• Mark Turnbull

Critical Power - An Important Fatigue Threshold in Exercise Physiology


Human physiologists and sport scientists are naturally interested in the link between the development of fatigue (and its mechanistic portents) and exercise performance. Fatigue is an ongoing dynamic process during high-intensity exercise involving central and peripheral mechanisms that temporarily limit the power-producing capabilities of the integrated neuromuscular system. Fatigue is distinct from task failure, which is defined as the point at which fatigue develops to the point at which it, or its symptoms, cause intolerance and therefore limits the desired exercise performance. The link between fatigue and performance has historically been regarded as elusive; however, in recent years, compelling evidence has indicated that it is enshrined within the concept of a critical power (CP). At its essence, this concept describes the tolerable duration of severe-intensity exercise. When the time to the limit of tolerance is plotted against particular constant speeds or power outputs, the relationship is not linear (as one might perhaps naively expect) but is rather curvilinear, with the ability to sustain exercise falling away more sharply at higher compared with lower power outputs (Fig. 1). Mathematically, this relationship is described as being hyperbolic. When exercise tolerance is considered, the power asymptote is known as CP (or critical speed [CS] when intensity is measured in units of speed rather than power) and the curvature constant is known asW′(i.e.,Wprime) and is measured in units of work done, that is, joules (orD′ when measured in units of distance, that is, meters). This hyperbolic power–duration relationship can be transformed into a linear relationship if work done is plotted against time, such that the slope of the line equals CP and the intercept equals W′.

Figure 1: The hyperbolic power/speed–duration curve that defines the limit of tolerance for whole-body exercise such as cycling or running as well as individual muscle, joint, or muscle group exercise. The curve is constructed by the subject exercising at constant power or speed to the limit of tolerance (points 1–4). Typically, these bouts are performed on different days and result in exhaustion within 2–15 min. This hyperbolic relationship is highly conserved across the realm of human physical activities and exercise modes and also across the animal kingdom and is defined by two parameters: the asymptote for power (CP, or speed, CS, and their metabolic equivalent, V˙O2) and the curvature constant W′ (denoted by the rectangular boxes above CP and expressed in kilojoules). Note that CP/CS defines the upper boundary of the heavy intensity domain and represents the highest power sustainable without drawing continuously on W′. Above CP (severe-intensity exercise) limit of tolerance occurs when W′ has been expended. Severe-intensity exercise is characterized by a V˙O2 profile that rises continuously to V˙O2 max and blood lactate that increases to limit of tolerance (see text for additional details). LT, lactate threshold, defined usefully during incremental or ramp exercise as the V˙O2 above which blood lactate begins its sustained increase; GET, gas exchange threshold, as identified from the nonlinearity of the V˙O2/V˙CO2 relationship.

A threshold in biological function

Credit for recognition of the inherent hyperbolicity between power output and its sustainability should be given to the British physiologist A. V. Hill. In a 1925 paper published in Nature, Hill plotted the relationship between mean speed and sustainable time using world record performance times over a variety of distances in men’s and women’s running and swimming, and showed that the relationship was hyperbolic in each case (42). This relationship remains evident when today’s world record performances are plotted in the same way. This is of interest because it indicates that the human power–duration relationship is hyperbolic in its nature, not only when the performance of a single individual is appraised but also when the best human performances are established by different individuals. It is also known that this hyperbolic power–duration relationship holds not just for individuals performing a wide range of whole-body activities (cycling, running, rowing, and swimming) but also when the exercise is confined to a single muscle or joint (16,48,61,77 rev. 47,48). Moreover, the power–duration relationship is an integral property of muscular performance in an array of other species including the lungless salamander, ghost crab, mouse, and thoroughbred racehorse (rev. 47). The consistency of these observations indicates that the power–duration relationship, and the bioenergetic features underpinning it, is an integral feature of exercise performance.

The characteristics of the power–duration relationship

It should be emphasized that the power–duration relationship describes exercise tolerance but does not, in itself, explain it. Nevertheless, the physiological responses to exercise performed below and above the CP asymptote may provide important insights into the fatigue process. CP was originally defined as the external power output that could be sustained “indefinitely” or for “a very long time without fatigue” (69). This definition should be considered theoretical, however, because it is clear that no exercise can ever be undertaken indefinitely. Rather, it is now understood that CP separates power outputs for which exercise tolerance is predictably limited (exercise >CP, which may be sustained for a maximum of perhaps 30 min) from those that can be sustained for longer periods (<CP). The actual time to intolerance (Tlim) for exercise performed above CP is defined, and therefore closely predicted, by the following equation:

Tlim = W'/(P-CP)

This equation highlights that the time to intolerance >CP is a function of 1) the proximity of the power output (P) being sustained to CP and 2) the size ofW′. When Pi s considerably above CP, the constant amount of work represented by theW′parameter will be used rapidly and Tlim will be short. Should P be closer to CP, then W′will be “used” more slowly andTlimwill be longer. A crucial consideration here is that W′is assumed to be constant for allPabove CP. This two-parameter power–time or power–duration model therefore implies that absolute exercise performance depends on simply the value of CP (in watts) and the value ofW′(in joules). Both CP and W′parameters can vary considerably among individuals as a function of health/disease, age, fitness, and training (102).



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