Numerical analysis of cooperative abduction muscle forces in a human shoulder joint

Because some shoulder muscles originate from a wide area, the modeling of such muscles has been a significant problem in a computer simulation. We demonstrated a new method of determining a vector for each of the muscles originating from a wide area. A 3-dimensional musculoskeletal model of a human shoulder was constructed from computed tomography data of a normal volunteer. Numerical analysis of 11 muscle forces and the joint reaction force during shoulder abduction from 10° to 150° was performed from the static equilibrium equations. An optimal origin point for the vector of the muscle with a wide origin area was determined in every analyzed position. Electromyography was carried out to validate the results of the simulation, and a significant correlation with the analyzed force was obtained in each muscle. The anatomic biomechanical model with the new muscle modeling method led to the results reflecting the actual muscle activities in a living body.