The importance of stretch and contractile activity in the prevention of connective tissue accumulation in muscle.

The loss of serial sarcomeres which results when muscles are immobilised in a shortened position is accompanied by an increase in the proportion of collagen and an increased muscle stiffness. In order to determine whether it is lack of stretch or lack of contractile activity which is the main factor involved in these changes experiments were carried out using different combinations of immobilisation and electrical stimulation. It was found that the connective tissue accumulation that occurs in inactive muscles can be prevented either by passive stretch or by active stimulation. It was also shown that in muscle that is working over a reduced range there is, as in muscle immobilised in the shortened position, a reduction in serial sarcomeres. In this case, however, there is no concomitant increase in connective tissue, again indicating that contractile activity is important for the maintenance of normal muscle compliance.