Abstract: A machine force system and control for an exercise device is disclosed. The device includes a drive motor and a brake the combination of which is mechanically fastened to the input shaft of a clutch. This clutch can take a variety of forms but is preferably an electrically controlled particle clutch. The output shaft of the clutch is in mechanical communication with the exercise arm of the exercise device. In the preferred embodiment, this mechanical communication is through a gear reduction and preferably a multiple reduction. This reduction increases the torque from the clutch to the exercise arm while reducing the speed of movement. A sensor to indicate the position of the exercise arm and a microprocessor unit to read, compare and operate the brake, motor and clutch is also included. The system includes a variety of exercises including isotonic, isokinetic and increased eccentric dynamic force, passive dynamic force and static isometric and stepped isometric resistance.