Muscle Adaptation to Extreme Endurance Training in Man

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Abstract
To evaluate the effect of extreme endurance training on muscle fiber composition and activities of oxidative enzymes in different fiber types biopsies were taken from vastus lateralis, gastrocnemius and deltoideus of elite orienteers. Comparisons were made between the (trained) leg muscles and the (relatively untrained) arm muscles and with leg muscles of 16-18 yr old boys. The orienteers had the same percentage type I fibers in vastus lateralis and gastrocnemius as in deltoideus, but higher percentage type I fibers in vastus lateralis compared with the controls. The similarity between trained and untrained muscle in the orienters suggests that training had not caused the high percentage type I fibers which rather may be the result of selection of individuals with the best prerequisites forhigh oxidative capacity. The distribution of type II subgroups in the leg muscles of the orienteers differed from both their own deltoideus and leg muscles of the controls, the relationship IIA/IIB was altered in favor of the more oxidative IIA. The leg muscles of the orienteers also showed an increased occurrence of the normally rare IIC fiber. There may be a training-induced alteration in the subgroup pattern. Unlike in the controls there was no significant difference in succinate dehydrogenase activity, measured in single fibers, between type I and II fibers in gastrocnemius of the orienteers. Type II fibers had the ability metabolically to adapt to high oxidative demands. This may to some extent be mediated by a conversion from IIB to IIA form.