Effects of stretch-shortening cycle training on mechanical properties and fibre type transition in the rat soleus muscle.

The effects of exercise training on mechanical properties and fibre type transitions have been investigated in rat soleus muscles. The exercise was a repetition of stretch-shortening cycles. A method of dual controlled releases was applied to obtain tension/extension curves, which characterize the elastic behaviour of the series elastic component (SEC), and the force/velocity relationship characterizing the contractile elements. Other contractile measurements included: contraction time (tc), half-relaxation time (t1/2) and twitch/tetanus ratio (Pt/Po). Additionally, the muscle fibre type composition was determined by a classical histochemical method. A 12-week period of training induced a significantly higher percentage of fast-twitch fibres and a lower percentage of slow-twitch fibres (P < 0.01). This fibre adaptation led to a significant (P < 0.01) decrease in tc and an increase in maximum shortening velocity (Vmax). An increase in compliance of the SEC was also observed. This elastic adaptation is interpreted in terms of modification of the active components of the SEC. All the histochemical and mechanical data presented in this study show that rat soleus muscles trained by stretch-shortening cycles acquired faster characteristics. Thus the results confirm that a slow-twitch to a fast-twitch fibre transition is obtainable in mature rats.