  Creating natural-looking running gaits for humanoid robots is a complex task
due to the underactuated degree of freedom in the trunk, which makes the motion
planning and control difficult. The research on trunk movements in human
locomotion is insufficient, and no formalism is known to transfer human motion
patterns onto robots. Related work mostly focuses on the lower extremities, and
simplifies the problem by stabilizing the trunk at a fixed angle. In contrast,
humans display significant trunk motions that follow the natural dynamics of
the gait. In this work, we use a spring-loaded inverted pendulum model with a
trunk (TSLIP) together with a virtual point (VP) target to create trunk
oscillations and investigate the impact of these movements. We analyze how the
VP location and forward speed determine the direction and magnitude of the
trunk oscillations. We show that positioning the VP below the center of mass
(CoM) can explain the forward trunk pitching observed in human running. The VP
below the CoM leads to a synergistic work between the hip and leg, reducing the
leg loading. However, it comes at the cost of increased peak hip torque. Our
results provide insights for leveraging the trunk motion to redistribute joint
loads and potentially improve the energy efficiency in humanoid robots.
