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The link between energy expenditure and fatigue

This is an excerpt from Physiology of Sport and Exercise 7th Edition With Web Study Guide-Loose-Leaf Edition by W. Larry Kenney,Jack Wilmore & David Costill.

Critical Power: The Link Between Energy Expenditure and Fatigue


An athlete who can sustain a high level of exercise intensity for a prolonged period without fatiguing will be successful. Exercise physiologists have a name for the link between optimal performance and fatigue: critical power. The critical power defines the tolerable duration of high-intensity exercise. If we graph the relation between power output (or exercise intensity, or speed) and the maximal time that intensity can be maintained, the line is curvilinear, as depicted in figure 5.13. At very high power outputs, exercise can be performed only for short durations. But as intensity is progressively decreased, exercise can be performed for longer and longer durations. At some point, this relation levels off and reaches an asymptote, defining the critical power for that activity—the maximal intensity that can be sustained without fatigue limiting performance.


Figure 5.13

Figure 5.13 The relation between power output (in watts [W]) and the time that power output can be maintained. The critical power is defined as the asymptote in the relation, i.e., the maximal power output that can be sustained without fatigue limiting the duration of performance.


Critical power represents the highest metabolic rate that is maintained entirely by oxidative metabolism. In that regard, it is related to the lactate threshold (discussed earlier in this chapter), but occurs at slightly higher intensities. Not surprisingly, critical power is increased with endurance or high-intensity interval training and decreased with aging and in chronic disease states. Hypoxia, such as that encountered at altitude (discussed in chapter 13), also reduces critical power, while breathing elevated oxygen concentrations elevates it.


Critical power is a useful measure in sport and exercise physiology because it correlates well with performance in running, rowing, swimming, and even team sport activities lasting from a few min to 2 h.28 However, while exercise at or below the critical power should theoretically be able to be continued indefinitely, in reality exercise at the critical power cannot be sustained beyond 30 min or so. With much attention being paid to breaking the 2 h barrier for the marathon, the critical power concept would dictate that a runner has to sustain a critical speed of only 21.1 km/h (13.1 mph, or slightly under 4.6 min miles); however, sustaining that heavy-intensity pace for 2 h has proven virtually impossible.28