Mechanical energy costs of human movement: an approach to evaluating the transfer possibilities of two-joint muscles.

Different methods of calculating the mechanical energy cost of a movement presented in the literature can give results differing by an order of magnitude. The assumptions made concerning the transfer of energy between different parts of the body are part of the problem. This investigation assesses the role of transfer in energy saving and specifically, the possibility of two-joint muscles reducing the mechanical energy cost of a movement compared to a system having one-joint muscles only. An algorithm was developed which recruited one-joint or both one- and two-joint muscles to supply the net joint moments. The work performed under these two conditions was then compared. It was found that activation of both one- and two-joint musculature reduced the mechanical work cost during walking by between 7 and 29% over that required by single-joint musculature alone. This investigation supports suggestions in the literature that one of the functions of two-joint musculature is to reduce the mechanical energy cost and probably the metabolic cost of movement.