Core myofibers and related alterations induced in rats' soleus muscle by immobilization in shortened position

Experimental induction of core myofibers by tenotomy or local tetanus suggests that mechanical factors such as muscle tension loss, shortening or immobilization may play a role in core fiber formation. To test this hypothesis, we investigated the morphologic alterations induced in soleus (SOL) and extensor digitorum longus (EDL) muscles following immobilization of rats' hindlimb in various positions. The SOL and EDL muscles were immobilized in either shortened or lengthened state by applying wire-meshed plaster cast for 1, 2 and 3 weeks. The muscles were dissected out, measured, weighed and examined by histochemistry and electron microscopy. Gross atrophy was noted in all muscles but was greatest in shortened SOL. The SOL atrophy was diffuse and associated with relative increase in type 2 fibers. In EDL, the atrophy selectively involved fibers with low oxidative enzyme activity. Core myofibers were seen mainly in shortened SOL and consisted of myofibrillar derangement, loss of myofilaments and streaming of Z bands. The preferential involvement of shortened SOL (tonic, fatigue-resistant, slow-twitch muscle) suggests that the functional length, loss of tension subsequent to shortening and intrinsic biochemical properties of the muscle are important in core fiber formation.