Work-exhaustion time relationships and the critical power concept. A critical review.

The present review is focused on the physiological meanings of the critical power concept proposed by Scherrer in 1954 and its applications to general exercises such as running, cycling and swimming. Since the first studies on the critical power of local exercises, many studies have found that critical power is correlated with indices which are related to aerobic endurance such as maximal oxygen uptake, ventilatory threshold, OBLA or maximal lactate steady state. In fact, the relationship between exhaustion time t(lim) and the Work Wlim (or Distance Dlim) performed at exhaustion is not exactly linear and, consequently, the power-t(lim) equation is not a true hyperbola. The effect of the range of t(lim), used in the calculation of the slope of the Wlim-t(lim) relationship (called critical power) are discussed. When critical power is calculated from short supramaximal exercises, this power is higher than the power output which corresponds to a lactate steady state (or an oxygen uptake steady state) and does not correspond to a power output which can be sustained a long time. The authors present experimental data collected during local (knee extension) and general (running and cycling) exercises which suggest that critical power could correspond to a steady state provided that critical power is calculated from heavy submaximal exercises only (t(lim) ranging between 6 and 30 min). It is difficult to predict exhaustion time from critical power or critical velocity because of the hyperbolic nature of the power-t(lim) relationship. On the other hand, a large error in the measure of t(lim) should have a small effect on the calculation of critical power or velocity. In contrast, the value of Y intercept of the Wlim-t(lim) (or Dlim-(t(lim)) relationship should be sensitive to errors in t(lim).

Relationship between the 4 mmol running velocity, the time-distance relationship and the Léger-Boucher's test.

The relationship between distance and best time is roughly linear for distances between 1500 and 5000 m. The slope of this relationship has the dimension of a velocity (Vlim) which can be sustained during a long time. The individual time-distance relationships and the resulting Vlim have been studied in 32 subjects practicing different athletic activities by measuring exhaustion time for 2 to 4 constant-velocity running exercises performed to exhaustion. The velocity corresponding to 4 mmol.l-1 of blood lactate (V4 mmol) has been compared with Vlim. As maximal oxygen uptake is a major factor determining V4 mmol, Vlim and V4 mmol have also been correlated with the result of a field test which is assumed to measure maximal aerobic power (Léger-Boucher's test). This test consists in running until exhaustion at a velocity which increases every two minutes. The higher the velocity at exhaustion (Vléger) is, the higher the maximal oxygen uptake is assumed. Both Vlim and Vléger were very well correlated with V4 mmol (r greater than 0.90) and the average value of Vlim was almost equal to the average value of V4 mmol (13.89 vs 13.71 km.h-1). However, it was not possible to estimate V4 mmol accurately from the values of Vlim or Vléger because the standard errors of estimates were too large.