[A study of trunk muscle in idiopathic scoliosis]

Isometric and isokinetic parameters related to trunk muscle function in patients with idiopathic scoliosis were measured by Cybex dynamometer with a trunk stabilization system. Activities of the paraspinal muscles were also determined by electromyographic observations. In scoliotic patients some parameters, especially endurance in lateral bending to the concave side of the trunk, were predominant. In electromyographic analysis, neurogenic changes were observed in 25.9% of patients with idiopathic scoliosis. In nonprogressive scoliosis, the mean amplitude of EMG did not differ between the convex and the concave. In progressive scoliosis, however, the mean amplitude of EMG was higher on the convex side than on the concave side. Thus, the compensatory reaction on the curvature of the spine caused higher amplitude of action potential of the back muscles on the convex side than in the concave side. These data suggest that the trunk muscle imbalance is one of the most important factors in the onset and progression of idiopathic scoliosis.