During human locomotion external and internal moments are developed about the major joints of the lower limbs. The internal torque is the result of active contraction of muscles, passive properties of the joint and its surrounding muscles, ligaments and soft tissues, and the reflexive contractions of muscles around a joint.

Although the total intenal torque about the major joints of the lower limb has been determined in the past, the components of this torque were unknown until recently.

The purpose of this study was to determine the passive, active, and reflexive components of the internal torque developed about the ankle joint during locomotion and to determine whether the active component of the internal torque can be indirectly estimated from the electrical activity of the muscles that cross the ankle joint.

The results of the study indicated that in normal locomotion the reflexive component of the internal torque is negligible and the passive component contributes about 6% to the total internal torque. Therefore, it was concluded that the major contribution to the internal torque developed about the ankle during normal locomotion is the result of active contraction of muscles. These results, however, were observed to be significantly affected by pathological changes in the properties of the structures surrounding the ankle. The method developed in this study to estimate the active component of the internal torque during locomotion from the myoelectric activity (EMG) of the muscles around the ankle was based on developing a transfer function between active torque and EMG that was based on Hill's muscle model. The results of this part of the study demonstrated that the active component of muscular torque can be estimated with less than 20% error from the EMG data at the instances during the walking cycle where the muscles (tricepts surae) were contracting isometrically. Correspondingly, based on this method the maximum value of the active component of the internal torque could be estimated.