  Legged locomotion in humans is governed by natural dynamics of the human body
and neural control. One mechanism that is assumed to contribute to the high
efficiency of human walking is the impulsive ankle push-off, which potentially
powers the swing leg catapult. However, the mechanics of the human lower leg
with its complex muscle-tendon units spanning over single and multiple joints
is not yet understood. Legged robots allow testing the interaction between
complex leg mechanics, control, and environment in real-world walking gait. We
developed a 0.49m tall, 2.2kg anthropomorphic bipedal robot with Soleus and
Gastrocnemius muscle-tendon units represented by linear springs, acting as
mono- and biarticular elastic structures around the robot's ankle and knee
joints. We tested the influence of three Soleus and Gastrocnemius spring-tendon
configurations on the ankle power curves, the coordination of the ankle and
knee joint movements, the total cost of transport, and walking speed. We
controlled the robot with a feed-forward central pattern generator, leading to
walking speeds between 0.35m/s and 0.57m/s at 1.0Hz locomotion frequency, at
0.35m leg length. We found differences between all three configurations; the
Soleus spring-tendon modulates the robot's speed and energy efficiency likely
by ankle power amplification, while the Gastrocnemius spring-tendon changes the
movement coordination between ankle and knee joints during push-off.
