The mechanisms underpinning the slow component of V˙ O 2 in humans

Purpose: When exercising above the lactic threshold (LT), the slow component of oxygen uptake (V˙O2sc) appears, mainly ascribed to the progressive recruitment of Type II fibers. However, also the progressive decay of the economy of contraction may contribute to it. We investigated oxygen uptake (V˙O2) during isometric contractions clamping torque (T) or muscular activation to quantify the contributions of the two mechanisms. Methods: We assessed for 7 min T of the leg extensors, net oxygen uptake (V˙O2net) and root mean square (RMS) from vastus lateralis (VL) in 11 volunteers (21 ± 2 yy; 1.73 ± 0.11 m; 67 ± 14 kg) during cyclic isometric contractions (contraction/relaxation 5 s/5 s): (i) at 65% of maximal voluntary contraction (MVC) (FB-Torque) and; (ii) keeping the level of RMS equal to that at 65% of MVC (FB-EMG). Results: V˙O2net after the third minute in FB-Torque increased with time (V˙O2net = 94 × t + 564; R2 = 0.99; P = 0.001), but not during FB-EMG. V˙O2net/T increased only during FB-Torque (V˙O2net/T = 1.10 × t + 0.57; R2 = 0.99; P = 0.001). RMS was larger in FB-Torque than in FB-EMG and significantly increased in the first three minutes of exercise to stabilize till the end of the trial, indicating that the pool of recruited MUs remained constant despite V˙O2sc. Conclusion: The analysis of the RMS, V˙O2 and T during FB-Torque suggests that the intrinsic mechanism attributable to the decay of contraction efficiency was responsible for an increase of V˙O2net equal to 18% of the total V˙O2sc.