MUSCLE-CONTRACTION AND FATIGUE - THE ROLE OF ADENOSINE 5'-DIPHOSPHATEAND INORGANIC-PHOSPHATE

Though many explanations are offered for the fatigue process in contra cting skeletal muscle (both central and peripheral factors), none comp letely explain the decline in force production capability because fati gue is specific to the activity being performed. However, one needs to look no further than the muscle contraction crossbridge cycle itself in order to explain a major contributor to the fatigue process in exer cise of any duration. The byproducts of adenosine 5'-triphosphate (ATP ) hydrolysis, adenosine 5'-diphosphate (ADP) and inorganic phosphate ( P-i) are released during the crossbridge cycle and can be implicated i n the fatigue process due to the requirement of their release for prop er crossbridge activity. P-i release is coupled to the powerstroke of the crossbridge cycle. The accumulation of P-i during exercise would l ead to a reversal of its release step, therefore causing a decrement i n force production capability. Due to the release of P-i with both the immediate (phosphagen) energy system and the hydrolysis of ATP, P-i a ccumulation is probably the largest contributor to the fatigue process in exercise of any duration. ADP release occurs near the end of the c rossbridge cycle and therefore controls the velocity of crossbridge de tachment. Therefore, ADP accumulation, which occurs during exercise of extended duration (or in ischaemic conditions), causes a slowing of t he rate constants (and therefore a decrease in the maximal velocity of shortening) in the crossbridge cycle and a reduced oscillatory power output. The combined effects of there accumulated hydrolysis byproduct s accounts for a large amount of the fatigue process in exercise of an y intensity or duration.