The metabolic capacity to generate energy for muscular activity consists of three separate energy sources, with each source predominating in a particular duration and intensity of physical activity. These three types of muscular activity, summarized in Table 6–2, are commonly measured as muscular strength, anaerobic power, and aerobic power. Muscular strength, defined as the maximal force that can be generated in one muscular contraction, derives its energy almost exclusively from stored high-energy phosphagens, ATP, and creatinine phosphate (CP). Anaerobic power, also commonly referred to as muscular endurance, is defined as the muscular force generated during brief, intense exercise (repetitive contractions) which derives its energy primarily from phosphagens (ATP and CP) replenished from the anaerobic glycolytic metabolic pathway. Aerobic power is defined as the rate at which energy can be generated from oxygen-requiring phosphorylation of food substrates to replenish ATP and CP. The aerobic metabolic system is used primarily during prolonged physical activity of low- to moderate-level intensity.