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Progressive recruitment of muscle fibers is not necessary for the slow component of O₂ kinetics

¹Department of Physiology and Biochemistry, University School of Physical Education, Krakow, Poland; ²Department of Kinesiology, Auburn University, Auburn, Alabama; ³Department of Medicine, University of California San Diego, La Jolla, California; ⁴Department of Physics, Jagiellonian University, Krakow, Poland; ⁵Dipartimento di Scienze e Tecnologie Biomediche, Università degli Studi di Udine, Udine, Italy; and ⁶Institute of Bioimaging and Molecular Physiology, Consiglio Nationale delle Ricerche, Milano, Italy

The "slow component" of O₂ uptake (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 60–70% of peak 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 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) O₂, uncorrected for force development; 2) O₂ normalized for peak force; 3) O₂ normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected O₂ was considered, was observed in all muscles when O₂/peak force and O₂/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for O₂/peak force (0.18 ± 0.06, means ± SE) and for O₂/force-time (0.22 ± 0.05) compared with uncorrected O₂ (0.04 ± 0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of O₂ kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.

skeletal muscle bioenergetics

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