Progressive recruitment of muscle fibers is not necessary for the slow component of V̇o₂kinetics

Abstract

The "slow component" of O2 uptake (VO2) 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 approximately 60-70% of peak VO2. 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 VO2 was calculated as blood flow multiplied by arteriovenous O2 content difference. The muscle fatigued (force decreased by approximately 20-25%) during contractions. Kinetics of adjustment were evaluated for 1) VO2, uncorrected for force development; 2) VO2 normalized for peak force; 3) VO2 normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected VO2 was considered, was observed in all muscles when VO2/peak force and VO2/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for VO2/peak force (0.18+/-0.06, means+/-SE) and for VO2/force-time (0.22+/-0.05) compared with uncorrected VO2 (0.04+/-0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of VO2 kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.

Performance and physiological responses during a sprint interval training session: relationships with muscle oxygenation and pulmonary oxygen uptake kinetics

“…5). This is in line with previous data showing greater energy requirement from fatigued muscle per unit of external work performed relative to nonfatigued muscle (Barclay 1996;Zoladz et al 2008).…”

Slow $$ \dot{V}{\text{O}}_{2} $$ kinetics during moderate-intensity exercise as markers of lower metabolic stability and lower exercise tolerance

“…Relatively faster _ VO 2 kinetics, strictly associated with the relatively lower [PCr] decrease, would then simply be an ''epiphenomenon'' of a relatively higher metabolic stability, which could be the relevant variable in terms of exercise tolerance (Zoladz et al 2006). Disturbances in metabolic stability may also be responsible for the decrease in muscle efficiency observed during high-intensity exercise (Woledge 1998), as well as for the slow component Whipp et al 2002) of _ VO 2 kinetics in skeletal muscle (Zoladz et al 2008). Evidence against a cause-effect relationship between _ VO 2 kinetics, the O 2 deficit and exercise tolerance derives also from the observation that factors which determine a faster _ VO 2 kinetics and a smaller O 2 deficit can be positively but also negatively associated with exercise tolerance.…”

Determinants of performance in 1,500-m runners

“…This suggests that athletes with the fastest _ VO 2 kinetics will be able to reach _ VO 2max in the shortest time, will have a smaller O 2 deficit, will accumulate less lactate and other metabolites associated with the fatigue process, and should therefore have better exercise tolerance ). In addition, due to the high average intensity during a 1,500-m race, there is likely to be the appearance of a _ VO 2 slow component, which represents a reduced muscle efficiency and is an index of the fatigue process (Whipp 1994;Zoladz et al 2008;Jones et al 2011). Consequently, it may be hypothesized that better 1,500-m performance will be associated with a smaller slow component.…”