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A computer model of the human lower extremity was developed to study how surgical lengthening of tendon affects the force- and moment-generating capacity of the muscles. This model computes the maximum isometric force and the resulting joint moments that each muscle-tendon complex can develop at any body position. Tendon lengthenings were simulated by increasing the tendon length of each muscle-tendon complex and computing the change in the maximum isometric muscle force and joint moments at a specific body position. These simulations showed that the forces and moments developed by the ankle plantarflexors are extremely sensitive to changes in tendon length. For example, at a body position corresponding to the midstance phase of gait, the maximum isometric moment generated by soleus decreased 30% with a 1-cm increase in tendon length, and 85% with a 2-cm increase in tendon length. In contrast, 1- and 2-cm increases in iliopsoas tendon length decreased its hip flexion moment by only 4% and 9%, respectively. This article quantifies the sensitivity of muscle force and joint moments to changes in tendon length for the most commonly lengthened lower-extremity tendons. These results indicate how much each of these tendons should be lengthened to achieve an incremental decrease in muscle force or joint moment.
Delp et al. (Sun,) studied this question.