Effects of passive stretching on muscle injury and HSP expression during recovery after immobilization in rats.

OBJECTIVE Stretching exercise is known to induce muscle hypertrophy and is implicated in the modulation of muscle fiber behavior. We aim to determine whether stretching exercise is protective against reloading-induced muscle damage in immobilized rat soleus muscle. METHODS Rat hindlimbs in 54 eight-week-old male Wistar rats were immobilized by cast for 4 weeks, followed by reloading alone through normal ambulation in 24 (group NS) and after passive stretch in 25 rats (group S). Stretching exercise (30 min each day) lasted 6 days. To determine if passive stretching affects expression of heat shock proteins (HSPs) in rat soleus muscle during reloading following cast immobilization, the ratio of invading muscle fibers and HSPs expression were measured following cast removal. RESULTS The ratio of invading muscle fibers increased during the first and second days of reloading in group NS. Compared with reloading alone, stretching exercise reduced invading muscle fibers at most time points following cast removal (group S). Additionally, expression of HSP25 and HSP72 increased with time during reloading only in the group without passive stretch (group NS). CONCLUSION Following immobilization, in the rat soleus muscle passive stretching exercise protects against injury induced by reloading. Furthermore, the protection provided by passive stretch is independent of HSPs.