Muscle metabolites and performance during high-intensity, intermittent exercise

Six men werestudied during four 30-s "all-out" exercise bouts on anair-braked cycle ergometer. The first three exercise bouts wereseparated by 4 min of passive recovery; after the third bout, subjectsrested for 4 min, exercised for 30 min at 30-35% peakO₂ consumption, and rested for afurther 60 min before completing the fourth exercise bout. Peak powerand total work were reduced (P < 0.05) during bout 3 765 ± 60 (SE) W; 15.8 ± 1.0 kJ] compared withbout 1 (1,168 ± 55 W, 23.8 ± 1.2 kJ), but no difference in exercise performance was observed betweenbouts 1 and4 (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, whilemuscle lactate and inosine 5'-monophosphate wereincreased. Muscle ATP and glycogen before bout4 remained lower than values beforebout 1 (P < 0.05), but there were no differences in muscle inosine 5'-monophosphate, lactate, pH, and SR Ca²⁺ uptake. Muscle CP levelsbefore bout 4 had increased aboveresting levels. Consistent with the decline in muscle ATP wereincreases in hypoxanthine and inosine before bouts3 and 4. The decline in exercise performance does not appear to be related to a reduction inmuscle glycogen. Instead, it may be caused by reduced CP availability, increased H⁺ concentration,impairment in SR function, or some other fatigue-inducing agent.