A reversibly attachable muscle trainer includes a multi-arm structure with motion sensors and force generators for exercising opposing muscles of a person moving an implement so as to maintain an ideal movement. When attached to the implement, the muscle trainer trains the opposing muscles to consistently move the implement in an ideal way. While the person moves the implement with the muscle trainer attached, a processor receives signals from the motion sensors and determines differences between the actual implement motion and an ideal implement motion. The difference indicates a dominating force direction in which the dominating muscles urge the implement. The processor activates the force generators to apply external forces to the implement to urge the implement in various types of training force directions.