Contractile work and biarticular mechanisms of the triceps surae muscles facilitate net ankle mechanical work at high walking speeds

Increasing walking speed is accompanied by an enhancement of the mechanical power and work performed at the ankle joint despite the decrease of the intrinsic muscle force potential. We measured Achilles tendon (AT) elongation and, based on an experimentally determined AT-force-elongation relationship; we quantified AT-force as a proxy of the triceps surae muscle force at four walking speeds (slow 0.7 m.s-1, preferred 1.4 m.s-1, transition 2.0 m.s-1 and maximum 2.6{+/-}0.3 m.s-1). Further, we investigated the mechanical power and work of the triceps surae muscles at the ankle joint (TSA) and the mechanical power and work of the biarticular gastrocnemii at the ankle and knee joint. We found a ~21% decrease of maximum AT-force at the two higher speeds compared to the preferred; however, the net TSA-work increased as a function of walking speed. An earlier plantarflexion accompanied by increased activation of the triceps surae muscles and a knee-to-ankle energy transfer via the biarticular gastrocnemii enhanced the net TSA-mechanical work by 1.7 and 2.4-fold in the transition and maximum walking speeds, respectively. Our findings provide first time evidence for different mechanistic participation of the monoarticular soleus muscle and the biarticular gastrocnemii for the speed-related enhancement of net TSA-work.