A method for the determination of the force-length relation of selected in-vivo human skeletal muscles

In this paper a method is presented to determine force-length relations of in-vivo human skeletal muscles. The method is experimental and can be used for selected multi-joint muscles. It contains three basic assumptions: (a) the maximal, isometric force a muscle can exert is constant for a given muscle length, (b) antagonistic muscle activity for the experimental contractions is constant, and (c) resultant joint moments obtained during the experiments are produced by muscular forces exclusively. Experimentally determined force-length relations of intact in-vivo human skeletal muscles have not been determined yet. Application of this method will allow the comparison of actual force-length relations of selected human skeletal muscles to force-length relations used previously. Proposed mechanisms responsible for the force-length characteristics of a muscle, such as the cross-bridge theory, may be critically evaluated. Differences of force-length relations obtained under in vivo and in vitro conditions may be quantified.