The influence of prolonged exhaustive exercise on mitochondrial oxidative function was investigated in ten men. Muscle biopsies were taken before and after exercise and mitochondrial respiration investigated in fibre bundles made permeable by pretreatment with saponin. After exercise, respiration in the absence of ADP increased by 18 % (P < 0.01), but respiration at suboptimal ADP concentration (0.1 mM) and maximal ADP‐stimulated respiration (1 mM ADP) remained unchanged. In the presence of creatine (20 mM), mitochondrial affinity for ADP increased markedly and respiration at suboptimal ADP concentration (0.1 mM) was similar (pre‐exercise) or higher (post‐exercise; P < 0.05) than with 1 mM ADP alone. The increase in respiratory rate with creatine was correlated to the relative type I fibre area (r= 0.84). Creatine‐stimulated respiration increased after prolonged exercise (P < 0.01). The respiratory control index (6.8 ± 0.4, mean ± s.e.m.) and the ratio between respiration at 0.1 and 1 mM ADP (ADP sensitivity index, 0.63 ± 0.03) were not changed after exercise. The sensitivity index was negatively correlated to the relative type I fibre area (r=−0.86). The influence of exercise on muscle oxidative function has for the first time been investigated with the skinned‐fibre technique. It is concluded that maximal mitochondrial oxidative power is intact or improved after prolonged exercise, while uncoupled respiration is increased. The latter finding may contribute to the elevated post‐exercise oxygen consumption. The finding that the sensitivity of mitochondrial respiration for ADP and creatine are related to fibre‐type composition indicates intrinsic differences in the control of mitochondrial respiration between fibres.

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Part of the difficulty in determining the nature of these relationships has been the inconsistency in research conducted regarding age groups and controlling for past and present activity patterns. Further difficulties comparing across studies arise due to the variety of methods and parameters used to sample and analyse muscle tissue. Standardizing methodology will allow future research to yield more consistent results.