Quantitative alterations in intramuscular connective tissue following immobilization: an experimental study in the rat calf muscles.

The purpose of the present investigation was to study the effect of immobilization in two different positions, shortened or lengthened, on the connective tissue and capillarization of the rat calf muscles. In 18 rats the left hind limb was immobilized so that the soleus and gastrocnemius muscles were in a lengthened position and tibialis anterior was in a shortened position, and in 18 rats vice versa. The right hind limbs were kept free to serve as controls. After 1, 2, and 3 weeks of immobilization, an equal number of rats were killed and the muscles were analyzed. The connective tissue muscle ratio was measured from histological sections by automatic image analyzer and the number of capillaries per 1000 muscle fibers was also recorded. In the intact muscles the proportions of intramuscular connective tissue in the soleus, gastrocnemius, and tibialis anterior were 2, 3, and 5%, respectively. During the immobilization period of 3 weeks the proportion of connective tissue increased significantly in all muscles, but the only significant difference between the positions was in the soleus muscle in which more connective tissue was found in the lengthened position, 54% vs 30% in the shortened position. The amount of capillaries decreased significantly during immobilization in all muscles to about 65% of normal capillar density, but the position of immobilization seemed to have no effect on this phenomenon. The increase in intramuscular connective tissue during immobilization seems to occur simultaneously with muscle atrophy and loss of muscle capillarity. The stretched position of immobilization seems to have an additional fibrotic effect if the immobilized muscle, such as soleus, consists mainly of type I fibers generally known to be more vulnerable to disuse atrophy than type II fibers.