Mid Sweden University

Downhill running poses challenges were the gradient is of importance for energy cost and oxygen uptake. While demonstrated that downhill running at a slight gradient is most efficient, the energy cost increases with a steeper gradient (1). However, the additional effect of running speed has not been studied for downhill running. Therefore, the aim of the study was to analyse the combined effect of gradient and speed in downhill running on oxygen cost and running economy. METHODS:Runners (n=6) were recruited for the study and performed 1) VO2max and running economy (J·kg-1·m-1) tests and 2) an experimental running protocol at two speeds,12 km·h-1 and 80% of the speed at VO2max (V80) and three gradients (0, -5° and -10°). V80 was higher than 12 km·h-1 for all participants. All testing was performed on a large treadmill 3x5 m (Rodby, Sweden) that were controlled for speed and gradient. The experimental protocol was performed continuously with 5 min at each workload in a randomized order, 30 min in total. VO2 was measured throughout the experimental protocol using a mixing chamber (Moxus Metabolic Cart, USA). RESULTS:VO2 expressed as ml·kg-1·min-1 increased because of speed (F1,5=27.8, p=0.003) and decreased with gradient (F1,5=87.6, p<0.001). Between -5° and -10°, VO2 decreased less during V80 compared to 12 km·h-1 shown by an interaction (F2,10=7.9, p=0.009). However, speed did not influence running economy (F1,5=0.9, p=0.38) while gradient increased running economy (F1,5=90.1, p<0.001). A non-significant interaction effect suggests a shift in running economy between -5° and -10° depending on speed (F2,10=3.5, p=0.07). The running economy at V80 was higher compared to 12 km·h-1 at -5° but reversed at -10°. While a relation between running economy at V80 -10°, V80 -5° and 12 km·h-1 -10° (rs>0.88, p<0.019) was found, no relations between running economy on level terrain and steep downhill running (-10°) were recognised. CONCLUSION:While we found no effect on running economy from speed alone, we did see a shift in the running economy for different speeds at an increased downhill gradient. This indicates that a high speed (V80) is more efficient at moderate downhill gradients, while a lower speed (12 km·h-1) is more efficient in steeper downhill gradients. While previous research demonstrate that gradient is of great influence to running economy, the findings of this study suggest that speed also affects the running economy in downhill running.