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Muscle [phosphocreatine] dynamics following the onset of exercise in humans: the influence of baseline work‐rate

The kinetics of pulmonary O2 uptake is known to be substantially slower when exercise is initiated from a baseline of lower‐intensity exercise rather than from rest. However, it is not known whether putative intracellular regulators of mitochondrial respiration (and in particular the phosphocreatine concentration, [PCr]) show similar non‐linearities in their response dynamics. The purpose of this study was therefore to investigate the influence of baseline metabolic rate on muscle [PCr] kinetics (as assessed using 31P‐magnetic resonance spectroscopy) following the onset of exercise. Seven male subjects completed ‘step’ tests to heavy‐intensity exercise (80% of peak work‐rate) from a resting baseline and also from a baseline of moderate‐intensity exercise (40% of peak work‐rate) using a single‐leg knee‐extensor ergometer situated inside the bore of a 1.5 T super‐conducting magnet. The time constant describing the kinetics of the initial exponential‐like fall in [PCr] was significantly different between rest‐to‐moderate (25 ± 14 s), rest‐to‐heavy (48 ± 11 s) and moderate‐to‐heavy exercise (95 ± 40 s) (P < 0.05 for all comparisons). A delayed‐onset ‘slow component’ in the [PCr] response was observed in all subjects during rest‐to‐heavy exercise, but was attenuated in the moderate‐to‐heavy exercise condition. These data indicate that muscle [PCr] kinetics does not conform to ‘linear, first‐order’ behaviour during dynamic exercise, and thus have implications for understanding the regulation of muscle oxidative metabolism.