Effects of Two Types of Fatigue on the V·O2 Slow Component

The aim of the study was to test the hypothesis of the involvement of type II fibres in the VO 2 slow component phenomenon by using two prior fatiguing protocols on the knee extensor muscles. Nine subjects performed three constant-load cycling exercises at a work rate corresponding to 80% of their VO 2 max: (i) preceded by a 20-min fatiguing protocol using electromyo-stimulation (EMS), (ii) preceded by a 20-min fatiguing protocol using voluntary contractions (VOL), and (iii) without fatiguing protocol (NFP). Voluntary and evoked neuromuscular properties of the knee extensor muscles were tested before (PRE) and after (POST) the two fatiguing protocols. Results show a significant reduction in voluntary force after both fatiguing protocols (-19.9% and -11.8%, in EMS and VOL, respectively p < 0.01). After EMS, this decrease was greater than after VOL (p < 0.05) and was combined with a slackening of muscle contractile properties which was absent after VOL (p < 0.05). Regarding the effects on oxygen uptake kinetics, the appearance of the slow component was delayed after EMS and its amplitude was lower than those obtained in VOL and NFP conditions (0.48 ±0.07 vs. 0.75 ±0.09 and 0.69 ± 0.08 L·min -1 , respectively; p<0.05). It can thus be concluded that exercises dedicated to preferentially fatiguing type II fibres may alter VO 2 kinetics.