Foot Force Directional Control Across the Workspace

Seated Pushing Task

The discovery of the human capability and preference for changing foot force magnitude while preserving foot force direction enabled characterization of a single push effort with a single quantity, the direction of the linear force path. Seated subjects exerted force against stationary force sensing pedals placed throughout the workspace. The pedal was mounted on a transverse frictionless pivot placed below the ball of the foot. The direction of the linear force path varied systematically with pedal location but was independent of the orientation of the foot.

Preferred Control Structure

Of the multiple explanations that could possibly explain the systematic variation of force direction with limb endpoint position, the best prediction is provided by a common intersection of the observed foot force lines of action which was located near the center of mass (CM). Well call this point a divergent point (DP), the geometric location from which human's preferred leg muscle coordination appears to produce force.   We also evaluated minimum joint torque and minimum muscle stress models. The significance of the seated DP model explanation suggests tuning to support control of whole body angular momentum by directing foot force near the CM. Our later research suggests that a similar control structure exists in walking.

While the force path is linear within a single push effort to a given force magnitude, the direction of a force path varies systematically when the magnitude target of the push changes. This effect is small but consistent across subjects. When humans push with the intent of producing greater force they use slightly different muscle coordination that angles the force further anterior. A possible significance of this behavior is that it could accommodate for the more crouched posture that is likely for larger magnitude push efforts, such as in a jump take-off. Relative to the leg the CM is further anterior for the more crouched postures that would be encountered in a more effortful jump. Thus, this behavior seems to be tuned to preserve a force direction aimed near the CM.

Relation to Upright Posture

In summary, various aspects of seated lower limb coordination are consistent with the need for control of upright posture when the legs are used for balance. Later studies have provided support for this powerful idea that seated pushing tasks can reveal coordination that is a fundamental component of walking but had been obscured by the complexity of the walking task.

Relevant Publications:

Gruben KG, Rogers LM, Schmidt MW: Direction of foot force for pushes against a fixed pedal: role of effort level. Motor Control, 7(3):229-41, 2003.

Schmidt MW, López-Ortiz C, Barrett PS, Rogers LM, Gruben KG: Foot force direction in an isometric pushing task: prediction by kinematic and musculoskeletal models. Experimental Brain Research, 150(2):245-54, 2003.

Gruben KG, López-Ortiz C, Giachetti RS: Muscular and postural components of foot forces during quasi-static extension efforts. J Applied Biomechanics, 19:239-245, 2003.

Gruben KG, Rogers LM, Schmidt MW, Tan L: Direction of foot force for pushes against a fixed pedal: variation with pedal position. Motor Control, 7(4):366-383, 2003.


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