Carbohydrate present in muscle (300 g), liver (90 g), and body fluids (30 g) is the major fuel for physical performance. The ATP stored in muscle cells can only give high-power output for a few seconds. It can be resynthesized anaerobically for a further few seconds (5-8) by using the phosphate from creatinine phosphate. These short, intense bursts of muscular activity are found in sprints (100 meters), track and field events, and sports such as tennis, hockey, football, gymnastics, and weightlifting. If the maximum effort lasts for 30 seconds, then breakdown of muscle glycogen can supply the energy, with buildup of muscle lactic acid. Most physical activity, however, requires an energy source that can power muscles for longer periods. The duration and intensity of exercise determines the mix of fuel used. In resting or light activity, about 60% is from free fatty acids (FFAs) and tri-glycerides in muscles. At moderate levels of activity (approximately 50% of the maximum O 2 uptake), fat and carbohydrate contribute about equal amounts as energy sources. Carbohydrate, a prime energy source, becomes more and more important as the intensity of the exercise increases. The change to using carbohydrate is not a linear response but accelerates with the intensity of the work. Endurance athletes use more fat and so conserve the carbohydrate in muscle and liver, but it is that level that ultimately limits continued performance. Fatigue arises when the store becomes depleted. The store of carbohydrate usually suffices for just 2 to 3 hours of physical exertion.
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