  Bipedal animals have diverse morphologies and advanced locomotion abilities.
Terrestrial birds, in particular, display agile, efficient, and robust running
motion, in which they exploit the interplay between the body segment masses and
moment of inertias. On the other hand, most legged robots are not able to
generate such versatile and energy-efficient motion and often disregard trunk
movements as a means to enhance their locomotion capabilities. Recent research
investigated how trunk motions affect the gait characteristics of humans, but
there is a lack of analysis across different bipedal morphologies. To address
this issue, we analyze avian running based on a spring-loaded inverted pendulum
model with a pronograde (horizontal) trunk. We use a virtual point based
control scheme and modify the alignment of the ground reaction forces to assess
how our control strategy influences the trunk pitch oscillations and energetics
of the locomotion. We derive three potential key strategies to leverage trunk
pitch motions that minimize either the energy fluctuations of the center of
mass or the work performed by the hip and leg. We suggest how these strategies
could be used in legged robotics.
