  Postural stability is one of the most crucial elements in bipedal locomotion.
Bipeds are dynamically unstable and need to maintain their trunk upright
against the rotations induced by the ground reaction forces (GRFs), especially
when running. Gait studies report that the GRF vectors focus around a virtual
point above the center of mass (VPA), while the trunk moves forward in pitch
axis during the stance phase of human running. However, a recent simulation
study suggests that a virtual point below the center of mass (VPB) might be
present in human running, since a VPA yields backward trunk rotation during the
stance phase. In this work, we perform a gait analysis to investigate the
existence and location of the VP in human running at 5 ms-1, and support our
findings numerically using the spring-loaded inverted pendulum model with a
trunk (TSLIP). We extend our analysis to include perturbations in terrain
height (visible and camouflaged), and investigate the response of the VP
mechanism to step-down perturbations both experimentally and numerically. Our
experimental results show that the human running gait displays a VPB of ~-30cm
and a forward trunk motion during the stance phase. The camouflaged step-down
perturbations affect the location of the VPB. Our simulation results suggest
that the VPB is able to encounter the step-down perturbations and bring the
system back to its initial equilibrium state.
