Is limb immobilization a model of muscle disuse?

Abstract

Limb immobilization is often used as a model of disuse-atrophy of skeletal muscle. Because the degree of atrophy is somewhat dependent on the length at which the muscle is immobilized, information regarding the effect of immobilization at various joint positions on the electrical activity of slow and fast muscles should provide a better understanding of the role of “use-disuse” in determining the degree of atrophy. In this study, the soleus muscle (SOL) and medial gastrocnemius muscle (MG) of adult female rats were chronically implanted with bipolar recording electrodes. The hind limbs were immobilized bilaterally using an external brace such that the muscles were fixed in a shortened, neutral, or lengthened position. Electromyograms (EMGs) of the SOL and MG were recorded simultaneously during a 15-min period each hour for 24 consecutive hours 3 and 9 days after electrode implantation, and 7, 17, and 28 days after immobilization. The weights of both muscles decreased significantly (P < 0.05) when fixed in a shortened or neutral, but not in a lengthened position after 28 days. As indicated by the 24-h integrated EMG (IEMG), the SOL, which was more active than the MG in unrestrained rats, showed the greatest decrease in postimmobilization activity. After 28 days of immobilization in a shortened position, the IEMG of the SOL and MG was significantly reduced (P < 0.05) by 77 and 50%, respectively. The IEMG of the SOL immobilized in a neutral position decreased 50% whereas no change was seen in the MG. There was no significant change in the IEMG for the SOL or MG fixed in a lengthened position. These data suggest that the EMG was dependent on the type of muscle and the length of fixation. Furthermore, it appeared that the degree of atrophy was not related closely to changes in the IEMG suggesting that limb immobilization was not necessarily a model of “disuse”.