Mid Sweden University

Aerobic exercise (AE) may interfere with muscle adaptations induced by resistance exercise (RE). Three experimental campaigns were conducted to explore the influence of AE on molecular, functional and muscular adaptations to acute and chronic RE. Twenty-nine men performed unilateral knee extensor RE preceded by AE (AE+RE). The contralateral leg did RE only. First, the influence of acute AE on muscle molecular responses to RE performed 6 h later was studied. Subsequently, this exercise regimen was implemented over 5 weeks training. The relationships between acute and chronic outcomes were examined and molecular responses to acute exercise were assessed in untrained and trained muscle. Finally, acute and chronic responses to AE+RE, interspersed by only 15 min recovery, were investigated.Phosphorylation of mTOR and p70S6K was greater after AE+RE than after RE. In parallel, myostatin was suppressed for a longer time after AE+RE. These results suggest that AE+RE enhance skeletal muscle anabolic environment more than RE alone (Paper I). After 5 weeks training, improvements in muscle strength and power were similar across legs. However, AE+RE prompted a greater increase in muscle size than RE, suggesting that AE potentiates the hypertrophic stimulus to RE training without altering muscle function progress (Paper II). Consistent with changes in whole-muscle size, AE+RE showed greater anabolic molecular responses than RE. As chronic training blunted this effect, it appears that AE offers a synergistic hypertrophic stimulus to RE only during short-term training (Paper III). Although putative regulators of hypertrophy such as p70S6K, myostatin and PGC-1a4 were examined, no molecular marker correlated with changes in muscle size, strength or power induced by training. Hence, this study challenges the concept that single molecular markers are viable predictors of training-induced muscle adaptations (Paper III–IV). When recovery time between exercise bouts was reduced to 15 min, AE+RE still produced a more substantial increase in muscle size than RE. However, progression of concentric strength was blunted. Thus, while restored muscle function between exercise bouts is a prerequisite for achieving maximal gains in strength and power, incomplete recovery appears not to compromise muscle hypertrophy (Paper V).Collectively, the results suggest that outcomes of AE+RE are impacted by chronic training and time allowed for recovery between exercise modes. Yet, the current study offers no support to the view that AE interferes with muscle hypertrophy induced by RE.