Simple SummaryThis study enhances our understanding of how different mechanisms in the triceps surae muscles contribute to increased mechanical power and work at the ankle joint during various walking speeds. The findings reveal that both the soleus (Sol) and gastrocnemii muscles play a role in the speed-related increase of mechanical work at the ankle joint but through distinct energetic processes and mechanisms. The Sol muscle primarily increases contractile work (62%), while the gastrocnemii muscles demonstrate an enhanced contribution through biarticular mechanisms (38%). This knowledge can inform the design of exercise interventions and customized assistance through bioinspired exoskeleton configurations for both healthy individuals and those with pathological conditions.AbstractIncreasing walking speed is accompanied by an increase of the mechanical power and work performed at the ankle joint despite the decrease of the intrinsic muscle force potential of the soleus (Sol) and gastrocnemius medialis (GM) muscles. In the present study, we measured Achilles tendon (AT) elongation and, based on an experimentally determined AT force–elongation relationship, quantified AT 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 AT force at the ankle joint and, separately, the mechanical power and work of the monoarticular Sol at the ankle joint and the biarticular gastrocnemii at the ankle and knee joints. We found a 21% decrease in maximum AT force at the two higher speeds compared to the preferred; however, the net work of the AT force at the ankle joint (ATF work) increased as a function of walking speed. An earlier plantar flexion accompanied by an increased electromyographic activity of the Sol and GM muscles and a knee-to-ankle joint energy transfer via the biarticular gastrocnemii increased the net ATF mechanical work by 1.7 and 2.4-fold in the transition and maximum walking speed, respectively. Our findings provide first-time evidence for a different mechanistic participation of the monoarticular Sol muscle (i.e., increased contractile net work carried out) and the biarticular gastrocnemii (i.e., increased contribution of biarticular mechanisms) to the speed-related increase of net ATF work.