Time Course of Changes in Human Skeletal Muscle Succinate Dehydrogenase and Cytochrome Oxidase Activities and Maximal Oxygen Uptake with Physical Activity and Inactivity

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Abstract
Succinate dehydrogenase (SDH) and cytochrome oxidase activities in the lateral vastus of the human quadriceps femoris muscle together with total body V́o2 max were followed during an 8–10 week period of endurance training (n=13) and a successive 6 week period without training (n = 8). During the training period there was a gradual increase in both Vo. max and muscle oxidative enzyme activities, all being significantly different from the pre‐training levels after 3 weeks of training. After 8 weeks of training V́o2. max was 19%, vastus lateralis SDH 32%, and cytochrome oxidase activity 35% above the pre‐training levels respectively. 6 weeks post‐training V́o3 max was still 16% above the pre‐training level, and not signifi‐ficantly different from the level at the end of training (p >0.2). In contrast vastus lateralis SDH activity had returned to the pre‐training level. Cytochrome oxidase activity had returned to the pre‐training level within two weeks post‐training. The significantly faster post‐training decline in skeletal muscle oxidative enzyme activities in contrast to that of the V́o2 max indicates that an enhancement of the oxidative potential in skeletal muscle is not a necessity for a high V́o2 max. Moreover, the fast return to the pre‐training level of both SDH and cytochrome oxidase activities indicate a high turnover rate of enzymes in the TCA cycle as well as the respiratory chain.