Connective tissue changes in immobilised muscle

The reduction in fibre length of muscles immobilised in a shortened position is accompanied by reduced compliance of the muscle. Since the intramuscular connective tissue framework distributes the forces passively imposed on a muscle by stretching, it was decided to investigate the amount and distribution of connective tissue in immobilised muscles. Biochemical analysis of the hydroxyproline content of muscles immobilised in the shortened position for different periods of time showed an increase in the ratio of collagen to muscle fibre tissue. This occurred during the first few days of immobilisation, before there was any significant loss of sarcomeres. Thus the increase in connective tissue appeared to result directly from immobilisation rather than from redistribution of connective tissue, following shortening of the fibres. A detailed histological analysis of muscle sections stained for connective tissue with Sirius Red showed that the early increase in connective tissue in immobilised muscles occurred in the perimysium rather than the endomysium, although after a longer period of immobilisation there was also a thickening of the endomysium. Ultrastructural analysis of the perimysium in normal muscle showed that the angle the collagen fibres made with the muscle fibres changed with the state of stretch of the muscle; when the muscle was shortened, the angle was larger than when the muscle was lengthened. In immobilised muscle, collagen fibres were found to be arranged at a more acute angle to the axis of the muscle fibres than was found in normal muscle; this would be expected to affect the compliance of the muscle. The experiments described indicate that the increased stiffness of immobilised muscles could result from both quantitative and qualitative changes in the connective tissue.