A device and method is shown for invoking a muscle's natural involuntary, reflexive response or stretch reflex by imparting a sudden increase in load on the muscle over a defined period of time from a predetermined base load at which the muscle has assumed a baseline tonus, and over a predetermined amplitude of motion. The muscle is stimulated by cycling the load with a frequency of between 1 and 60 Hz, or more preferably between 10 and 30 Hz, and an amplitude of displacement of the muscle between 2 and 50 mm, or more preferably between 5 and 10 mm. The force input to the muscle can be provided by either the mass of the body to which the muscle is connected or by an external mass or resistance to motion. A seesaw platform can be oscillated in a vertical direction at the correct frequency and amplitude. Alternatively, a surface adapted to be fixed to a portion of the body can be oscillated relative to an external mass or other element that resists motion due to gravitational, frictional or inertial forces.