Changes in stiffness induced by hindlimb suspension in rat soleus muscle

Soleus muscle atrophy was induced by hindlimb suspension of rats for 3 weeks with the intention of inducing a relative increase in the percentage of fast twitch fibres and assessing modifications in muscle stiffness. A method of dual controlled releases was used to obtain tension/extension curves and force/velocity relationships characterizing the mechanical behaviour of the soleus. Fibre typing was achieved by myofibrillar adenosine 5-triphosphatase staining. Results showed that hindlimb suspension decreased the percentage of slowtwitch fibres (–31%) to the profit of fast-twitch fibres (+370%) and intermediate fibres (+255%). This led to an increase in maximal shortening velocity. Tension/extension curves indicated a decrease in soleus stiffness after 3 weeks of unloading. Changes in elastic properties are interpreted in terms of modifications occurring in the active part and the passive part of the so-called series elastic component. These changes also suggest that the parameters derived from a twitch are inappropriate to account for modifications in speed-related properties of muscle.