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Kreg G. Gruben, Lynn M. Rogers, and Matthew W. Schmidt

Control of the force exerted by the foot on the ground is critical to human locomotion. During running on a treadmill and pushing against a fixed pedal, humans increased foot force in a linear manner in sagittal plane force space. However, for pushes against a moving pedal, force output was linear for some participants but slightly curved for others. A primary difference between the static and dynamic pedaling studies was that the dynamic study required participants to push with varying peak effort levels, whereas a constant peak effort level was used for the fixed pedal pushes. The present study evaluated the possibility that force direction varied with level of effort. Seated humans pushed against a fixed pedal to a series of force magnitude targets. The force direction varied systematically with effort level consistent with the force path curvature observed for dynamic pedaling.

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Kreg G. Gruben, Lynn M. Rogers, Matthew W. Schmidt, and Liming Tan

The force that healthy humans generated against a fixed pedal was measured and compared with that predicted by four models. The participants (n = 11) were seated on a stationary bicycle and performed brief pushing efforts against an instrumented pedal with the crank fixed. Pushes were performed to 10 force magnitude targets and at 12 crank angles. The increasing magnitude portion of the sagittal-plane force path for each push effort was fitted with a line to determine the direction of the muscle component of the foot force. Those directions varied systematically with the position of the pedal (crank angle) such that the force path lines intersected a common region superior and slightly anterior to the hip. The ability of four models to predict force path direction was tested. All four models captured the general variation of direction with pedal position. Two of the models provided the best performance. One was a musculoskeletal model consisting of nine muscles with parameters adjusted to provide the best possible ft. The other model was derived from (a) observations that the lines-of-action of the muscle component of foot force tended to intersect in a common region near the hip, and (b) the corresponding need for foot force to intersect the center-of-mass during walking. Thus, this model predicted force direction at each pedal position as that of a line intersecting the pedal pivot and a common point located near the hip (divergent point). The results suggest that the control strategy employed in this seated pushing task reflects the extensive experience of the leg in directing force appropriately to maintain upright posture and that relative muscle strengths have adapted to that pattern of typical activation.

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Kreg G. Gruben, Citlali López-Ortiz, and Robert S. Giachetti

The forces acting within and upon a limb are derived from three sources: postural (gravitational), inertial, and muscular. A method for decomposition has been established for free limb movements (Hoy & Zernicke, 1986); however, that method does not apply to kinematically constrained tasks whereby the limb exerts force on the environment. Presented here is a method for calculating the muscular and postural components for a quasi-static limb during a kinematically constrained task. It is a modified form of the inverse dynamic method reported by Kautz and Hull (1993) combined with the technique of Gruben and López-Ortiz (2000). This method stabilizes the limb against gravity with moments at each joint of the limb. Data from quasi-static lower limb extension efforts in one individual were analyzed to compare predictions of our method with those of the Kautz and Hull (1993) method. Differences in the postural component of foot force between the two methods increased with knee extension. The novelty of the method presented here was the use of an experimentally derived direction for the muscle component of foot force and the inclusion of a physiologically-based criterion for determining the support of the limb against gravity.