THE EFFECTS OF ENDURANCE EXERCISE ON DYSTROPHIC MDX MICE .2. CONTRACTILE PROPERTIES OF SKINNED MUSCLE-FIBERS

Dystrophic (mdx) mice were subjected to a 15 week exercise programme c onsisting of endurance swimming. Single fibres from the extensor digit orum longus (EDL, fast-twitch) and soleus (SOL, mixed fast- and slow-t witch) muscles were attached to a sensitive force-recording apparatus, and activated in Ca2+- and Sr2+-buffered solutions. In addition to th e 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 f ibres from the EDL and type IIA fibres from the soleus of the Swim gro up were significantly less sensitive to Ca2+ and Sr2+ compared with th ose fibres sampled from the sedentary (Sedent) group, suggesting that endurance exercise was able to modify Ca2+- and Sr2+-activated contrac tile characteristics. The swim-trained (Swim) group's increased incide nce of SOL fibres with characteristics intermediate to those of the fa st- and slow-twitch fibre types suggests a possible exercise-induced f ibre type transformation as an adaptation to the functional demand.