Foot Roll & Force Direction

Contribution of Ankle Torque to Foot Force Direction

Standing and walking involve the foot force having a center-of-pressure (CP) that changes location with respect to the foot. While the CP patterns have been long recognized as important components of human postural control, the effect of CP variation on foot force direction had not been studied.

While the foot force vector (magnitude, direction, and CP) is a function of torque about multiple joints within the body, the ankle torque has the largest effect on CP. Thus, we investigated the effect that ankle torque modulation has on CP and foot force direction. To control the influence of the other joints we used a sagittal plane linked-segment model of the human. We show that when hip and knee joint torques are held constant and ankle torque is varied (three instances in left panels of figure), the instantaneous foot force has a systematic relationship between direction and CP characterized by an exact divergent point located near the knee joint (right panel of figure, DP labeled as Pi). That location depends on body spatial configuration, hip and knee joint torque values, and anthropometric parameters. For parameters taken from walking the divergent point is located below the knee joint throughout the stance phase.


This finding is significant because other studies have shown that ankle torque may be controlled independently of hip and knee torques. Thus, this force direction behavior may be a critical component of how the various controllers interact to produce walking. See a later section for the details.

Relevant Publications:

Gruben KG, Boehm WL: Mechanical interaction of center of pressure and force direction in the upright human. J Biomechanics, 45(9), 1661-1665, 2012.

Gruben KG, Boehm WL: Response to letter to the editor: ‘The ground reaction vector in walking passes always (almost) through the same point.’ J Biomechanics, 46, 632-633, 2012.

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.


>Back to All Research