Skeletal muscle fatigue precedes the slow component of oxygen uptake kinetics during exercise in humans

During constant work rate (CWR) exercise above the lactate threshold (LT), the exponential kinetics of oxygen uptake ((V) over dot(O2)) are supplemented by a (V) over dot(O2) slow component ((V) over dot(O2)) which reduces work efficiency. This has been hypothesised to result from 'fatigue and recruitment', where muscle fatigue during supra-LT exercise elicits recruitment of additional, but poorly efficient, fibres to maintain power production. To test this hypothesis we characterised changes in the power-velocity relationship during sub- and supra-LT cycle ergometry in concert with (V) over dot(O2) kinetics. Eight healthy participants completed a randomized series of 18 experiments consisting of: (1) a CWR phase of 3 or 8 min followed immediately by; (2) a 5 s maximal isokinetic effort to characterize peak power at 60, 90 and 120 rpm. CWR bouts were: 20 W (Con); 80% LT (Mod); 20% Delta (H); 60% Delta (VH); where Delta is the difference between the work rate at LT and (V) over dot(O2) (max). The (V) over dot(O2sc) was 238 +/- 128 and 686 +/- 194 ml min(-1) during H and VH, with no discernible (V) over dot(O2sc) during Mod. Peak power in Con was 1025 +/- 400, 1219 +/- 167 and 1298 +/- 233 W, at 60, 90 and 120 rpm, respectively, and was not different after Mod (P > 0.05). Velocity-specific peak power was significantly reduced (P < 0.05) by 3 min of H (-103 +/- 46 W) and VH (-216 +/- 60 W), with no further change by 8 min. The (V) over dot(O2sc) was correlated with the reduction in peak power (R-2 = 0.49; P < 0.05). These results suggest that muscle fatigue is requisite for the (V) over dot(O2). However, the maintenance of velocity-specific peak power between 3 and 8 min suggests that progressive muscle recruitment is not obligatory. Rather, a reduction in mechanical efficiency in fatigued fibres is implicated.