The mechanisms underpinning the slow component of VO₂ in humans

Purpose When exercising above the lactic threshold (LT), the slow component of oxygen uptake (VO2sc) 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 (VO2) 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 (VO (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.VO(2)net after the third minute in FB-Torque increased with time (VO2net = 94 x t + 564; R-2 = 0.99; P = 0.001), but not during FB-EMG.V O-2net / T increased only during FB-Torque (VO2net/T = 1.10 x 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.VO2sc. Conclusion The analysis of the RMS,VO2 and T during FB-Torque suggests that the intrinsic mechanism attributable to the decay of contraction efficiency was responsible for an increase of VO2net equal to 18% of the total.VO2sc.