There is provided a machine for rehabilitation or exercise, comprising: a frame; a first arm movably secured to the frame via a first actuator; a first force sensor for measuring a force exerted by a user on the first arm; and a control unit adapted for controlling a displacement speed for the first arm via the first actuator as a function of the force and for increasing the displacement speed of the first arm via the first actuator when the force is superior to a target force. In one embodiment, there is further provided an electromyograph for location on the exercised muscle for measuring an electrical potential generated by the muscle and for lowering the target force when the electrical potential is superior to a predetermined maximum electrical potential. There is further provided a method for operating an exercise machine and a system for exercising a muscle.