The effect of prolonged submaximal exercise on gas exchange kinetics and ventilation during heavy exercise in humans.

This study compared ventilation, gas exchange (oxygen uptake, VO(2)) and the surface electromyogram (EMG) activity of four major lower limb muscles during heavy exercise before (Pre-Ex) and after (Post-Ex) a sustained 90-min cycling exercise at 60% VO(2peak). The 90-min exercise was incorporated under the hypothesis that sustained exercise would alter substrate availability in the second exercise bout causing differences in fibre recruitment patterns, gas exchange and ventilation. Nine trained male subjects [VO(2peak)=60.2 (1.7) ml.kg(-1).min(-1)] completed two identical 6-min bouts of cycling performed at high intensity [approximately 90% VO(2peak); 307 (6) W, mean (SE)]. Ventilation and gas exchange were measured breath-by-breath and the EMG was recorded during the last 12 s of each minute of the two 6-min bouts. EMG signals were analysed to determine integrated EMG (iEMG) and mean power frequency (MPF). VO(2) at min 3 and min 6 in Post-Ex were significantly higher (i.e., +201 and 141 ml.min(-1), respectively, P<0.05) than in Pre-Ex but there was a approximately 25% decrease of the slow component, taken as the difference between min 6 and min 3 [187 (27) vs 249 (35) ml.min(-1), respectively, P<0.05]. The greater whole-body VO(2) after 3 min of exercise in Post-Ex was not accompanied by clear alterations in the iEMG and MPF of the examined leg muscles. Ventilation and heart rate were elevated (approximately 12-16 l.min(-1) and approximately 10 beats.min(-1), respectively, P<0.05) as were the ratios V(E)/O(2) and V(E)/VCO(2) in the Post-Ex tests. It was concluded that the VO(2) and ventilation responses to high-intensity exercise can be altered following prolonged moderate intensity exercise in terms of increased amplitude without associated major changes in either iEMG or MPF values among conditions.