EMG-force relations of a single skeletal muscle acting across a joint: Dependence on joint angle

The electromyogram (EMG)-force relations of a single skeletal muscle (soleus) acting on a joint set isometrically at various flexion angles was studied using electrical nerve stimulation as the experimental method. The EMG-force relationships were linear at extremes of joint extension and became progressively nonlinear as the flexion angle increased. Joint angles at extreme extension showed the least passive and active force whereas their corresponding EMG versus force relations were linear. At extremes of joint flexion large passive force was accompanied by minimal active force. The largest active forces were recorded at midrange of the joints' excursion (90 degrees ). The source of the increasing nonlinearity was traced to large variability in the active and passive muscle forces as a function of joint angle, excluding all but minor variability in the EMG with joint position. We concluded that length-dependent variations of active and passive forces of the muscle (length-tension relations) compounded with the variation of its moment arm when acting across the joint require major consideration in biomechanical studies in which EMG is used to represent muscle force indirectly.