MUSCLE METABOLITES AND PERFORMANCE DURING HIGH-INTENSITY, INTERMITTENT EXERCISE

Six men were studied during four 30-s ''all-out'' exercise bouts on an air-braked cycle ergometer. The first three exercise bouts were separ ated by 4 min of passive recovery; after the third bout, subjects rest ed for 4 min, exercised for 30 min at 30-35% peak O-2 consumption, and rested for a further 60 min before completing the fourth exercise bou t. Peak power and total work were reduced (P < 0.05) during bout 3 [76 5 +/- 60 (SE) W; 15.8 +/- 1.0 kJ] compared with bout 1 (1,168 +/- 55 m T, 23.8 +/- 1.2 kJ), but no difference in exercise performance was obs erved between bouts 1 and 4 (1,094 +/- 64 W, 23.2 +/- 1.4 kJ). Before bout 3, muscle ATP, creatine phosphate (CP), glycogen, pH, and sarcopl asmic reticulum (SR) Ca2+ uptake were reduced, while muscle lactate an d inosine 5'-monophosphate were increased. Muscle ATP and glycogen bef ore bout 4 remained lower than values before bout I (P < 0.05), but th ere were no differences in muscle inosine 5'-monophosphate, lactate, p H, and SR Ca2+ uptake. Muscle CP levels before bout 4 had increased ab ove resting levels. Consistent with the decline in muscle ATP were inc reases in hypoxanthine and inosine before bouts 3 and 4. The decline i n exercise performance does not appear to be related to a reduction in muscle glycogen. Instead, it may be caused by reduced CP availability , increased H+ concentration, impairment in SR function, or some other fatigue-inducing agent.