  In this paper, we investigate whether applying ankle torques during
mid-stance can be a more effective way to reduce energetic cost of locomotion
than actuating leg length alone. Ankles are useful in human gaits for many
reasons including static balancing. In this work, we specifically avoid the
heel-strike and toe-off benefits to investigate whether the progression of the
center of pressure from heel-to-toe during mid-stance, or some other approach,
is beneficial in and of itself. We use an "Ankle Actuated Spring Loaded
Inverted Pendulum" model to simulate the shifting center of pressure dynamics,
and trajectory optimization is applied to find limit cycles that minimize cost
of transport. The results show that, for the vast majority of gaits, ankle
torques do not affect cost of transport. Ankles reduce the cost of transport
during a narrow band of gaits at the transition from grounded running to aerial
running. This suggests that applying ankle torque during mid-stance of a steady
gait is not a directly beneficial strategy, but is most likely a path between
beneficial heel-strikes and toe-offs.
