Abstract: A method for non-invasive in vivo determination of muscle fiber composition and apparatus for accomplishing the same. The method includes the steps of electrically stimulating a chosen muscle; determining the stimulation current; measuring the electrical potential of the muscle; the contraction time; and the force produced by the contraction; and by intercorrelating the data by multiple regression, determining the type, percentage and size of muscle fibers within the muscle stimulated. Apparatus for determining the muscle composition includes a muscle stimulator of controlled voltage; electromyogram equipment; and a force transducer providing a tension curve as well as force measurements.