Muscle Contracture Induced by Exercise

An unusual disorder of muscle function, characterized by painless contractures occurring with exercise, was present in a patient since childhood, and muscle contraction was normal, but the response to exercise was abnormal.

Abstract

An unusual disorder of muscle function, characterized by painless contractures occurring with exercise, was present in a patient since childhood. Muscle contraction was normal, but the relaxation phase became increasingly slow during exercise. Myotonia was ruled out by the absence of muscle action potentials during the slow relaxation. The most important biochemical finding was a marked decrease in the ability of the isolated sarcoplasmic reticulum to accumulate calcium ions. The concentration of muscle phosphorylase was normal, but the proportion of the active form was unusually high, and there was a high level of venous-blood lactate during rest. The slow relaxation of this patient's muscles may be due to the decreased uptake of calcium by the sarcoplasmic reticulum. The persistently high level of free calcium expected from such a defect may explain the persistent activation of muscle phosphorylase and hence the increased production of lactate during rest. The disease is thus attributable to a selective defect of relaxing factor.