A local growth factor or a local tissue repair factor. Combining physiological and molecular biology methods have indicated how a factor expressed by stressed muscle induces local muscle fiber repair and adaptation. Mechano Growth Factor (MGF) is derived from the insulin-like growth factor (IGF-I), but its sequence differs from the systemic IGF-I produced by the liver. MGF is expressed by mechanically overloaded muscle and is involved in tissue repair and adaptation. It is expressed as a pulse following muscle damage and is apparently involved in the activation of muscle satellite (stem) cells. These donate nuclei to the muscle fibers that are required for repair and for the hypertrophy processes which may have similar regulatory mechanisms.