The curvilinear relationship between power output and the time for which it can be sustained is a fundamental and well-known feature of high-intensity exercise performance. This relationship ‘levels off’ at a ‘critical power’ (CP). The CP may be functionally defined as the highest power output that can be sustained without progressively drawing on W', where the latter represents, at the onset of exercise, a fixed amount of work that can be done when CP is exceeded. Therefore, CP separates power outputs that can be sustained with stable values of, for example, muscle phosphocreatine, blood lactate, and pulmonary oxygen uptake (V?O2), from power outputs where these variables change continuously with time until their respective minimum and maximum values are reached and exercise intolerance occurs. The amount of work that can be done during exercise above CP (the so-called W') is constant but may be utilized at different rates depending on the proximity of the exercise power output to CP . CP is obtained from a minimum of 2 maximum trials lasting between 2 and 15 minutes  and it is a power that can be sustained from 20 to 40 minutes until exhaustion in relation to the level and characteristics of the athlete .