Abstract. In order to implement the high-efficiency resistance training for a specific muscle of human shoulders using the rehabilitation robots, a muscle-specific rehabilitation training method based on the optimal load orientation concept (OLOC) was proposed. A 3D mathematical musculoskeletal model of the shoulder complex was used to predict the muscle forces. In this model, 31 muscle bundles were used to represent all the muscles contributing to the shoulder function, and the Hill-type model was used to characterize the mechanical property of the muscles. The calculation results show that, for a specific muscle, there is always an optimal load orientation (OLO) of the external load which lead the activation of muscle to its maximum. Moreover, the distribution of the OLO is significantly consistent as the movement under different magnitudes of load. Thus the optimal load orientation cluster for a specific muscle, which can be used to specify a muscle-specific rehabilitation strategy, was determined. Simultaneously, the analysis suggests that the muscle-specific rehabilitation training method based on the OLOC could improve the training efficiency of specific muscles significantly.