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Intensity-dependent tolerance to exercise after attaining O_{2 max} in humans

¹Department of Physiology, St. George's Hospital Medical School, Cranmer Terrace, London, United Kingdom; ²Department of Medicine, Division of Physiology, University of California, San Diego, California 92093-0623; ³Department of Exercise Science and Physiology, Hiroshima Women's University, Hiroshima 724-8558, Japan; and ⁴Division of Respiratory and Critical Care Physiology and Medicine, Harbor-University of California at Los Angeles Medical Center, Torrance, California 90502

Submitted 11 December 2002 ; accepted in final form 25 March 2003

The tolerable duration of high-intensity, constant-load cycle ergometry is a hyperbolic function of power, with an asymptote termed critical power (CP) and a curvature constant (W') with units of work. It has been suggested that continued exercise after exhaustion may only be performed below CP, where predominantly aerobic energy transfer can occur and W' can be partially replenished. To test this hypothesis, six volunteers each performed cycle-ergometer exercise with breath-by-breath determination of ventilatory and pulmonary gas exchange variables. Initially, four exercise tests to exhaustion were made: 1) a ramp-incremental and 2) three high-intensity constant-load bouts at different work rates, to estimate lactate (_L) and CP thresholds, W', and maximum oxygen uptake (O_{2 max}). Subsequently, subjects cycled to the limit of tolerance (for 360 s) on three occasions, each followed by a work rate reduction to 1) 110% CP, 2) 90% CP, and 3) 80% _L for a 20-min target. W' averaged 20.9 ± 2.35 kJ or 246 ± 30 J/kg. After initial fatigue, 110% CP was tolerated for only 30 ± 12 s. Each subject completed 20 min at 80% _L, but only two sustained 20 min at 90% CP; the remaining four subjects fatigued at 577 ± 306 s, with oxygen consumption at 89 ± 8% O_{2 max}. The results support the suggestion that replenishing W' after fatigue necessitates a sub-CP work rate. The variation in subjects' responses during 90% CP was unexpected but consistent with mechanisms such as reduced CP consequent to prior high-intensity exercise, variation in lactate handling, and/or regional depletion of energy substrates, e.g., muscle glycogen.

maximal oxygen uptake; fatigue; gas exchange dynamics

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