The physiological energy expenditure involved in common activities such as running, walking, or cycling can be influenced by a variety of biomechanical factors. In evaluating changes in mechanical energy in order to derive a measure of mechanical power which is more directly related to metabolic energy cost, it is necessary to be able to identify the source of these changes. Factors such as concentric and eccentric muscular contractions, transfer of energy, elastic storage and reuse of energy, and joint range of motion limitations can all change the mechanical energy of a segment, but each involves a different metabolic energy expenditure. While a number of computational methods have been suggested for the calculation of mechanical power, each incorporates a different set of assumptions involving the factors just mentioned, and widely varying results for mechanical power have been obtained. The lack of definitive information concerning the relationship between mechanical and physiological energy changes limits the accuracy, meaningfulness, and usefulness of measures of mechanical power and muscular efficiency.