Abstract: A shoulder prosthesis restores the complex dynamics of the arm for whole-arm amputees. While most prosthesis for arm amputees focus on restoring the user's capabilities for dexterous manipulation, the disclosed prosthesis remains affixed to the shoulder and exerts a moment on the user's trunk similar to that of the arm during walking for dynamic motion assistance. The prosthesis includes a rotating, gimbaled mass oriented based on gait and stride for emulating forces that would have been provided by the amputee arm. The size, ease of use, and relatively low cost manufacture of the proposed device makes it an attractive complement or alternative to standard prosthesis, particularly for amputees who pursue rigorous or prolonged physical activity.