An exercise system adapted to provide a truly interactive level of exercise with timed exercise intervals that are shown to be optimal in various physiological studies. An example of this invention provides the following unique features: (1) electronically controlled, continuously variable resistance in the positive stroke that responds to the user's efforts and varies the resistance according to the user's current physiological needs; (2) electronically controlled, continuously variable resistance force in the negative stroke that incrementally overcomes the positive muscle contraction and returns to the original position to complete each repetition; and (3) a sophisticated feedback control system that (a) monitors distance, time, and applied force by a user, (b) systematically controls the resistance force to complete each repetition in the specified time intervals, (c) maintains a smooth transition throughout the range of motion, and (d) virtually eliminates the variations of resistance caused by biomechanics, friction, inertia, etc. As a result, an example of the present invention provides the optimum in various types of exercise programs, including but not limited to (1) muscle building, (2) muscle toning, and (3) physical rehabilitation.