What does the fatigue index measure in terms of exercise performance?

The fatigue index is used to assess how performance changes or deteriorates over the course of an exercise test or physical activity. Specifically, it provides insight into an individual's capacity to sustain power output during high-intensity efforts.

When discussing the components of the fatigue index, it is measured as the lowest power output over a specific time segment (in this case, a 5-second segment) relative to the peak power achieved during the exercise. This ratio illustrates how much an individual’s performance declines from their maximum output, effectively highlighting their endurance and the onset of fatigue.

This means that a higher fatigue index indicates less fatigue resistance, while a lower fatigue index suggests greater endurance and the ability to maintain performance levels. Therefore, identifying the fatigue index using the formula of the lowest segment divided by the peak power provides crucial data about an athlete's performance under fatigue conditions. The other choices focus on general outputs or performance measures which do not directly address the relationship between peak power and segmented performance decline.