Serum enzyme, myoglobin and muscle strength relationships in polymyositis and dermatomyositis

Strength of the quadriceps and hamstring groups was biomechanically assessed in terms of isometric torque production in 14 patients with inflammatory myopathy. Eleven had polymyositis and 3, dermatomyositis. Determinations of serum creatine kinase, lactate dehydrogenase, transaminase, and myoglobin were simultaneously obtained over an average period of observation of 1.8 years. In certain individual patients, there were significant correlations between laboratory indices and strength during the entire course of illness. In others, this was not the case. In the total group of patients, absolute values of the laboratory indices did not correlate well with strength except in the case of serum myoglobin, where there a significant inverse relationship. Logarithmic transformations of the laboratory data increased the inverse correlations. High strength and low myoglobin were related to high prednisone dose. Since laboratory guides are not always related to disease activity, quantitative assessment of muscle strength is necessary.