The effects of endurance exercise on dystrophic mdx mice. II. Contractile properties of skinned muscle fibres

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Abstract
Dystrophic (mdx) mice were subjected to a 15 week exercise programme consisting of endurance swimming. Single fibres from the extensor digitorum longus (EDL, fast-twitch) and soleus (SOL, mixed fast- and slow-twitch) muscles were attached to a sensitive force-recording apparatus, and activated in Ca$^{2+}$- and Sr$^{2+}$-buffered solutions. In addition to the normal well-defined fibre types in these muscles, a small number of fibres were also sampled from the soleus of both experimental groups, which were `Intermediate' to the other two SOL fibre types. Type IIB fibres from the EDL and type IIA fibres from the soleus of the Swim group were significantly less sensitive to Ca$^{2+}$ and Sr$^{2+}$ compared with those fibres sampled from the sedentary (Sedent) group, suggesting that endurance exercise was able to modify Ca$^{2+}$- and Sr$^{2+}$-activated contractile characteristics. The swim-trained (Swim) group's increased incidence of SOL fibres with characteristics intermediate to those of the fast- and slow-twitch fibre types suggests a possible exercise-induced fibre type transformation as an adaptation to the functional demand.

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