Progressive recruitment of muscle fibers is not necessary for the slow component of V̇o2 kinetics
The “slow component” of O2 uptake (V̇o2) kinetics during constant-load heavy-intensity exercise is traditionally thought to derive from a progressive recruitment of muscle fibers. In this study, which represents a reanalysis of data taken from a previous study by our group (Grassi B, Hogan MC, Greenhaff PL, Hamann JJ, Kelley KM, Aschenbach WG, Constantin-Teodosiu D, Gladden LB. J Physiol 538: 195–207, 2002) we evaluated the presence of a slow component-like response in the isolated dog gastrocnemius in situ ( n = 6) during 4 min of contractions at ∼60–70% of peak V̇o2. In this preparation all muscle fibers are maximally activated by electrical stimulation from the beginning of the contraction period, and no progressive recruitment of fibers is possible. Muscle V̇o2 was calculated as blood flow multiplied by arteriovenous O2 content difference. The muscle fatigued (force decreased by ∼20–25%) during contractions. Kinetics of adjustment were evaluated for 1) V̇o2, uncorrected for force development; 2) V̇o2 normalized for peak force; 3) V̇o2 normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected V̇o2 was considered, was observed in all muscles when V̇o2/peak force and V̇o2/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for V̇o2/peak force (0.18 ± 0.06, means ± SE) and for V̇o2/force-time (0.22 ± 0.05) compared with uncorrected V̇o2 (0.04 ± 0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of V̇o2 kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.