Walking Stability & the Divergent Point

Systematic Foot Force Variation During Walking

We discovered that the remarkable ability to retain an upright posture while walking is aided by a systematic relationship between foot force (F) direction and center-of-pressure characterized as having a common intersection point we call a divergent point (DP). That point is located above the CM in the vicinity of the shoulder joint.

The benefit of this pattern is that the torque about the CM provided by the foot force accelerates the body back toward upright. That is, when the body happens to tip too far forward, body mechanics locate the CP further anterior and this DP behavior directs the foot force anterior to the CM which produces a posteriorly pitching torque that accelerates the body back toward upright. Conversely for posterior pitch of the body. This pattern of force direction and the resulting stability advantage is also observed for a rigid block sitting on a horizontal surface and for a ship floating in water. However humans place the DP higher than for a rigid block of the same mechanical parameters.

In humans, this DP behavior is the product of active muscle control and not purely mechanics. Humans can adjust relative torques at the hip, knee, and ankle torques to produce foot force with a wide range of CP and force direction. That capability could produce a force that always intersects the CM.

Benefits of the DP Strategy

Directing foot force at a point above the CM reduces the feedback control burden. Directing the foot force at the CM produces not torque about the CM and would thus not alter whole body angular momentum. Because a key objective of walking is to control whole body attitude, zero torque about the CM may seem optimal because it minimizes the torque necessary for error correction. That approach has been widely touted in the control of robotic bipeds. However, any disturbance that induces angular motion must then be detected and corrected with an alteration of the foot force. An alternate strategy is to use a feedforward controller that continuously directs the force at the supra-CM DP. Errors are then automatically corrected without feedback control intervention. This frees human cognitive resources for other tasks.

Increased stabilization can be provided by raising the height of the DP. We have observed that cognitive distraction or threat of mechanical perturbation leads to elevated DP location.

While the advantages of this behavior are evident, the mechanism by which humans accomplish this control requires a more detailed analysis (see later section).

Relevant Publications:

Gruben KG, Boehm WL: Force direction pattern stabilizes sagittal plane mechanics of human walking. Human Movement Science, 31(3), 649-659, 2012.

 

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