Fibre type transition and stiffness modification of soleus muscle of trained rats.

Rat soleus muscles were subjected to two types of overload with intent to induce fibre type transitions and modifications in muscular stiffness. The overloading techniques were respectively an endurance training program and a strength training program. The method of controlled releases was used to obtain tension-extension curves characterizing the elastic behaviour of the soleus. Fibre typing was made by myofibrillar ATPase staining and the effectiveness of the training programs was also evaluated by assessing enzymatic activities. Endurance training resulted in an increase of stiffness associated with a decrease of type II fibers. The opposite change was found as a result of strength training. These results demonstrate that endurance training and strength training induce opposite modifications in muscular stiffness which are correlated with fibre type transitions.