Are the effects of training on fat metabolism involved in the improvement of performance during high-intensity exercise?

The objective of the present study was to relate changes in certain muscle characteristics and indicators of metabolism in response to endurance training to the concomitant changes in time to exhaustion (T_lim) at a work rate corresponding to maximal oxygen uptake \(\left( {{\dot {\text{V}}O}_{{\text{2peak}}} } \right).\) Eight healthy sedentary subjects pedalled on a cycle ergometer 2 h a day, 6 days a week, for 4 weeks. Training caused increases in \(\dot{\text{V}}\)O_2peak (by 8%), T_lim (from 299±23 s before to 486±63 s after training), citrate synthase and 3-hydroxyl-acyl-CoA dehydrogenase (HAD) activities (by 54% and 16%, respectively) and capillary density (by 31%). Decreases in activity of lactate dehydrogenase (LDH) and muscle type of LDH (by 24% and 28%, respectively) and the phosphofructokinase/citrate synthase ratio (by 37%) were also observed. Respiratory exchange ratio (RER) tended to be lower (P<0.1) at all relative work rates after training while the corresponding ventilation rates ( \(\dot{\text{V}}\)E) were unchanged. At the same absolute work rate, RER and \(\dot{\text{V}}\)E were lower after training (P<0.05). The improvement of T_lim with training was related to the increases in HAD activity (r=0.91, P=0.0043), and to the decreases in RER calculated for Pa_peak (r=0.71, P=0.0496). The present results suggest that the training-induced adaptations in fat metabolism might influence T_lim at a work rate corresponding to \(\dot{\text{V}}\)O_2peak and stimulate the still debated and incompletely understood role of fat metabolism during short high-intensity exercise