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1 Dept. Physiology and Biochemistry, AWF, Krakow, Poland
2 Kinesiology, Auburn University, Auburn, Alabama, United States
3 Dept. Medicine, UCSD, San Diego, California, United States
4 Physics, Jagiellonian University, Krakow, Poland
5 Dip. Scienze Tecnol. Biomediche, Universita degli Studi di Udine, Udine, Italy
* To whom correspondence should be addressed. E-mail: bruno.grassi{at}unimi.it.
The slow component of O2 uptake (V'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 re-analysis of data taken from a previous study by our group (Grassi et al., 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 V'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'O2 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) V'O2, uncorrected for force development; 2) V'O2 normalized for peak force; 3) V'O2 normalized for force-time integral. A slow component-like response, described in only one muscle out of 6 when uncorrected V'O2 was considered, was observed in all muscles when V'O2/peak force and V'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'O2/peak force (0.18±0.06, x±SE) and for V'O2/force-time (0.22±0.05) as compared to uncorrected V'O2 (0.04±0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of V'O2 kinetics, which may be caused by the metabolic factors which induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.
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