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1 Department of Physiology, St. George's Hospital Medical School, London, United Kingdom
2 Department of Physiology, St. George's Hospital Medical School, London, United Kingdom; Division of Physiology, Department of Medicine, University of California, San Diego, San Diego, CA, USA
3 Department of Exercise Science and Physiology, Hiroshima Women's University, Hiroshima, Japan
4 Department of Physiology, St. George's Hospital Medical School, London, United Kingdom; Division of Respiratory and Critical Care Physiology, Harbor-UCLA Medical Center, Torrance, CA, USA
* To whom correspondence should be addressed. E-mail: hrossiter{at}ucsd.edu.
The tolerable duration of high-intensity, constant-load cycle ergometry is a hyperbolic function of the power, with an asymptote termed 'critical power' (CP) and a curvature constant (W') equivalent to a constant amount 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' be (at least) partially restored or replenished. To test this hypothesis six volunteers each performed 7 cycle-ergometer exercise tests with breath-by-breath determination of ventilatory and pulmonary gas exchange variables. Subjects initially performed four exercise tests to exhaustion: (1) a ramp-incremental, and (2) three high-intensity constant-load bouts at different work rates, for estimation of lactate (LT) and CP thresholds, W', and maximum oxygen uptake. Subsequently, subjects cycled to the limit of tolerance (for ~360s) on three occasions each being followed by a work rate reduction, to: (a) 110%CP, (b) 90%CP, and (c) 80%LT for a 20 minute target recovery period. W' averaged 20.9 KJ (± 2.35, SD) or 246 J.kg-1 (± 30). After initial fatigue, exercise at 110%CP was only tolerated for 30s (± 12). While each subject completed 20 minutes at 80%LT, only two were able to complete the target 20 minutes at 90%CP: the remaining 4 fatiguing at 785s (± 400) with VO2 having decreased to 89% (± 8) of that at the limit of tolerance. The differences in the tolerable durations for recovery bouts (a), (b), and (c) were significantly different (P<0.05) and dependent on intensity. The results support the suggestion that recovery of W' after fatigue necessitated a reduction in the work rate to a sub-CP level. The variation of subjects' responses in the 90%CP trial was unexpected but were consistent with mechanisms such as CP being reduced consequent to the prior high-intensity bout, variation in mechanisms of lactate handling and/or regional depletion of energy substrates, e.g. muscle glycogen.
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