Postural stability in human running with step-down perturbations: an experimental and numerical study

Abstract

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 centre of mass (VP A ), 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 centre of mass (VP B ) might be present in human running, because a VP A 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 m s −1 , and support our findings numerically using the spring-loaded inverted pendulum model with a trunk. 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 VP B of ≈−30 cm and a forward trunk motion during the stance phase. The camouflaged step-down perturbations affect the location of the VP B . Our simulation results suggest that the VP B is able to encounter the step-down perturbations and bring the system back to its initial equilibrium state.

Walking like a robot: do the ground reaction forces still intersect near one point when humans imitate a humanoid robot?

“…In addition, an increasing dispersion between the participants with increasing perturbation of upright gait can also be observed in other studies, e.g. walking down visible and camouflaged curbs [12] or running down camouflaged drops [20].…”

“…In those experiments, the GRFs point to the VPP with a small spread. In simulations of running humans, humanoid robots and birds, there were stable solutions for the VPP position both below and above the CoM [18][19][20][21]. However, the question arises whether the VPP is a target driving the gait strategy or merely a consequence of the complex dynamics and control during gait.…”

Positions of pivot points in quadrupedal locomotion: limbs and trunk global control in four different dog breeds

“…The VPP template can perhaps be understood as a global representation of coupled contralateral central pattern generators (CPGs) (Brown, 1914; Grillner and Manira, 2020; Grillner and Zangger, 1979). Simulation studies showed that stable bipedal gaits emerge for certain values of VPP position in body frame and leg impedance (Andrada et al, 2014; Drama et al, 2020; Maus et al, 2010). Note that these parameters were adapted by our analyzed dogs when changing from walk to trot (see e.g., Tab.…”

“…Finally, if the VPP is placed caudally to the proximal pivot, as depicted in dogs' thoracic limb, the torque generated will absorb energy. In this case the leg must add energy axially to the system for the sake of periodicity (Blickhan et al, 2015;Drama and Badri-Spröwitz, 2020). Accordingly, the dogs under study here, except Whippets during walk, add energy axially to the system.…”

“…In bipeds, VPP as the target of control simplifies the balance of the trunk by changing an inverted pendulum control problem into a suspended one. Moreover, simulation studies based on this bio-inspired strategy showed that VPP control can balance not only the trunk but also generate stable bipedal gaits (Andrada et al, 2014;Drama and Badri-Spröwitz, 2020;Maus et al, 2010). So far, the existence of a VPP in quadrupedal locomotion has not been demonstrated experimentally.…”