Balance responses to lateral perturbations in human treadmill walking

Abstract

SUMMARYDuring walking on a treadmill 10 human subjects (mean age 20 years) were perturbed by 100ms pushes or pulls to the left or the right, of various magnitudes and in various phases of the gait cycle. Balance was maintained by (1) a stepping strategy (synergy), in which the foot at the next step is positioned a fixed distance outward of the 'extrapolated centre of mass', and (2) a lateral ankle strategy, which comprises a medial or lateral movement of the centre of pressure under the foot sole. The extrapolated centre of mass is defined as the centre of mass position plus the centre of mass velocity multiplied by a parameter related to the subject's leg length. The ankle strategy is the fastest, with a mechanical delay of about 200ms (20% of a stride), but it can displace the centre of pressure no more than 2cm. The stepping strategy needs at least 300ms (30% of a stride) before foot placement, but has a range of 20cm. When reaction time is sufficient, the magnitude of the total response is in good agreement with our hypothesis: mean centre of pressure (foot) position is a constant distance outward of the extrapolated centre of mass. If the reaction time falls short, a further correction is applied in the next step. In the healthy subjects studied here, no further corrections were necessary, so balance was recovered within two steps (one stride). Supplementary material available online at

Stepping in the direction of the fall: the next foot placement can be predicted from current upper body state in steady-state walking

“…10: 20140405 from steady-state data can reliably predict consequences of external perturbations. Performing external perturbations [3,4] and inferring the subsequent inputs to the leg muscles (from electromyography or inverse dynamics) can delineate the relative importance of feedback and feed-forward control.…”

“…Mathematical models [1][2][3] suggest the effectiveness of appropriate foot placement to avoid falling forward or sideways: a person falling rightwards could produce a corrective leftward force effectively by placing the next foot to the right of its usual position. Experiments involving external mechanical [3] and visual perturbations [4] have found some evidence for such foot placement dynamics, likely due to both active control and passive dynamics. In this article, we infer plausible foot placement dynamics without such external perturbations, using natural step-to-step variability in steady-state walking data; see figure 1a for foot placement variability.…”

Limb and Trunk Mechanisms for Balance Control during Locomotion in Quadrupeds

“…These oscillations reflect the anticipatory postural adjustment, which causes the COM to be propelled toward the stance-limb side before the lifting of the swing foot (McIlroy and Maki, 1999). From Figure 2B it is seen that the COM projection always occurred within the step width (also termed "the support area"), which is an important condition for postural stability (Hof et al, 2010).…”

Modeling Walking with an Inverted Pendulum Not Constrained to the Sagittal Plane. Numerical Simulations and Asymptotic Expansions

“…A survey regarding the analysis and control of biped robots walking can be found in [40]. Experimental studies of the responses of humans walking in a treadmill subject to lateral oscillations are reported in [41]. Lateral stability of walking is studied both theoretically and experimentally in [42] [43] [44] [45] [46].…”