During constant work rate (CWR) exercise above the lactate threshold (LT), the exponential kinetics of oxygen uptake (VO2) are supplemented by the VO2 slow component (VO2SC) which reduces work efficiency compared to sub-LT exercise (2). The majority of the additional O2-cost occurs within the active locomotor muscles (3) due to an increase in the ATP-cost of force production (4). Whether this results from an increase in motor unit recruitment or an increased ATP turnover in active fibres remains uncertain (5). Nonetheless, these mechanisms are posited to originate from muscle fatigue consequent to supra-LT exercise. We hypothesised that skeletal muscle fatigue would precede the VO2SC. We therefore determined the relationship between velocity-specific peak torque and the VO2SC using instantaneous switching from CWR to maximal isokinetic cycle ergometry with pedal force measurement (Excalibur Sport PFM, Lode BV, Groningen). Eight healthy participants completed ramp-incremental exercise to determine LT and VO2max. This was followed by a randomised series of 18 exercise tests consisting of 1) a CWR phase of 3 or 8 min followed immediately by 2) a 5 s maximal isokinetic effort to characterise peak torque at 60, 90 and 120 rpm. CWR bouts were: 20W (CON); 80%LT (MOD); 20%Δ (H); 60%Δ (VH); where Δ is the difference between LT and VO2max. VO2 was measured breath-by-breath (Oxycon Mobile, VIASYS GmbH, Höchberg). There was no discernible VO2SC during MOD (difference in VO2 between 3 and 8 min; 35 (± 92) mL.min-1). The VO2SC was 162 (± 133), and 449 (± 164) mL.min-1 in H and VH. Peak torque in CON was 145 (± 24), 129 (± 18), and 103 (± 19) Nm, at 60, 90, and 120 rpm respectively, and was not different after MOD (F = 1.25; P > 0.05; η2 = 0.15). Peak torque was significantly reduced at all frequencies (P < 0.05) by 3 minutes of H (-9 ± 7 Nm) and VH (-21 ± 7 Nm), with no further change (P > 0.05) by 8 minutes. Additionally a time x frequency interaction revealed greater fatigue at higher frequencies in VH (F = 3.14; P < 0.05; η2 = 0.31). Overall, the VO2SC was correlated (P < 0.05) with the reduction in peak-torque at all pedalling frequencies: 60 (R2 = 0.37), 90 (R2 = 0.24) and 120 rpm (R2 = 0.43). These results are consistent with the hypothesis that a VO2SC is consequent to muscle fatigue; both fatigue and the VO2SC being absent in MOD. The greater fatigue at high pedalling frequencies in VH is consistent with a weighted contribution of type II muscle fibres in the fatigue (1) and VO2SC processes. While muscle fatigue preceded the VO2SC, surprisingly the peak torque was unchanged between 3 and 8 minutes. This suggests that increased ATP turnover may be temporally dissociated from muscle fatigue; the mechanism(s) for which are currently unclear. Nevertheless, muscle fatigue generated early in the transient appears to be requisite for the VO2SC in humans.