A proportional resistance exercise servo device. User interfacng means is connected to a drive shaft so that the user applies force to said drive shaft and vice versa. The device applies braking force to the drive shaft as it is rotated in a first direction by user-exerted force on the interfacing means, in a braking mode; and it applies power to drive the drive shaft in a second direction and thereby exerts force on the interfacing means, in a power mode. Direction reversal means automatically stops the braking at a first limit and thereafter applies power thereto, and automatically stops the power at a second limit and thereafter begins braking it. Both the braking and powering are programmed, but feedback alters the program in accordance with the user's performance. Acceleration and deceleration are controlled. Various performance parameters are displayed or recorded.