Progressive recruitment of muscle fibers is not necessary for the slow component of (V)over-dotO2 kinetics

The slow component of O-2 uptake ((V) over dot(O2)) 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 similar to 60-70% of peak (V) over dot(O2). 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 (V) over dot(O2) was calculated as blood flow multiplied by arteriovenous O-2 content difference. The muscle fatigued (force decreased by similar to 20-25%) during contractions. Kinetics of adjustment were evaluated for 1) (V) over dot(O2), uncorrected for force development; 2) (V) over dot(O2) normalized for peak force; 3) (V) over dot(O2) normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected (V) over dot(O2) was considered, was observed in all muscles when (V) over dot(O2)/peak force and (V) over dot(O2)/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for (V) over dot(O2)/peak force (0.18 +/- 0.06, means +/- SE) and for (V) over dot(O2)/force-time (0.22 +/- 0.05) compared with uncorrected (V) over dot(O2) (0.04 +/- 0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of (V) over dot(O2) kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.