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

Abstract

Non-technical summary The mechanisms determining exercise intolerance are poorly understood. A reduction in work efficiency in the form of an additional energy cost and oxygen requirement occurs during high-intensity exercise and contributes to exercise limitation. Muscle fatigue and subsequent recruitment of poorly efficient muscle fibres has been proposed to mediate this decline. These data demonstrate in humans, that muscle fatigue, generated in the initial minutes of exercise, is correlated with the increasing energy demands of high-intensity exercise. Surprisingly, however, while muscle fatigue reached a plateau, oxygen uptake continued to increase throughout 8 min of exercise. This suggests that additional recruitment of inefficient muscle fibres may not be the sole mechanism contributing to the decline in work efficiency during high-intensity exercise.Abstract During constant work rate (CWR) exercise above the lactate threshold (LT), the exponential kinetics of oxygen uptake (V O 2 ) are supplemented by aV O 2 slow component (V O 2 sc ) 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 withV O 2 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% (H); 60% (VH); where is the difference between the work rate at LT andV O 2 max . TheV O 2 sc was 238 ± 128 and 686 ± 194 ml min −1 during H and VH, with no discernibleV O 2 sc 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. TheV O 2 sc was correlated with the reduction in peak power (R 2 = 0.49; P < 0.05). These results suggest that muscle fatigue is requisite for theV O 2 sc . 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.

Knee Extensor Fatigue Threshold Is Related to Whole-Body V˙O2max

“…This slow component of VO2 kinetics during "very heavy" exercise has been well documented, and progressive motor unit recmitment has been proposed as a potential underlying cause (e.g., Poole et al [26]). Recent findings suggest that a reduction of mechanical efficiency of early recmited motor units may also contribute to the steady rise in VO2 (6). This is in line with data from isolated frog fibers showing reduced force production relative to oxygen consumption when slowly fatiguing fibers were fatigued (18).…”

“…Although great care should be taken to interpret changes in rsEMG in terms of motor unit recmitment and rate coding (14) when rsEMG does increase, it is usually paralleled by additional motor unit recmitment and/ or increased rate coding (10,19). It is important to note that recent findings support the idea that reduced efficiency of early recmited motor units may also contribute to the rise in oxygen consumption during very heavy exercise (6,18). The concurrent decrease in mean power frequency during contractions above fatigue threshold may relate to decreases in muscle flber action potential conduction velocity, which, in tum, have been linked to fatigue-related metabolic changes such as accumulation of potassium and decreased pH (33).…”

Effects of aerobic fitness on oxygen uptake kinetics in heavy intensity swimming

“…The _ VO 2 slow component reflects an increase in the O 2 cost of supra-VT exercise, and thus a reduction in work efficiency, that may be attributed to the recruitment of additional (mostly type II, i.e. less efficient) muscle fibres or to a reduction in the efficiency of the working fibres (Barstow et al 1996;Cannon et al 2011;Pringle et al 2003). A reduction in the amplitude of the _ VO 2 slow component would both signify an improved exercise economy/efficiency, and be influenced by the elevation of the VT.…”

Conventional testing methods produce submaximal values of maximum oxygen consumption

“…This finding was unexpected. One possible speculation is that the body retained the adaptation that caused it to use more oxygen during DEC, possibly an altered recruitment of different skeletal muscle fibre types, during the INC F 79. Indeed, a recent study80 showed that a progressive increase in VO 2 , the so-called slow-component phase, can occur during an all-out 3-min cycling protocol despite decreasing levels of muscle electromyographic activity and power output throughout the trial.…”