Increasing the moment arm of the tibialis anterior induces structural and functional adaptation: implications for tendon transfer.

Previous studies on the functional effects of tendon transfer have not examined possible muscle adaptation following transfer. The purpose of the present study was to test the hypothesis that muscle adapts to increased moment arm and excursion such that joint torque is maintained near normal levels. The moment arm and excursion of the tibialis anterior (TA) were increased by releasing the TA from its retinacular restraint at the ankle joint in growing (4-week-old) rabbits. Twelve weeks post-release, in vivo TA force during hopping was smaller in released compared with control rabbits, compensating for the increased moment arm, and thus TA torque at the ankle joint was not significantly different between groups. Physiological cross-sectional area was smaller, and the number of sarcomeres in series was larger, in the released TA compared with the control TA. These adaptations may result from chronically decreased in vivo TA force production, and chronically increased TA excursion, respectively. In addition, these adaptations were consistent with the smaller in vivo force for the released TA. Comparisons between control and sham-operated rabbits showed no significant differences for in vivo TA force, torque, or muscle architecture. Thus, muscle appears capable of adapting to increased moment arm and excursion such that joint torque is maintained near normal levels. These findings have important implications for tendon transfer procedures that increase the moment arm and/or excursion of the released muscle.