Abstract: A control system for an exercise machine includes a closed loop hydraulic circuit. The single control system allows an exercise machine to operate in either isokinetic or isodynamic modes. The system utilizes a hydraulic cylinder to allow an exercise machine to provide both positive and negative isodynamic resistance. The control system utilizes rotary and linear cylinders which allow for the proper ratio of torque resistance between joints to be automatically incorporated into the machine. By presetting an adjustment mechanism, a different concentric resistance and eccentric resistance on each opposing muscle group of a given exercise is achieved due to the fact that the applied air pressure automatically changes at the instant the exercise velocity reverses as the user changes the direction of a given exercise. The control system also allows for a limited range of motion for a given exercise to be present by a user. The control system allows for a given exercise machine to utilize three different modes.