The mechanisms underpinning the slow component of V˙O2 in humans

Purpose When exercising above the lactic threshold (LT), the slow component of oxygen uptake ( V ˙ O 2sc ) 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 ˙ O 2 ) 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 ˙ O 2net ) 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 ˙ O 2net after the third minute in FB-Torque increased with time ( V ˙ O 2net = 94 × t + 564; R 2 = 0.99; P = 0.001), but not during FB-EMG. V ˙ O 2net /T increased only during FB-Torque ( V ˙ O 2net /T = 1.10 × t + 0.57; R 2 = 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 ˙ O 2sc . Conclusion The analysis of the RMS, V ˙ O 2 and T during FB-Torque suggests that the intrinsic mechanism attributable to the decay of contraction efficiency was responsible for an increase of V ˙ O 2net equal to 18% of the total V ˙ O 2sc .