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 separated by 4 min of passive recovery; after the third bout, subjects rested for 4 min, exercised for 30 min at 30-35% peak O₂ consumption, and rested for a further 60 min before completing the fourth exercise bout. Peak power and total work were reduced (P < 0.05) during bout 3 [765 ± 60 (SE) W; 15.8 ± 1.0 kJ] compared with bout 1 (1,168 ± 55 W, 23.8 ± 1.2 kJ), but no difference in exercise performance was observed 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 sarcoplasmic reticulum (SR) Ca²⁺ uptake were reduced, while muscle lactate and inosine 5'-monophosphate were increased. Muscle ATP and glycogen before bout 4 remained lower than values before bout 1 (P < 0.05), but there were no differences in muscle inosine 5'-monophosphate, lactate, pH, and SR Ca²⁺ uptake. Muscle CP levels before bout 4 had increased above resting levels. Consistent with the decline in muscle ATP were increases in hypoxanthine and inosine before bouts 3 and 4. The decline in 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.