Intensity-dependent tolerance to exercise after attaining VO2 max in humans

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 ((θ) over cap (L)) and CP thresholds, W', and maximum oxygen uptake ((V)over dotO(2 max)). Subsequently, subjects cycled to the limit of tolerance (for similar to360 s) on three occasions, each followed by a work rate reduction to 1) 110% CP, 2) 90% CP, and 3) 80% (θ) over cap (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% (θ) over cap (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% (V)over dotO(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.