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Research Papers

# Controlling Propulsive Forces in Gait Initiation in Transfemoral Amputees

[+] Author and Article Information
Helco G. van Keeken, Bert Otten

Center for Human Movement Sciences,  University of Groningen, A. Deusinglaan 1, 9713 AV Groningen, The Netherlands

Aline H. Vrieling, Jan P. Halbertsma, Tanneke Schoppen, Klaas Postema

Center for Rehabilitation, University Medical Center Groningen, University of Groningen, P.O. Box 30001, 9700 RB Groningen, The Netherlands

At L. Hof

Center for Human Movement Sciences, University of Groningen, A. Deusinglaan 1, 9713 AV Groningen, The Netherlands; Center for Rehabilitation, University Medical Center Groningen, University of Groningen, P.O. Box 30001, 9700 RB Groningen, The Netherlands

J Biomech Eng 130(1), 011002 (Feb 05, 2008) (9 pages) doi:10.1115/1.2838028 History: Received June 29, 2006; Revised May 30, 2007; Published February 05, 2008

## Abstract

During prosthetic gait initiation, transfemoral (TF) amputees control the spatial and temporal parameters that modulate the propulsive forces, the positions of the center of pressure (CoP), and the center of mass (CoM). Whether their sound leg or the prosthetic leg is leading, the TF amputees reach the same end velocity. We wondered how the CoM velocity build up is influenced by the differences in propulsive components in the legs and how the trajectory of the CoP differs from the CoP trajectory in able bodied (AB) subjects. Seven TF subjects and eight AB subjects were tested on a force plate and on an $8m$ long walkway. On the force plate, they initiated gait two times with their sound leg and two times with their prosthetic leg. Force measurement data were used to calculate the CoM velocity curves in horizontal and vertical directions. Gait initiated on the walkway was used to determine the leg preference. We hypothesized that because of the differences in propulsive components, the motions of the CoP and the CoM have to be different, as ankle muscles are used to help generate horizontal ground reaction force components. Also, due to the absence of an active ankle function in the prosthetic leg, the vertical CoM velocity during gait initiation may be different when leading with the prosthetic leg compared to when leading with the sound leg. The data showed that whether the TF subjects initiated a gait with their prosthetic leg or with their sound leg, their horizontal end velocity was equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound leg, both when the prosthetic leg was leading and when the sound leg was leading. In the vertical CoM velocity, a tendency for differences between the two conditions was found. When initiating gait with the sound leg, the downward vertical CoM velocity at the end of the gait initiation was higher compared to when leading with the prosthetic leg. Our subjects used a gait initiation strategy that depended mainly on the active ankle function of the sound leg; therefore, they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase. Both legs were controlled in one single system of gait propulsion. The shape of the CoP trajectories, the applied forces, and the CoM velocity curves are described in this paper.

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Copyright © 2008 by American Society of Mechanical Engineers
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## Figures

Figure 1

Posterior displacement of the CoP creating a forward momentum

Figure 2

Schematic overview of typical CoMy and CoMz velocities, the forces (GRFy, GRFz), and the CoPy motion during gait initiation of TF subjects leading with their sound leg or the prosthetic leg and AB subjects. The stick figures show five sub-phases of gait initiation: (1) the start of the Apa phase, (2) the middle of the Apa phase, (3) the Apa-Exe phase transition, (4) the middle of the Exe phase, and (5) the end of the Exe phase. The CoP position during these sub-phases is represented by a triangle. The stick figures are based on video images. Two typical GRFy curves were found in the TF sound leg leading group. These curves are represented by the solid and the dotted line.

Figure 3

Typical time courses of GRFz and GRFy, CoPy motion, and the resulting velocities of typical TF subjects ((a) sound leg leading; (b) prosthesis leg leading) and AB subjects (c). The time course is divided in an Apa phase and an Exe phase. The start of the Apa phase is defined as the moment when GRFy is bigger than 1% of the body weight in Newton. The start of the Exe phase is defined as the moment when the CoPx reaches the highest velocity when moving from the leading leg toward the trailing leg (not in the figure). The end of the Exe phase is defined as the moment when GRFz is at its maximum, before the leading swinging leg touches the floor (heel strike). The end velocities at the moment the leading leg touches the floor are marked with the dashed lines. To fit GRFz in the figure, a scaling factor of 0.1 is used.

Figure 4

Deviator from the TF amputee group, prosthetic leg (a) leading, and the AB group (b). The time course of GRFz and GRFy, CoPy motion, and the resulting velocities of the deviating subjects, divided into an Apa phase and an Exe phase. To fit GRFz in the figure, a scaling factor of 0.1 is used.

Figure 5

Schematic overview of typical CoMy and CoMz velocities, the forces (GRFy; GRFz), and the CoPy motion during gait initiation of TF amputee deviators leading with their prosthetic leg and AB deviators. The stick figures show five sub-phases of gait initiation: (1) the start of the Apa phase, (2) the middle of the Apa phase, (3) the Apa-Exe phase transition, (4) the middle of the Exe phase, and (5) the end of the Exe phase. The CoP position during these sub-phases is represented by a triangle. The stick figures are based on video images.

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