STTs were equally fast (228 ± 10 s), and ~ 1.3% faster than the second and the third STTs (P < 0.05). These two fastest STTs were associated with a 14% greater O2 deficit (P < 0.05), while the average V˙O2 was similar during all four STTs (86 ± 3% of V˙O2max). Positive pacing was used throughout all STTs, with significantly less time spent on the first than second course half. In addition, metabolic rates were substantially higher (~_30%) for uphill than for flat skiing, indicating that pacing was regulated to the terrain. CONCLUSIONS: The fastest STTs were characterized primarily by a greater anaerobic energy production, which also explained 69% of the individual variation in performance. Moreover, the skiers employed positive pacing and a variable exercise intensity according to the course profile, yielding an irregular distribution of anaerobic energy production.

  • 4.
    Andersson, Erik
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Willis, Sarah J.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Energy System Contributions And Determinants Of Performance In Classical Sprint Cross-Country Skiing2014In: Proceedings for the 19th ECCS in Amsterdam, The Netherlands, 2014Conference paper (Refereed)
  • 5.
    Cheng, Arthur J.
    et al.
    Karolinska Institutet.
    Willis, Sarah J.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Zinner, Christoph
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Chaillou, Thomas
    Karolinska Institutet; Örebro universitet.
    Ivarsson, Niklas
    Karolinska Institutet.
    Ørtenblad, Niels
    University of Southern Denmark, Odense, Denmark.
    Lanner, Johanna T.
    Karolinska Institutet.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Karolinska Institutet.
    Westerblad, Håkan
    Karolinska Institutet.
    Post-exercise recovery of contractile function and endurance in humans and mice is accelerated by heating and slowed by cooling skeletal muscle2017In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 595, no 24, p. 7413-7426Article in journal (Refereed)
    Abstract [en]

    Key points: We investigated whether intramuscular temperature affects the acute recovery of exercise performance following fatigue-induced by endurance exercise. Mean power output was better preserved during an all-out arm-cycling exercise following a 2 h recovery period in which the upper arms were warmed to an intramuscular temperature of ˜ 38°C than when they were cooled to as low as 15°C, which suggested that recovery of exercise performance in humans is dependent on muscle temperature. Mechanisms underlying the temperature-dependent effect on recovery were studied in intact single mouse muscle fibres where we found that recovery of submaximal force and restoration of fatigue resistance was worsened by cooling (16-26°C) and improved by heating (36°C). Isolated whole mouse muscle experiments confirmed that cooling impaired muscle glycogen resynthesis. We conclude that skeletal muscle recovery from fatigue-induced by endurance exercise is impaired by cooling and improved by heating, due to changes in glycogen resynthesis rate.

    Manipulation of muscle temperature is believed to improve post-exercise recovery, with cooling being especially popular among athletes. However, it is unclear whether such temperature manipulations actually have positive effects. Accordingly, we studied the effect of muscle temperature on the acute recovery of force and fatigue resistance after endurance exercise. One hour of moderate-intensity arm cycling exercise in humans was followed by 2 h recovery in which the upper arms were either heated to 38°C, not treated (33°C), or cooled to ∼15°C. Fatigue resistance after the recovery period was assessed by performing 3 × 5 min sessions of all-out arm cycling at physiological temperature for all conditions (i.e. not heated or cooled). Power output during the all-out exercise was better maintained when muscles were heated during recovery, whereas cooling had the opposite effect. Mechanisms underlying the temperature-dependent effect on recovery were tested in mouse intact single muscle fibres, which were exposed to ∼12 min of glycogen-depleting fatiguing stimulation (350 ms tetani given at 10 s interval until force decreased to 30% of the starting force). Fibres were subsequently exposed to the same fatiguing stimulation protocol after 1-2 h of recovery at 16-36°C. Recovery of submaximal force (30 Hz), the tetanic myoplasmic free [Ca2+] (measured with the fluorescent indicator indo-1), and fatigue resistance were all impaired by cooling (16-26°C) and improved by heating (36°C). In addition, glycogen resynthesis was faster at 36°C than 26°C in whole flexor digitorum brevis muscles. We conclude that recovery from exhaustive endurance exercise is accelerated by raising and slowed by lowering muscle temperature.

  • 6.
    Fredsted, A.
    et al.
    Aarhus Univ, Dept Biomed, DK-8000 Aarhus C, Denmark.
    Gissel, H.
    Aarhus Univ, Dept Biomed, DK-8000 Aarhus C, Denmark.
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Clausen, T.
    Aarhus Univ, Dept Biomed, DK-8000 Aarhus C, Denmark.
    Effects of beta(2)-agonists on force during and following anoxia in rat extensor digitorum longus muscle2012In: Journal of applied physiology, ISSN 8750-7587, E-ISSN 1522-1601, Vol. 112, no 12, p. 2057-2067Article in journal (Refereed)
    Abstract [en]

    Fredsted A, Gissel H, Ortenblad N, Clausen T. Effects of beta(2)-agonists on force during and following anoxia in rat extensor digitorum longus muscle. J Appl Physiol 112: 2057-2067, 2012. First published April 5, 2012; doi:10.1152/japplphysiol.01558.2011.-Electrical stimulation of isolated muscles may lead to membrane depolarization, gain of Na+, loss of K+ and fatigue. These effects can be counteracted with beta(2)-agonists possibly via activation of the Na+-K+ pumps. Anoxia induces loss of force; however, it is not known whether beta(2)-agonists affect force and ion homeostasis in anoxic muscles. In the present study isolated rat extensor digitorum longus (EDL) muscles exposed to anoxia showed a considerable loss of force, which was markedly reduced by the beta(2)-agonists salbutamol (10(-6) M) and terbutaline (10(-6) M). Intermittent stimulation (15-30 min) clearly increased loss of force during anoxia and reduced force recovery during reoxygenation. The beta(2)-agonists salbutamol (10(-7)-10(-5) M) and salmeterol (10(-6) M) improved force development during anoxia (25%) and force recovery during reoxygenation (55-262%). The effects of salbutamol on force recovery were prevented by blocking the Na+-K+ pumps with ouabain or by blocking glycolysis with 2-deoxyglucose. Dibutyryl cAMP (1 mM) or theophylline (1 mM) also improved force recovery remarkably. In anoxic muscles, salbutamol decreased intracellular Na+ and increased Rb-86 uptake and K+ content, indicating stimulation of the Na+-K+ pumps. In fatigued muscles salbutamol induced recovery of excitability. Thus beta(2)-agonists reduce the anoxia-induced loss of force, leading to partial force recovery. These data strongly suggest that this effect is mediated by cAMP stimulation of the Na+-K+ pumps and that it is not related to recovery of energy status (PCr, ATP, lactate).

  • 7.
    Gejl, K. D.
    et al.
    Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark .
    Hvid, L. G.
    Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark .
    Frandsen, U.
    Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark .
    Jensen, Kurt
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark .
    Sahlin, K.
    Åstrand Laboratory, Swedish School of Sport and Health Sciences, Stockholm, Sweden .
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark .
    Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes2014In: Medicine & Science in Sports & Exercise, ISSN 0195-9131, E-ISSN 1530-0315, Vol. 46, no 3, p. 496-505Article in journal (Refereed)
    Abstract [en]

    Purpose: The aim of the present study was to investigate the influence of muscle glycogen content on sarcoplasmic reticulum (SR) function and peak power output (Wpeak) in elite endurance athletes. Methods: Fourteen highly trained male triathletes (V̇O2max = 66.5 ± 1.3 mL O2·kg·min), performed 4 h of glycogen-depleting cycling exercise (HRmean = 73% ± 1% of maximum). During the first 4 h of recovery, athletes received either water (H2O) or carbohydrate (CHO), separating alterations in muscle glycogen content from acute changes affecting SR function and performance. Thereafter, all subjects received CHO-enriched food for the remaining 20-h recovery period. Results: Immediately after exercise, muscle glycogen content and SR Ca release rate was reduced to 32% ± 4% (225 ± 28 mmol·kg dw) and 86% ± 2% of initial levels, respectively (P < 0.01). Glycogen markedly recovered after 4 h of recovery with CHO (61% ± 2% of preexercise) and SR Ca release rate returned to preexercise level. However, in the absence of CHO during the first 4 h of recovery, glycogen and SR Ca release rate remained depressed, with the normalization of both parameters at the end of the 24 h of recovery after receiving a CHO-enriched diet. Linear regression demonstrated a significant correlation between SR Ca release rate and muscle glycogen content (P < 0.01, r = 0.30). The 4 h of cycling exercise reduced Wpeak by 5.5%-8.9% at different cadences (P < 0.05), and Wpeak was normalized after 4 h of recovery with CHO, whereas Wpeak remained depressed (P < 0.05) after water provision. Wpeak was fully recovered after 24 h in both the H2O and the CHO group. Conclusion: In conclusion, the present results suggest that low muscle glycogen depresses muscle SR Ca release rate, which may contribute to fatigue and delayed recovery of Wpeak 4 h postexercise. © 2014 by the American College of Sports Medicine.

  • 8.
    Gejl, Kasper D.
    et al.
    Univ Southern Denmark, Odense, Denmark.
    Vissing, Kristian
    Aarhus Univ, Aarhus, Denmark.
    Hansen, Mette
    Aarhus Univ, Aarhus, Denmark.
    Thams, Line
    Univ Southern Denmark, Odense, Denmark.
    Rokkedal-Lausch, Torben
    Aalborg Univ, Aalborg, Denmark.
    Plomgaard, Peter
    Rigshosp, Copenhagen, Denmark; Univ Copenhagen, Rigshosp, Copenhagen, Denmark.
    Lundby, Anne-Kristine Meinild
    Univ Copenhagen, Rigshosp, Copenhagen, Denmark.
    Nybo, Lars
    Univ Copenhagen, Copenhagen, Denmark.
    Jensen, Kurt
    Univ Southern Denmark, Odense, Denmark.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Swedish Olymp Comm, Stockholm.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Odense, Denmark .
    Changes in metabolism but not myocellular signaling by training with CHO-restriction in endurance athletes2018In: Physiological Reports, E-ISSN 2051-817X, Vol. 6, no 17, article id e13847Article in journal (Refereed)
    Abstract [en]

    Carbohydrate (CHO) restricted training has been shown to increase the acute training response, whereas less is known about the acute effects after repeated CHO restricted training. On two occasions, the acute responses to CHO restriction were examined in endurance athletes. Study 1 examined cellular signaling and metabolic responses after seven training-days including CHO manipulation (n = 16). The protocol consisted of 1 h high-intensity cycling, followed by 7 h recovery, and 2 h of moderate-intensity exercise (120SS). Athletes were randomly assigned to low (LCHO: 80 g) or high (HCHO: 415 g) CHO during recovery and the 120SS. Study 2 examined unaccustomed exposure to the same training protocol (n = 12). In Study 1, muscle biopsies were obtained at rest and 1 h after 120SS, and blood samples drawn during the 120SS. In Study 2, substrate oxidation and plasma glucagon were determined. In Study 1, plasma insulin and proinsulin C-peptide were higher during the 120SS in HCHO compared to LCHO (insulin: 0 min: +37%; 60 min: +135%; 120 min: +357%, P = 0.05; proinsulin C-peptide: 0 min: +32%; 60 min: +52%; 120 min: +79%, P = 0.02), whereas plasma cholesterol was higher in LCHO (+15-17%, P = 0.03). Myocellular signaling did not differ between groups. p-AMPK and p-ACC were increased after 120SS (+35%, P = 0.03; +59%, P = 0.0004, respectively), with no alterations in p-p38, p-53, or p-CREB. In Study 2, glucagon and fat oxidation were higher in LCHO compared to HCHO during the 120SS (+26-40%, P = 0.03; +44-76%, P = 0.01 respectively). In conclusion, the clear respiratory and hematological effects of CHO restricted training were not translated into superior myocellular signaling after accustomization to CHO restriction.

  • 9.
    Gejl, Kasper D.
    et al.
    Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster, University of Southern Denmark, Odense, Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Denmark.
    Andersson, Erik
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Plomgaard, Peter
    Department of Clinical Biochemistry, Rigshospitalet, Copenhagen, Denmark.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Swedish Olymp Comm, Stockholm.
    Nielsen, Joachim
    Department of Pathology, SDU Muscle Research Cluster, Odense University Hospital, Odense.
    Local depletion of glycogen with supra-maximal exercise in human skeletal muscle fibres2017In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 595, no 9, p. 2809-2821Article in journal (Refereed)
    Abstract [en]

    Skeletal muscle glycogen is heterogeneous distributed in three separated compartments (intramyofibrillar, intermyofibrillar and subsarcolemmal). Although only constituting 4-15% of the total glycogen volume, the availability of intramyofibrillar glycogen has been shown to be of particular importance to muscle function. The present study was designed to investigate the depletion of these three sub-cellular glycogen compartments during repeated supra-maximal exercise in elite athletes. Ten elite cross-country skiers (age: 25 +/- 4 yrs., VO2 max : 65 +/- 4 ml kg-1 min-1 , mean +/- SD) performed four approximately 4-minute supra-maximal sprint time trials (STT 1-4) with 45 min recovery. The sub-cellular glycogen volumes in m. triceps brachii were quantified from electron microscopy images before and after both STT 1 and STT 4. During STT 1, the depletion of intramyofibrillar glycogen was higher in type I fibres (-52% [-89:-15%]) than type 2 fibres (-15% [-52:22%]) (P = 0.02), while the depletion of intermyofibrillar glycogen (main effect: -19% [-33:0], P = 0.006) and subsarcolemmal glycogen (main effect: -35% [-66:0%], P = 0.03) was similar between fibre types. In contrast, only intermyofibrillar glycogen volume was significantly reduced during STT 4, in both fibre types (main effect: -31% [-50:-11%], P = 0.002). Furthermore, for each of the sub-cellular compartments, the depletion of glycogen during STT 1 was associated with the volumes of glycogen before STT 1. In conclusion, the depletion of spatially distinct glycogen compartments differs during supra-maximal exercise. Furthermore, the depletion changes with repeated exercise and is fibre type-dependent. 

  • 10.
    Gejl, Kasper Degn
    et al.
    Univ Southern Denmark, Dept Sports Sci & Clin Biomech, Odense M, Denmark.
    Thams, Line Bork
    Univ Southern Denmark, Dept Sports Sci & Clin Biomech, Odense M, Denmark.
    Hansen, Mette
    Aarhus Univ, Sect Sport Sci, Dept Publ Hlth, Aarhus, Denmark.
    Rokkedal-Lausch, Torben
    Aalborg Univ, Dept Hlth Sci & Technol, SMI, Fac Med, Aalborg, Denmark.
    Plomgaard, Peter
    Rigshosp, Dept Clin Biochem, Copenhagen, Denmark; Univ Copenhagen, Dept Infect Dis, Ctr Phys Act Res, Rigshosp, Copenhagen, Denmark.
    Nybo, Lars
    Univ Copenhagen, Dept Nutr Exercise & Sports, Copenhagen, Denmark.
    Larsen, Filip J.
    Karolinska Inst, Dept Physiol & Pharmacol, Stockholm; Swedish Sch Sport & Hlth Sci, Stockholm.
    Cardinale, Daniele A.
    Swedish Sch Sport & Hlth Sci, Stockholm; Swedish Sports Confederat, Elite Performance Ctr, Stockholm.
    Jensen, Kurt
    Univ Southern Denmark, Dept Sports Sci & Clin Biomech, Odense M, Denmark.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Swedish Olymp Comm, Stockholm.
    Vissing, Kristian
    Aarhus Univ, Sect Sport Sci, Dept Publ Hlth, Aarhus, Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Dept Sports Sci & Clin Biomech, Odense M, Denmark.
    No Superior Adaptations to Carbohydrate Periodization in Elite Endurance Athletes2017In: Medicine & Science in Sports & Exercise, ISSN 0195-9131, E-ISSN 1530-0315, Vol. 49, no 12, p. 2486-2497Article in journal (Refereed)
    Abstract [en]

    Purpose The present study investigated the effects of periodic carbohydrate (CHO) restriction on endurance performance and metabolic markers in elite endurance athletes. Methods Twenty-six male elite endurance athletes (maximal oxygen consumption (VO2max), 65.0 mL O(2)kg(-1)min(-1)) completed 4 wk of regular endurance training while being matched and randomized into two groups training with (low) or without (high) CHO manipulation 3 dwk(-1). The CHO manipulation days consisted of a 1-h high-intensity bike session in the morning, recovery for 7 h while consuming isocaloric diets containing either high CHO (414 2.4 g) or low CHO (79.5 1.0 g), and a 2-h moderate bike session in the afternoon with or without CHO. VO2max, maximal fat oxidation, and power output during a 30-min time trial (TT) were determined before and after the training period. The TT was undertaken after 90 min of intermittent exercise with CHO provision before the training period and both CHO and placebo after the training period. Muscle biopsies were analyzed for glycogen, citrate synthase (CS) and -hydroxyacyl-coenzyme A dehydrogenase (HAD) activity, carnitine palmitoyltransferase (CPT1b), and phosphorylated acetyl-CoA carboxylase (pACC). Results The training effects were similar in both groups for all parameters. On average, VO2max and power output during the 30-min TT increased by 5% +/- 1% (P < 0.05) and TT performance was similar after CHO and placebo during the preload phase. Training promoted overall increases in glycogen content (18% +/- 5%), CS activity (11% +/- 5%), and pACC (38% +/- 19%; P < 0.05) with no differences between groups. HAD activity and CPT1b protein content remained unchanged. Conclusions Superimposing periodic CHO restriction to 4 wk of regular endurance training had no superior effects on performance and muscle adaptations in elite endurance athletes.

  • 11.
    Hostrup, M.
    et al.
    Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark .
    Kalsen, A.
    Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark .
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Department of Sports Science and Biomechanics, University of Southern Denmark, Denmark .
    Juel, C.
    Department of Biology, University of Copenhagen, Denmark .
    Mørch, K.
    Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark .
    Rzeppa, S.
    Norwegian Doping Control Laboratory, Oslo University Hospital, Norway .
    Karlsson, S.
    Department of Respiratory Research, Bispebjerg University Hospital, Denmark .
    Backer, V.
    Department of Respiratory Research, Bispebjerg University Hospital, Denmark .
    Bangsbo, J.
    Department of Nutrition, Exercise and Sports, Section of Integrated Physiology, University of Copenhagen, Denmark .
    β2-Adrenergic stimulation enhances Ca2+ release and contractile properties of skeletal muscles, and counteracts exercise-induced reductions in Na+-K+-ATPase Vmax in trained men2014In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 592, no 24, p. 5445-5459Article in journal (Refereed)
    Abstract [en]

    The aim of the present study was to examine the effect of β2-adrenergic stimulation on skeletal muscle contractile properties, sarcoplasmic reticulum (SR) rates of Ca2+ release and uptake, and Na+-K+-ATPase activity before and after fatiguing exercise in trained men. The study consisted of two experiments (EXP1, n = 10 males, EXP2, n = 20 males), where β2-adrenoceptor agonist (terbutaline) or placebo was randomly administered in double-blinded crossover designs. In EXP1, maximal voluntary isometric contraction (MVC) of m. quadriceps was measured, followed by exercise to fatigue at 120% of maximal oxygen uptake (V˙O2, max ). A muscle biopsy was taken after MVC (non-fatigue) and at time of fatigue. In EXP2, contractile properties of m. quadriceps were measured with electrical stimulations before (non-fatigue) and after two fatiguing 45 s sprints. Non-fatigued MVCs were 6 ± 3 and 6 ± 2% higher (P &lt; 0.05) with terbutaline than placebo in EXP1 and EXP2, respectively. Furthermore, peak twitch force was 11 ± 7% higher (P &lt; 0.01) with terbutaline than placebo at non-fatigue. After sprints, MVC declined (P &lt; 0.05) to the same levels with terbutaline as placebo, whereas peak twitch force was lower (P &lt; 0.05) and half-relaxation time was prolonged (P &lt; 0.05) with terbutaline. Rates of SR Ca2+ release and uptake at 400 nm [Ca2+] were 15 ± 5 and 14 ± 5% (P &lt; 0.05) higher, respectively, with terbutaline than placebo at non-fatigue, but declined (P &lt; 0.05) to similar levels at time of fatigue. Na+-K+-ATPase activity was unaffected by terbutaline compared with placebo at non-fatigue, but terbutaline counteracted exercise-induced reductions in maximum rate of activity (Vmax) at time of fatigue. In conclusion, increased contractile force induced by β2-adrenergic stimulation is associated with enhanced rate of Ca2+ release in humans. While β2-adrenergic stimulation elicits positive inotropic and lusitropic effects on non-fatigued m. quadriceps, these effects are blunted when muscles fatigue.

  • 12.
    Hostrup, Morten
    et al.
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark.;Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Kalsen, Anders
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark.;Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Onslev, Johan
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark.;Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Jessen, Soren
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark.;Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Haase, Christoffer
    Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Habib, Sajad
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark..
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Dept Sports Sci & Biomech, Odense, Denmark.;Mid Sweden Univ, Swedish Winter Sports Res Ctr, Sundsvall, Sweden..
    Backer, Vibeke
    Bispebjerg Hosp, Dept Resp Res, Copenhagen, Denmark..
    Bangsbo, Jens
    Univ Copenhagen, Dept Nutr Exercise & Sports, Sect Integrated Physiol, DK-2100 Copenhagen, Denmark..
    Mechanisms underlying enhancements in muscle force and power output during maximal cycle ergometer exercise induced by chronic beta(2)-adrenergic stimulation in men2015In: Journal of applied physiology, ISSN 8750-7587, E-ISSN 1522-1601, Vol. 119, no 5, p. 475-486Article in journal (Refereed)
    Abstract [en]

    The study was a randomized placebo-controlled trial investigating mechanisms by which chronic beta(2)-adrenergic stimulation enhances muscle force and power output during maximal cycle ergometer exercise in young men. Eighteen trained men were assigned to an experimental group [oral terbutaline 5 mg/30 kg body weight (bw) twice daily (TER); n = 9] or a control group [placebo (PLA); n = 9] for a 4-wk intervention. No changes were observed with the intervention in PLA. Isometric muscle force of the quadriceps increased (P <= 0.01) by 97 +/- 29 N (means +/- SE) with the intervention in TER compared with PLA. Peak and mean power output during 30 s of maximal cycling increased (P <= 0.01) by 32 +/- 8 and 25 +/- 9 W, respectively, with the intervention in TER compared with PLA. Maximal oxygen consumption ((V) over dotO(2)max) and time to fatigue during incremental cycling did not change with the intervention. Lean body mass increased by 1.95 +/- 0.8 kg (P <= 0.05) with the intervention in TER compared with PLA. Change in single fiber cross-sectional area of myosin heavy chain (MHC) I (1,205 +/- 558 mu m(2); P <= 0.01) and MHC II fibers (1,277 +/- 595 mu m(2); P <= 0.05) of the vastus lateralis muscle was higher for TER than PLA with the intervention, whereas no changes were observed in MHC isoform distribution. Expression of muscle proteins involved in growth, ion handling, lactate production, and clearance increased (P <= 0.05) with the intervention in TER compared with PLA, with no change in oxidative enzymes. Our observations suggest that muscle hypertrophy is the primary mechanism underlying enhancements in muscle force and peak power during maximal cycling induced by chronic beta(2-)adrenergic stimulation in humans.

  • 13.
    Hvid, L. G.
    et al.
    Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark .
    Gejl, K.
    Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark .
    Bech, R. D.
    Department of Orthopaedic Surgery, Odense University Hospital, Odense, Denmark .
    Nygaard, T.
    Department of Orthopaedic Surgery, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark .
    Jensen, K.
    Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark .
    Frandsen, U.
    Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark .
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Institute of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark .
    Transient impairments in single muscle fibre contractile function after prolonged cycling in elite endurance athletes2013In: Acta Physiologica, ISSN 1748-1708, E-ISSN 1748-1716, Vol. 208, no 3, p. 265-273Article in journal (Refereed)
    Abstract [en]

    Aim Prolonged muscle activity impairs whole-muscle performance and function. However, little is known about the effects of prolonged muscle activity on the contractile function of human single muscle fibres. The purpose of this study was to investigate the effects of prolonged exercise and subsequent recovery on the contractile function of single muscle fibres obtained from elite athletes. Methods Nine male triathletes (26 +/- 1years, 68 +/- 1mL O2min-1 kg-1, training volume 16 +/- 1hweek-1) performed 4h of cycling exercise (at 73% of HRmax) followed by 24h of recovery. Biopsies from vastus lateralis were obtained before and following 4h exercise and following 24h recovery. Measurements comprised maximal Ca2+-activated specific force and Ca2+ sensitivity of slow type I and fast type II single muscle fibres, as well as cycling peak power output. Results Following cycling exercise, specific force was reduced to a similar extent in slow and fast fibres (-15 and -18%, respectively), while Ca2+ sensitivity decreased in fast fibres only. Single fibre-specific force was fully restored in both fibre types after 24h recovery. Cycling peak power output was reduced by 4-9% following cycling exercise and fully restored following recovery. Conclusion This is the first study to demonstrate that prolonged cycling exercise transiently impairs specific force in type I and II fibres and decreases Ca2+ sensitivity in type II fibres only, specifically in elite endurance athletes. Further, the changes in single fibre-specific force induced by exercise and recovery coincided temporally with changes in cycling peak power output.

  • 14.
    Hvid, Lars G.
    et al.
    Aarhus University, Aarhus, Denmark.
    Aagaard, Per
    University of Southern Denmark (SDU), Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. University of Southern Denmark (SDU), Denmark.
    Kjaer, Michael
    University of Copenhagen, Denmark.
    Suetta, Charlotte
    Rigshospitalet Glostrup, University of Copenhagen, Denmark.
    Plasticity in central neural drive with short-term disuse and recovery - effects on muscle strength and influence of aging2018In: Experimental Gerontology, ISSN 0531-5565, E-ISSN 1873-6815, Vol. 106, p. 145-153Article in journal (Refereed)
    Abstract [en]

    While short-term disuse negatively affects mechanical muscle function (e.g. isometric muscle strength) little is known of the relative contribution of adaptions in central neural drive and peripheral muscle contractility. The present study investigated the relative contribution of adaptations in central neural drive and peripheral muscle contractility on changes in isometric muscle strength following short-term unilateral disuse (4 days, knee brace) and subsequent active recovery (7 days, one session of resistance training) in young (n = 11, 24 yrs) and old healthy men (n = 11, 67 yrs). Maximal isometric knee extensor strength (MVC) (isokinetic dynamometer), voluntary muscle activation (superimposed twitch technique), and electrically evoked muscle twitch force (single and doublet twitch stimulation) were assessed prior to and after disuse, and after recovery. Following disuse, relative decreases in MVC did not differ statistically between old (16.4 ± 3.7%, p < 0.05) and young (−9.7 ± 2.9%, p < 0.05) (mean ± SE), whereas voluntary muscle activation decreased more (p < 0.05) in old (−8.4 ± 3.5%, p < 0.05) compared to young (−1.1 ± 1.0%, ns) as did peak single (−25.8 ± 6.6%, p < 0.05 vs −7.6 ± 3.3%, p < 0.05) and doublet twitch force (−23.2 ± 5.5%, p < 0.05 vs −2.0 ± 2.6%, ns). All parameters were restored in young following 7 days recovery, whereas MVC and peak twitch force remained suppressed in old. Regression analysis revealed that disuse-induced changes in MVC relied more on changes in single twitch force in young (p < 0.05) and more on changes in voluntary muscle activation in old (p < 0.05), whereas recovery-induced changes in MVC mainly were explained by gains in voluntary muscle activation in both young and old. Altogether, the present data demonstrate that plasticity in voluntary muscle activation (~central neural drive) is a dominant mechanism affecting short-term disuse- and recovery-induced changes in muscle strength in older adults. 

  • 15.
    Hvid, Lars G.
    et al.
    University of Southern Denmark (SDU), Denmark.
    Brocca, Lorenza
    University of Pavia, Italy.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. University of Southern Denmark (SDU), Denmark.
    Suetta, Charlotte
    University of Copenhagen, Denmark.
    Aagaard, Per
    University of Southern Denmark (SDU), Denmark.
    Kjaer, Michael
    University of Copenhagen, Denmark.
    Bottinelli, Roberto
    University of Pavia, Italy.
    Pellegrino, Maria Antonietta
    University of Pavia, Italy.
    Myosin content of single muscle fibers following short-term disuse and active recovery in young and old healthy men2017In: Experimental Gerontology, ISSN 0531-5565, E-ISSN 1873-6815, Vol. 87, no Part A, p. 100-107Article in journal (Refereed)
    Abstract [en]

    Short-term disuse and subsequent recovery affect whole muscle and single myofiber contractile function in young and old. While the loss and recovery of single myofiber specific force (SF) following disuse and rehabilitation has been shown to correlate with alterations in myosin concentrations in young, it is unknown whether similar relationships exist in old. Therefore, the purpose of the present study was to examine the effect of 14 days lower limb disuse followed by 28 days of active recovery on single muscle fiber myosin content in old (68 yrs) and young (24 yrs) recreationally physically active healthy men. Following disuse, myosin content decreased (p < 0.05) in MHC 1 (young − 28%, old − 19%) and 2a fibers (young − 23%, old − 32%). In old, myosin content decreased more (p < 0.05) in MHC 2a vs 1 fibers. Following recovery, myosin content increased (p < 0.05) and returned to pre-disuse levels for both young and old in both fiber types, with MHC 2a fibers demonstrating an overshooting in young (+ 31%, p < 0.05) but not old. Strong correlations were observed between myosin content and single fiber SF in both young and old, with greater slope steepness in MHC 2a vs 1 fibers indicating an enhanced intrinsic contractile capacity of MHC 2a fibers. In conclusion, adaptive changes in myofiber myosin content appear to occur rapidly following brief periods of disuse (2 wks) and after subsequent active recovery (4 wks) in young and old, which contribute to alterations in contractile function at the single muscle fiber level. Changes in myosin content appear to occur independently of age, while influenced by fiber type (MHC isoform) in young but not old.

  • 16.
    Jensen, Line
    et al.
    University of Southern Denmark, Odense, Denmark.
    Gejl, Kasper D.
    University of Southern Denmark, Odense, Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. University of Southern Denmark, Odense, Denmark.
    Nielsen, Jakob L.
    University of Southern Denmark, Odense, Denmark.
    Bech, Rune D.
    Department of Orthopedic Surgery, Odense University Hospital, Odense, Denmark.
    Nygaard, Tobias
    Department of Orthopedic Surgery, Rigshospitalet, Copenhagen, Denmark.
    Sahlin, Kent
    The Swedish School of Sport and Health Sciences, Stockholm.
    Frandsen, Ulrik
    University of Southern Denmark, Odense, Denmark.
    Carbohydrate restricted recovery from long term endurance exercise does not affect gene responses involved in mitochondrial biogenesis in highly trained athletes2015In: Physiological Reports, E-ISSN 2051-817X, Vol. 3, no 2, article id e12184Article in journal (Refereed)
    Abstract [en]

    The aim was to determine if the metabolic adaptations, particularly PGC-1a and downstream metabolic genes were affected by restricting CHO following an endurance exercise bout in trained endurance athletes. A second aim was to compare baseline expression level of these genes to untrained. Elite endurance athletes (VO2max 66 ± 2 mL·kg-1·min-1, n = 15) completed 4 h cycling at ~56% VO2max. During the first 4 h recovery subjects were provided with either CHO or only H2O and thereafter both groups received CHO. Muscle biopsies were collected before, after, and 4 and 24 h after exercise. Also, resting biopsies were collected from untrained subjects (n = 8). Exercise decreased glycogen by 67.7 ± 4.0% (from 699 ± 26.1 to 239 ± 29.5 mmol·kg-1·dw-1) with no difference between groups. Whereas 4 h of recovery with CHO partly replenished glycogen, the H2O group remained at post exercise level; nevertheless, the gene expression was not different between groups. Glycogen and most gene expression levels returned to baseline by 24 h in both CHO and H2O. Baseline mRNA expression of NRF-1, COX-IV, GLUT4 and PPAR-α gene targets were higher in trained compared to untrained. Additionally, the proportion of type I muscle fibers positively correlated with baseline mRNA for PGC-1α, TFAM, NRF-1, COX-IV, PPAR-α, and GLUT4 for both trained and untrained. CHO restriction during recovery from glycogen depleting exercise does not improve the mRNA response of markers of mitochondrial biogenesis. Further, baseline gene expression of key metabolic pathways is higher in trained than untrained.

  • 17.
    Kent, Jane A.
    et al.
    Univ Massachusetts, Dept Kinesiol, Totman 160A, Amherst, MA 01003 USA.
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Inst Sports Sci & Clin Biomech, Odense, Denmark.
    Hogan, Michael C.
    Univ Calif San Diego, Dept Med, San Diego, CA 92103 USA.
    Poole, David C.
    Kansas State Univ, Dept Kinesiol, Manhattan, KS 66506 USA.
    Musch, Timothy I.
    Kansas State Univ, Dept Kinesiol, Manhattan, KS 66506 USA.
    No Muscle Is an Island: Integrative Perspectives on Muscle Fatigue2016In: Medicine & Science in Sports & Exercise, ISSN 0195-9131, E-ISSN 1530-0315, Vol. 48, no 11, p. 2281-2293Article in journal (Refereed)
    Abstract [en]

    Muscle fatigue has been studied with a variety approaches, tools and technologies. The foci of these studies have ranged tremendously, from molecules to the entire organism. Single cell and animal models have been used to gain mechanistic insight into the fatigue process. The theme of this review is the concept that the mechanisms of muscle fatigue do not occur in isolation in vivo: muscular work is supported by many complex physiological systems, any of which could fail during exercise and thus contribute to fatigue. To advance our overall understanding of fatigue, a combination of models and approaches is necessary. In this review, we examine the roles that neuromuscular properties, intracellular glycogen, oxygen metabolism, and blood flow play in the fatigue process during exercise and pathological conditions.

  • 18.
    Nielsen, Joachim
    et al.
    Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense M, DK-5230, Denmark .
    Cheng, Arthur J.
    Department of Physiology and Pharmacology, Karolinska Institutet, SE-171 77, Stockholm, Sweden .
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense M, DK-5230, Denmark .
    Westerblad, Håkan
    Department of Physiology and Pharmacology, Karolinska Institutet, SE-171 77, Stockholm, Sweden .
    Subcellular distribution of glycogen and decreased tetanic Ca2+ in fatigued single intact mouse muscle fibres2014In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 592, no 9, p. 2003-2012Article in journal (Refereed)
    Abstract [en]

    Key points Muscle glycogen (the storage form of glucose) is consumed during muscle work and the depletion of glycogen is thought to be a main contributor to muscle fatigue. In this study, we used a novel approach to first measure fatigue-induced reductions in force and tetanic Ca2+ in isolated single mouse muscle fibres following repeated contractions and subsequently quantify the subcellular distribution of glycogen in the same fibre. Using this approach, we investigated whether the decreased tetanic Ca2+ induced by repeated contractions was associated with glycogen depletion in certain subcellular regions. The results show a positive correlation between depletion of glycogen located within the myofibrils and low tetanic Ca2+ after repetitive stimulation. We conclude that subcellular glycogen depletion has a central role in the decrease in tetanic Ca2+ that occurs during repetitive contractions. In skeletal muscle fibres, glycogen has been shown to be stored at different subcellular locations: (i) between the myofibrils (intermyofibrillar); (ii) within the myofibrils (intramyofibrillar); and (iii) subsarcolemmal. Of these, intramyofibrillar glycogen has been implied as a critical regulator of sarcoplasmic reticulum Ca2+ release. The aim of the present study was to test directly how the decrease in cytoplasmic free Ca2+ ([Ca2+](i)) during repeated tetanic contractions relates to the subcellular glycogen distribution. Single fibres of mouse flexor digitorum brevis muscles were fatigued with 70Hz, 350ms tetani given at 2s (high-intensity fatigue, HIF) or 10s (low-intensity fatigue, LIF) intervals, while force and [Ca2+](i) were measured. Stimulation continued until force decreased to 30% of its initial value. Fibres were then prepared for analyses of subcellular glycogen distribution by transmission electron microscopy. At fatigue, tetanic [Ca2+](i) was reduced to 70 +/- 4% and 54 +/- 4% of the initial in HIF (P<0.01, n=9) and LIF (P<0.01, n=5) fibres, respectively. At fatigue, the mean inter- and intramyofibrillar glycogen content was 60-75% lower than in rested control fibres (P<0.05), whereas subsarcolemmal glycogen was similar to control. Individual fibres showed a good correlation between the fatigue-induced decrease in tetanic [Ca2+](i) and the reduction in intermyofibrillar (P=0.051) and intramyofibrillar (P=0.0008) glycogen. In conclusion, the fatigue-induced decrease in tetanic [Ca2+](i), and hence force, is accompanied by major reductions in inter- and intramyofibrillar glycogen. The stronger correlation between decreased tetanic [Ca2+](i) and reduced intramyofibrillar glycogen implies that sarcoplasmic reticulum Ca2+ release critically depends on energy supply from the intramyofibrillar glycogen pool.

  • 19.
    Nielsen, Joachim
    et al.
    Univ Southern Denmark, Denmark.
    Gejl, Kasper D.
    Univ Southern Denmark, Denmark.
    Hey-Mogensen, Martin
    Univ Southern Denmark, Denmark.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Suetta, Charlotte
    Univ Copenhagen, Denmark.
    Krustrup, Peter
    Univ Southern Denmark, Denmark; Univ Exeter, England.
    Elemans, Coen P. H.
    Univ Southern Denmark, Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Denmark.
    Plasticity in mitochondrial cristae density allows metabolic capacity modulation in human skeletal muscle2017In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 595, no 9, p. 2839-2847Article in journal (Refereed)
    Abstract [en]

    Mitochondrial energy production involves the movement of protons down a large electrochemical gradient via ATP synthase located on the folded inner membrane, known as cristae. In mammalian skeletal muscle, the density of cristae in mitochondria is assumed to be constant. However, recent experimental studies have shown that respiration per mitochondria varies. Modelling studies have hypothesized that this variation in respiration per mitochondria depends on plasticity in cristae density, although current evidence for such a mechanism is lacking. In the present study, we confirm this hypothesis by showing that, in human skeletal muscle, and in contrast to the current view, the mitochondrial cristae density is not constant but, instead, exhibits plasticity with long-term endurance training. Furthermore, we show that frequently recruited mitochondria-enriched fibres have significantly increased cristae density and that, at the whole-body level, muscle mitochondrial cristae density is a better predictor of maximal oxygen uptake rate than muscle mitochondrial volume. Our findings establish an elevating mitochondrial cristae density as a regulatory mechanism for increasing metabolic power in human skeletal muscle. We propose that this mechanism allows evasion of the trade-off between cell occupancy by mitochondria and other cellular constituents, as well as improved metabolic capacity and fuel catabolism during prolonged elevated energy requirements.

  • 20.
    Nielsen, Joachim
    et al.
    Univ Southern Denmark, Denmark.
    Gejl, Kasper D.
    Univ Southern Denmark, Denmark.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Denmark.
    Is there plasticity in mitochondrial cristae density with endurance training?: Reply2017In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 595, no 9, p. 2987-2988Article in journal (Refereed)
  • 21. Nielsen, Joachim
    et al.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Schröder, Henrik
    Saltin, Bengt
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    The importance of being in the right spot2012In: Physiology News, ISSN 1476-7996, Vol. 87, p. 23-26Article in journal (Other academic)
  • 22.
    Nielsen, Joachim
    et al.
    Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, 5230 Odense M, Denmark.
    Krustrup, Peter
    Section of Human Physiology, Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark.
    Nybo, Lars
    Section of Human Physiology, Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark.
    Gunnarsson, Thomas P.
    Section of Human Physiology, Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark.
    Madsen, Klavs
    Department of Sport Science, Aarhus University, Aarhus, Denmark.
    Schröder, Henrik
    Institute of Pathology, University of Southern Denmark, Odense M, Denmark.
    Bangsbo, Jens
    Section of Human Physiology, Department of Exercise and Sport Sciences, University of Copenhagen, Copenhagen, Denmark.
    Örtenblad, Niels
    Univ So Denmark, Inst Sports Sci & Clin Biomech, DK-5230 Odense M, Denmark.
    Skeletal muscle glycogen content and particle size of distinct subcellular localizations in the recovery period after a high-level soccer match2012In: European Journal of Applied Physiology, ISSN 1439-6319, E-ISSN 1439-6327, Vol. 112, no 10, p. 3559-3567Article in journal (Refereed)
    Abstract [en]

    Whole muscle glycogen levels remain low for a prolonged period following a soccer match. The present study was conducted to investigate how this relates to glycogen content and particle size in distinct subcellular localizations. Seven high-level male soccer players had a vastus lateralis muscle biopsy collected immediately after and 24, 48, 72 and 120 h after a competitive soccer match. Transmission electron microscopy was used to estimate the subcellular distribution of glycogen and individual particle size. During the first day of recovery, glycogen content increased by ~60% in all subcellular localizations, but during the subsequent second day of recovery glycogen content located within the myofibrils (Intramyofibrillar glycogen, a minor deposition constituting 10–15% of total glycogen) did not increase further compared with an increase in subsarcolemmal glycogen (−7 vs. +25%, respectively, P = 0.047). Conversely, from the second to the fifth day of recovery, glycogen content increased (53%) within the myofibrils compared to no change in subsarcolemmal or intermyofibrillar glycogen (P < 0.005). Independent of location, increment in particle size preceded increment in number of particles. Intriguingly, average particle size decreased; however, in the period from 3 to 5 days after the match. These findings suggest that glycogen storage in skeletal muscle is influenced by subcellular localization-specific mechanisms, which account for an increase in number of glycogen particles located within the myofibrils in the period from 2 to 5 days after the soccer match.

  • 23. Nielsen, Joachim
    et al.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Physiological aspects of the subcellular localization of glycogen in skeletal muscle2013In: Applied Physiology, Nutrition and Metabolism, ISSN 1715-5312, E-ISSN 1715-5320, Vol. 38, no 2, p. 91-99Article, review/survey (Refereed)
    Abstract [en]

    Glucose is stored in skeletal muscle fibers as glycogen, a branched-chain polymer observed in electron microscopy images as roughly spherical particles (known asβ-particles of 10-45 nm in diameter),which are distributed in distinct localizations within the myofibers and are physically associated with metabolic and scaffolding proteins. Although the subcellular localization of glycogen has been recognized for more than 40 years, the physiological role of the distinct localizations has received sparse attention. Recently, however, studies involving stereological, unbiased, quantitative methods have investigated the role and regulation of these distinct deposits of glycogen. In this report, we review the available literature regarding the subcellular localization of glycogen in skeletal muscle as investigated by electron microscopy studies and put this into perspective in terms of the architectural, topological, and dynamic organization of skeletal muscle fibers. In summary, the distribution of glycogen within skeletal muscle fibers has been shown to depend on the fiber phenotype, individual training status, short-term immobilization, and exercise and to influence both muscle contractility and fatigability. Based on all these data, the available literature strongly indicates that the subcellular localization of glycogen has to be taken into consideration to fully understand and appreciate the role and regulation of glycogen metabolism and signaling in skeletal muscle. A full understanding of these phenomena may prove vital in elucidating the mechanisms that integrate basic cellular events with changing glycogen content.

  • 24.
    Santalla, Alfredo
    et al.
    Universidad Pablo Olavide, Sevilla, Spain.
    Nogales-Gadea, Gisela
    Neuromuscular Diseases Unit, Neurology Department, Hospital de la Santa Creu i Sant Pau, Universitat Autónoma de Barcelona, Barcelona, Spain .
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC), University of Southern Denmark, Odense, Denmark.
    Brull, Astrid
    Departament de Patologia Mitocondrial i Neuromuscular, Hospital Universitari Vall d'Hebron, Institut de Recerca (VHIR), Universitat Autónoma de Barcelona, Pg. Vall d'Hebron 119, Barcelona, 08035, Spain .
    de Luna, Noemi
    Departament de Patologia Mitocondrial i Neuromuscular, Hospital Universitari Vall d'Hebron, Institut de Recerca (VHIR), Universitat Autónoma de Barcelona, Pg. Vall d'Hebron 119, Barcelona, 08035, Spain .
    Pinos, Tomas
    Departament de Patologia Mitocondrial i Neuromuscular, Hospital Universitari Vall d'Hebron, Institut de Recerca (VHIR), Universitat Autónoma de Barcelona, Pg. Vall d'Hebron 119, Barcelona, 08035, Spain .
    Lucia, Alejandro
    Universidad Europea e Instituto de Investigación Hospital 12 de Octubre (i + 12), Madrid, Spain .
    McArdle Disease: A Unique Study Model in Sports Medicine2014In: Sports Medicine, ISSN 0112-1642, E-ISSN 1179-2035, Vol. 44, no 11, p. 1531-1544Article, review/survey (Refereed)
    Abstract [en]

    McArdle disease is arguably the paradigm of exercise intolerance in humans. This disorder is caused by inherited deficiency of myophosphorylase, the enzyme isoform that initiates glycogen breakdown in skeletal muscles. Because patients are unable to obtain energy from their muscle glycogen stores, this disease provides an interesting model of study for exercise physiologists, allowing insight to be gained into the understanding of glycogen-dependent muscle functions. Of special interest in the field of muscle physiology and sports medicine are also some specific (if not unique) characteristics of this disorder, such as the so-called 'second wind' phenomenon, the frequent exercise-induced rhabdomyolysis and myoglobinuria episodes suffered by patients (with muscle damage also occurring under basal conditions), or the early appearance of fatigue and contractures, among others. In this article we review the main pathophysiological features of this disorder leading to exercise intolerance as well as the currently available therapeutic possibilities. Patients have been traditionally advised by clinicians to refrain from exercise, yet sports medicine and careful exercise prescription are their best allies at present because no effective enzyme replacement therapy is expected to be available in the near future. As of today, although unable to restore myophosphorylase deficiency, the 'simple' use of exercise as therapy seems probably more promising and practical for patients than more 'complex' medical approaches.

  • 25.
    Severinsen, Kaare
    et al.
    Aarhus Univ Hosp, Dept Neurol, Noerrebrogade 44, DK-8000 Aarhus C, Denmark.
    Dalgas, Ulrik
    Aarhus Univ, Sect Sport Sci, Dept Publ Hlth, DK-8000 Aarhus C, Denmark.
    Overgaard, Kristian
    Aarhus Univ, Sect Sport Sci, Dept Publ Hlth, DK-8000 Aarhus C, Denmark.
    Pedersen, Asger R.
    Aarhus Univ, Hammel Neurorehabil & Res Ctr, DK-8000 Aarhus C, Denmark.
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Dept Sports Sci & Clin Biomech, SDU Muscle Res Cluster, Odense, Denmark.
    Lund, Caroline
    VIA Univ Coll, Aarhus N, Denmark.
    Jakobsen, Johannes K.
    Rigshosp, Dept Neurol, Copenhagen, Denmark.
    Andersen, Henning
    Aarhus Univ Hosp, Dept Neurol, Noerrebrogade 44, DK-8000 Aarhus C, Denmark.
    Skeletal muscle fiber characteristics and oxidative capacity in hemiparetic stroke survivors2016In: Muscle and Nerve, ISSN 0148-639X, E-ISSN 1097-4598, Vol. 53, no 5, p. 748-754Article in journal (Refereed)
    Abstract [en]

    Introduction: Skeletal muscle is changed after stroke, but conflicting data exist concerning muscle morphology and oxidative enzyme capacity. Methods: In 36 chronic stroke patients bilateral rectus femoris muscle biopsies were analyzed, and fiber type proportions and cross-sectional areas were determined by ATPase histochemistry. Enzymatic concentrations of citrate synthase (CS) and 3-Hydroxyacyl-coenzymeA-dehydrogenase (HAD) were determined using freeze-dried muscle tissue. Findings were correlated with clinical outcomes. Results: In the paretic muscles the mean fiber area was smaller (P=0.0004), and a lower proportion of type 1 fibers (P=0.0016) and a higher proportion of type 2X fibers (P=0.0002) were observed. The paretic muscle had lower CS (P=0.013) and HAD concentrations (P=0.037). Mean fiber area correlated with muscle strength (r=0.43; P=0.041), and CS concentration correlated with aerobic capacity (r=0.47; P=0.01). Conclusions: In stroke survivors there is a phenotypic shift toward more fatigable muscle fibers with reduced oxidative enzymatic capacity that relates to clinical outcomes. Muscle Nerve53: 748-754, 2016

  • 26.
    Willis, Sarah J
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Hébert-Losier, Kim
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Andersson, Erik
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Ørtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    DOUBLE POLING MUSCLE ACTIVATION AND FATIGUE DURING A SIMULATED CLASSIC SPRINT CROSS-COUNTRY SKIING COMPETITION2013In: Proceedings for the 6th International Congress on Science and Skiing / [ed] Erich Mueller, Josef Kröll, Stefan Josef Lindinger, Jurgen Pfusterschmied, Thomas Stöggl, 2013, p. 16-16Conference paper (Refereed)
  • 27.
    Zinner, Christoph
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Wurzburg, Dept Sport Sci, Wurzburg, Germany.
    Morales-Alamo, David
    Univ Las Palmas Gran Canaria, Las Palmas Gran Canaria, Spain.
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Inst Sports Sci & Clin Biomech, Odense, Denmark.
    Larsen, Filip J.
    Swedish Sch Sport & Hlth Sci, Stockholm.
    Schiffer, Tomas A.
    Linköping Univ, Dept Med & Hlth Sci, Linköping.
    Willis, Sarah J.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Gelabert-Rebato, Miriam
    Univ British Columbia, Sch Kinesiol, Vancouver, BC, Canada.
    Perez-Valera, Mario
    Univ Las Palmas Gran Canaria, Las Palmas Gran Canaria, Spain.
    Boushel, Robert
    Univ British Columbia, Sch Kinesiol, Vancouver, BC, Canada.
    Calbet, Jose A. L.
    Univ British Columbia, Sch Kinesiol, Vancouver, BC, Canada; Univ Las Palmas Gran Canaria, Las Palmas Gran Canaria, Spain.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ British Columbia, Sch Kinesiol, Vancouver, BC, Canada; UiT Arctic Univ Norway, Sch Sport Sci, Tromso, Norway.
    The Physiological Mechanisms of Performance Enhancement with Sprint Interval Training Differ between the Upper and Lower Extremities in Humans2016In: Frontiers in Physiology, ISSN 1664-042X, E-ISSN 1664-042X, Vol. 7, no SEP, article id 426Article in journal (Refereed)
    Abstract [en]

    To elucidate the mechanisms underlying the differences in adaptation of arm and leg muscles to sprint training, over a period of 11 days 16 untrained men performed six sessions of 4-6 x 30-s all-out sprints (SIT) with the legs and arms, separately, with a 1-h interval of recovery. Limb-specific VO(2)peak, sprint performance (two 30-s Wingate tests with 4-min recovery), muscle efficiency and time-trial performance (TT, 5-min all-out) were assessed and biopsies from the m. vastus lateralis and m. triceps brachii taken before and after training. VO(2)peak and Wmax increased 3-11% after training, with a more pronounced change in the arms (P < 0.05). Gross efficiency improved for the arms (+8.8%, P < 0.05), but not the legs (-0.6%). Wingate peak and mean power outputs improved similarly for the arms and legs, as did TT performance. After training, VO2 during the two Wingate tests was increased by 52 and 6% for the arms and legs, respectively (P < 0.001). In the case of the arms, VO2 was higher during the first than second Wingate test (64 vs. 44%, P < 0.05). During the TT, relative exercise intensity, HR, VO2, VCO2, V-E, and V-t were all lower during arm-cranking than leg-pedaling, and oxidation of fat was minimal, remaining so after training. Despite the higher relative intensity, fat oxidation was 70% greater during leg-pedaling (P = 0.017). The aerobic energy contribution in the legs was larger than for the arms during the Wingate tests, although VO2 for the arms was enhanced more by training, reducing the O-2 deficit after SIT. The levels of muscle glycogen, as well as the myosin heavy chain composition were unchanged in both cases, while the activities of 3-hydroxyacyl-CoA-dehydrogenase and citrate synthase were elevated only in the legs and capillarization enhanced in both limbs. Multiple regression analysis demonstrated that the variables that predict TT performance differ for the arms and legs. The primary mechanism of adaptation to SIT by both the arms and legs is enhancement of aerobic energy production. However, with their higher proportion of fast muscle fibers, the arms exhibit greater plasticity.

  • 28.
    Zinner, Christoph
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Willis, Sara
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Morales-Alamo, D
    U of Las Palmas de Gran Canaria.
    Larsen, F
    Karolinska Institutet.
    Schiffer, T
    Karolinska Institutet.
    Boushel, R
    Gymnastik och Idrottshögskolan i Stockholm.
    Örtenblad, Niels
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Syddanskt Universitet.
    Calbet, J
    U of Las Palmas de Gran Canaria.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Sprint interval training or arms and legs elevates peak VO2 and improves arm exercise economy2015Conference paper (Refereed)
    Abstract [en]

    Introduction

    Interval cycle sprint training (SIT) has been shown to improve anaerobic capacity, VO2max and biomarkers of muscle oxidative capacity in as little as 2 weeks in previously untrained adults. The present study was designed to characterize and compare systemic VO2 and exercise performance after SIT engaging the arms and legs.

     

    Methods

    Sixteen healthy, untrained men (23.9 ± 3.7 yrs; 183.8 ± 6.8 cm; 80.3 ± 14.1 kg) performed six sessions of 4-6x30 sec all-out sprints with the legs then arms (or vice versa) separated by a 1-h recovery over an 11-day period. Limb-specific VO2peak, anaerobic capacity (2x30-sec Wingate tests with 4 min of recovery), a 4-min submaximal work economy test, and a 5-min all-out time trial (TT) were conducted before and after the training program. Muscle biopsies (from the m. vastus lateralis and m. triceps brachii) were taken before and after the training period.

     

    Results

    VO2peak increased by 10.6% and 5.9% with arm and leg training, respectively (p<0.05), with the increase in the arms significantly greater than in the legs (p=0.02). Work economy was improved for the arms (-9.8%, p<0.05), but not for the legs (-0.9%). Mean power during the TT rose by 13.5% for the arms and 11.8% for the legs (p<0.05). Peak power output and mean power during the two Wingate tests were elevated in both the arms (PPO: 6.7% (p<0.01) and 13.3% (p<0.01); MPO: 6.1% (p<0.01) and 8.4% (p<0.01)) and legs (PPO: 3.1% (p=0.07) and 7.1% (p=0.02); MPO: 3.3% (p<0.01) and 5.6% (p<0.01)). The activity of 3-hydroxyacyl-CoA dehydrogenase (HAD) and levels of muscle glycogen were unchanged in both limbs.

     

    Discussion

    Sprint interval training with arm or leg cycling exercise increased peak pulmonary VO2 during their respective modes over an 11-day training period with a greater increase in the arms. Sprint performance rose to a similar extent in both extremities, yet work economy was improved only in the arms. These findings suggest some limb-specific responsiveness to SIT training.

  • 29.
    Örtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Hvid, Lars G
    University of Southern Denmark, Odense, Denmark.
    Jensen, Rasmus
    University of Southern Denmark, Odense, Denmark.
    Andersson, Erik
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Willis, Sarah J
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Gejl, Kasper D
    University of Southern Denmark, Odense, Denmark.
    Repeated sprint exercise impairs contractile force of isolated single human muscle fibers2013In: Proceedings for the 6th International Congress on Science and Skiing / [ed] Erich Mueller, Josef Kröll, Stefan Josef Lindinger, Jurgen Pfusterschmied, Thomas Stöggl, 2013, p. 93-Conference paper (Refereed)
    Abstract [en]

    INTRODUCTION: The purpose of the present study was, to examine the effects of repeated sprint skiing on the contractile apparatus of single muscle fibres obtained from a group of elite skiers. We have recently demonstrated that prolonged cycling exercise impairs the contractile apparatus of single muscle fibres, and that this can be restored following recovery. However, little is known about the effect of repeated high intensity exercise on single fibre properties, as i.e. during cross-country (cc) sprint competitions. We hypothesize that repeated high intensity exercise in highly trained subjects will impair the contractile apparatus maximum force output.

    METHOD: Eleven elite male sprint talented cc skiers (age 24 ± 4 years; VO2max 5.1 ± 0.5 (diagonal skiing, DIA), 4.9 ± 0.5 (double pooling, DP) L·min-1)) volunteered for the study. The skiers performed a simulated intermittent classic sprint roller skiing competition on a treadmill. The sprint exercise included 4 times1300m, with 45 min recovery between sprints. Each sprint consisted of 3 DP sections (1° uphill) and 2 DIA sections (7° uphill). Muscle biopsies were obtained in arm muscle (m. biceps brachii) before and after the sprint exercises. Muscle fibre bundles were cooled and skinned in a glycerinating solution and stored until analyzed. Single muscle fibre segments (n=232) were isolated and attached to a sensitive force recording transducer, and activated by Ca2+ buffered solutions at pH 7.1 to measure mechanically properties (maximum force Po and Po/cross sectional area (CSA)) and fibre typed by the Sr2+ sensitivity (Hvid et al. 2013).

    RESULTS: Average sprint time was 3min 49s ± 9s, with no difference between sprints. A total of 232 fibres were analysed (150 type I and 82 type II fibres). Type II fibres had a sign. (P<0.05) higher CSA (8103 ± 2334 µm2 (type I) and 8852 ± 2288 µm2 (type II) and Po (0.82 ± 0.43 and 1.24 ± 0.50 mN) than type I fibres. Also type II fibres had a 31% higher Po/CSA (108 ± 55 vs 142 ± 45 kN/m2). Following the intermittent sprint exercise, type II fibres exhibited a sign. (P = 0.01) 20% decrease in Po, with no difference in type I fibres. To test if the decrease in the single fibre Po were associated with oxidative stress we tested if this could be reversed with a strong reducing agent (dithiothreitol, DTT). DTT did not alter Po at pre nor the decrease in type II fibres following sprint exercise.

    DISCUSSION: By using a translational approach from whole body exercise to single fibre measurements, we here we demonstrate that type II fibres from highly trained cross country skiers, has a 20% decrease in Po following repeated sprint. Thus, part of the experienced fatigue following sprint competitions is due to impairments at the level of the contractile apparatus. Further, we did not find any evidence for oxidative stress as a causative component in the observed decrease in Po.

    CONCLUSION: Here we demonstrate for the first time, in highly trained sprint skiers, that repeated sprint impairs single fibre maximum force at the level of the contractile apparatus, which may have a significant impact on muscle function and fatigue.

    REFERENCES: Gejl K, Hvid LG, Ulrik Frandsen U, Jensen K, Sahlin K and Ørtenblad N. Muscle glycogen content modifies SR Ca2+ release rate in elite endurance athletes. Med. Sci. Sports Ex. (2013).

  • 30.
    Örtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, SDU Muscle Res Cluster, Dept Sports Sci & Clin Biomech, DK-5230 Odense, Denmark.;Mid Sweden Univ, Dept Hlth Sci, Swedish Winter Sports Res Ctr, Ostersund, Sweden..
    Nielsen, J.
    Univ Southern Denmark, SDU Muscle Res Cluster, Dept Sports Sci & Clin Biomech, DK-5230 Odense, Denmark..
    Muscle glycogen and cell function - Location, location, location2015In: Scandinavian Journal of Medicine and Science in Sports, ISSN 0905-7188, E-ISSN 1600-0838, Vol. 25, p. 34-40Article, review/survey (Refereed)
    Abstract [en]

    The importance of glycogen, as a fuel during exercise, is a fundamental concept in exercise physiology. The use of electron microscopy has revealed that glycogen is not evenly distributed in skeletal muscle fibers, but rather localized in distinct pools. In this review, we present the available evidence regarding the subcellular localization of glycogen in skeletal muscle and discuss this from the perspective of skeletal muscle fiber function. The distribution of glycogen in the defined pools within the skeletal muscle varies depending on exercise intensity, fiber phenotype, training status, and immobilization. Furthermore, these defined pools may serve specific functions in the cell. Specifically, reduced levels of these pools of glycogen are associated with reduced SR Ca2+ release, muscle relaxation rate, and membrane excitability. Collectively, the available literature strongly demonstrates that the subcellular localization of glycogen has to be considered to fully understand the role of glycogen metabolism and signaling in skeletal muscle function. Here, we propose that the effect of low muscle glycogen on excitation-contraction coupling may serve as a built-in mechanism, which links the energetic state of the muscle fiber to energy utilization.

  • 31.
    Örtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Nielsen, Joachim
    Saltin, Bengt
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Glycogen resynthesis rate following cross-country skiing is closely related to skeletal muscle content2012In: Sciance and Skiing V / [ed] Erich Mueller, Stefan Lindinger, Thomas Stöggl, Meyer & Meyer Sport, 2012, p. 549-556Chapter in book (Refereed)
  • 32.
    Ørtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Gejl, Kasper D
    Holmberg, H-C
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Andersson, Erik
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Plomgaard, Peter
    Nielsen, Joachim
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Subcellular glycogen pools is utilized in a fibre type specific manner during acute and repeated high intensity exercise in elite athletes2016Conference paper (Refereed)
  • 33.
    Ørtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Schrøder, HD
    University of Southern Denmark, Odense, Denmark.
    Saltin, Bengt
    University of Copenhagen, Copenhagen, Denmark.
    Nielsen, Joachim
    University of Southern Denmark, Odense, Denmark.
    Differences in the mitochondrial content of different fibre types in the leg and arm muscles of elite cross-country skiers2013In: Proceedings for the 18th Annual Congress of the ECSS, 2013Conference paper (Refereed)
  • 34.
    Ørtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Hvid, L
    University of Southern Denmark, Odense, Denmark.
    Jensen, R
    University of Southern Denmark, Odense, Denmark.
    Andersson, Erik
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Willis, Sarah
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Holmberg, Hans-Christer
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences.
    Gejl, K
    University of Southern Denmark, Odense, Denmark.
    Repeated sprint exercise affects contractile apparatus and force production of isolated human muscle fibers2014In: Science & Skiing VI / [ed] Erich Muller, Josef Kröll, Stefan Lindinger m.fl., Meyer & Meyer Sport, 2014, p. 446-452Chapter in book (Refereed)
  • 35.
    Ørtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. University of Southern Denmark, Denmark.
    Jensen, Kurt
    University of Southern Denmark, Denmark.
    Gross efficiency predicts a 6-min double-poling ergometer performance in recreational cross-country skiers2017In: Sports Engineering, ISSN 1369-7072, E-ISSN 1460-2687, Vol. 20, no 4, p. 329-333Article in journal (Refereed)
    Abstract [en]

    The purpose of the study was to investigate which physiological parameters would most accurately predict a 6-min, all-out, double-poling (DP) performance in recreational cross-country skiers. Twelve male recreational cross-country skiers performed tests consisting of three series lasting 10 s, one lasting 60 s, plus a 6-min, all-out, DP performance test to estimate mean and peak power output. On a separate day, gross mechanical efficiency (GE) was estimated from a 10-min, submaximal, DP test and maximal oxygen consumption (VO2 max) was estimated from an incremental treadmill running test. Power was measured after each stroke from the acceleration and deceleration of the flywheel that induced the friction on the ergometer. The power was shown to the skier on a small computer placed on the ergometer. A multivariable correlation analysis showed that GE most strongly predicted 6-min DP performance (r = 0.79) and interestingly, neither DP VO2 max, nor treadmill-running VO2 max, correlated with 6-min DP performance. In conclusion, GE correlated most strongly with 6-min DP performance and GE at the ski ergometer was estimated to be 6.4 ± 1.1%. It is suggested that recreational cross-country skiers focus on skiing technique to improve gross mechanical efficiency during intense DP.

  • 36.
    Ørtenblad, Niels
    et al.
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Inst Sports Sci & Clin Biomech, SDU Muscle Res Cluster SMRC, DK-5230 Odense M, Denmark.
    Westerblad, Håkan
    Karolinska Inst, Dept Physiol & Pharmacol, SE-17177 Stockholm, Sweden.
    Nielsen, Joachim
    Mid Sweden University, Faculty of Human Sciences, Department of Health Sciences. Univ Southern Denmark, Inst Sports Sci & Clin Biomech, SDU Muscle Res Cluster SMRC, DK-5230 Odense M, Denmark.
    Muscle glycogen stores and fatigue2013In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793, Vol. 591, no 18, p. 4405-4413Article, review/survey (Refereed)
    Abstract [en]

    Studies performed at the beginning of the last century revealed the importance of carbohydrate as a fuel during exercise, and the importance of muscle glycogen on performance has subsequently been confirmed in numerous studies. However, the link between glycogen depletion and impaired muscle function during fatigue is not well understood and a direct cause-and-effect relationship between glycogen and muscle function remains to be established. The use of electron microscopy has revealed that glycogen is not homogeneously distributed in skeletal muscle fibres, but rather localized in distinct pools. Furthermore, each glycogen granule has its own metabolic machinery with glycolytic enzymes and regulating proteins. One pool of such glycogenolytic complexes is localized within the myofibrils in close contact with key proteins involved in the excitation-contraction coupling and Ca2+ release from the sarcoplasmic reticulum (SR). We and others have provided experimental evidence in favour of a direct role of decreased glycogen, localized within the myofibrils, for the reduction in SR Ca2+ release during fatigue. This is consistent with compartmentalized energy turnover and distinctly localized glycogen pools being of key importance for SR Ca2+ release and thereby affecting muscle contractility and fatigability.

1 - 36 of 36
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