Walking Mechanics & Control- Xi Model

Requirements of Walking

Walking requires the movement of the human body through space driven by the complex interaction of neural, muscular, skeletal, and environmental systems. The interactions of these systems have been measured and modeled at many levels of analysis. A critical level is that of whole body mechanics which are driven by two forces, gravity and ground on feet (foot force). The foot force is the net common output of the neuromusculo-skeletal system and thus directly reflects the structure of the controller. The foot force is thus the critical variable interposed between the controller and the physical demands of the task.

Successful steady-state walking requires satisfying two mechanical demands: 1) don’t fall down, and 2) don’t fall over. Falling down results from the foot force being of insufficient magnitude to oppose the downward pull of gravity. Falling over results from the foot force producing inappropriate torque about the center of mass (CM) and leads to excess angular motion of the body as a whole. When humans have difficulty walking it is usually due to the later, angular motion control. Thus, analysis of walking must focus on the torque about the CM produced by foot force. Unfortunately only a handful of studies have even attempted to quantify this critical component of walking.

Contributions to Center of Pressure

Angular motion control during walking requires that the foot force produce appropriate torque about the CM throughout the stance phase. We previously showed that humans have a preference to direct the force at a divergent point located above the CM when walking, and that the advantage of this pattern is that is produces a torque that restores upright posture. That pattern involves precise coordination between center-of-pressure (CP) and force direction. CP location through the stance phase is due to two factors: 1) leg motion relative to the torso from anterior to posterior, and 2) foot roll which shifts the CP from posterior to anterior with respect to the foot. We have previously characterized the effect of CP shifts (due to ankle torque changes) on force direction due to body mechanics. Thus, we used that model to reveal the control of the other leg joints (hip and knee).

Contributions to Foot Force

By removing the foot roll effect from walking we discovered that the hip and knee joint torques are controlled so as to direct the foot force at the CM. This behavior is not surprising because we have previously extensively characterized it during seated pushing tasks. This CM-centric controller is of tremendous importance because it also means that a person can jump using the same controller that is used to walk and still land on their feet. This discovery also helps us understand the development of walking in infants, the disruption of walking after neurological insults, and the design of therapeutic, assistive, and autonomous walking apparatuses.

Observed foot force vectors passing through a DP are shown in bold. Those foot force vectors with the contribution of CP shift removed are shown as dashed lines, which intersect near the CM.

Relevant Publications:

Gruben KG, Boehm WL: Ankle torque control that shifts the center of pressure from heel to toe contributes non-zero sagittal plane angular momentum during human walking. J Biomechanics, 47(6), 1389-1394, 2014.


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