Skeletal muscle energy metabolism and fatigue during intense exercise in man

Adenosine triphosphate (ATP) is the sole fuel for muscle contraction. During near maximal intense exercise the muscle store of ATP will be depleted in < 1s, therefore, to maintain normal contractile function ATP must be continually resynthesized. During intense exercise (from approximately 75% VO2 max to near maximal workloads) this is achieved principally by the oxidation of carbohydrate and the anaerobic utilisation of phosphocreatine (PCr) and carbohydrate. The relative contribution of carbohydrate oxidation to total energy provision decreases, while that from anaerobic utilization increases. During prolonged intense exercise (approximately 75% VO2 max), the oxidation of glucose derived from skeletal muscle and liver glycogen stores is the primary pathway for ATP resynthesis. It is widely accepted that the availability of carbohydrate limits performance during this type of exercise as the point of exhaustion has been shown to be closely related to the depletion of muscle and liver glycogen stores. It is probable that carbohydrate depletion results in the inability of skeletal muscle to maintain the required rate of ATP resynthesis and therefore, the work intensity must be reduced for exercise to continue. During short lasting near maximal exercise (0-30 s), the anaerobic utilization of muscle PCr and glycogen will fuel muscle contraction. Evidence is available to indicate that fatigue during this type of exercise is related to the inability of type II fibres to maintain the required very high rate of ATP resynthesis. This has been suggested to result from a rapid depletion of type II fibre PCr stores and an insufficiency of the glycogenolytic rate to compensate for the fall in ATP production when the PCr store is depleted. In this situation the force generation has to decrease due to insufficient energy supplies.