Abstract
1-EMG revealed a reduction of the power to abduct the scapula in cases of so-called unstable shoulder (loose shoulder) . We checked the lower fibers of the serratus anterior and upper fibers of the trapezius of such patients (9 cases, 17 shoulders) and of normal volunteers (8 cases, 13 shoulders) . These two groups could be clearly differentiated by simple X-ray findings. Using the RBSM (Rigid Body Spring Model) method of Kawai, a normal shoulder and an unstable shoulder were computer simulated. The simulation showed that at 1500 abduction there was a large reduction in the raising power of the scapula of the unstable shoulder; the capsule was weaker and the resultant force of the humeral head was directed toward the inferior edge of the glenoid.
