Abstract: An inertial training apparatus and method utilized for strength training and rehabilitation strength training of muscles. The inertial training apparatus and method allows for generation of rotational inertial forces by accelerating a mass by a concentric contraction of a muscle and converting the rotational inertial forces of the mass into a substantially equivalent eccentric action of the muscle used to decelerate the mass. A tether is coupled between the mass and a body part to transfer the tension forces provided by the body part to accelerate the mass and, in turn, those provided by the body part to decelerate the mass. A swing arm and a mass on the swing arm are rotatably mounted to a frame to preferably allow for low-friction rotation about a substantially vertical axis. A primary tether guide is preferably coupled to the frame and can be positioned near an intermediate rotational position of the swing arm where acceleration forces are transferred into deceleration forces.