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Research Papers

Optimization of Prosthetic Foot Stiffness to Reduce Metabolic Cost and Intact Knee Loading During Below-Knee Amputee Walking: A Theoretical Study

[+] Author and Article Information
Nicholas P. Fey

Department of Mechanical Engineering,  The University of Texas at Austin, Austin, TX 78712

Glenn K. Klute

 Department of Veterans Affairs, Puget Sound Health Care System, Seattle, WA 98108

Richard R. Neptune1

Department of Mechanical Engineering,  The University of Texas at Austin, Austin, TX 78712rneptune@mail.utexas.edu

1

Corresponding author.

J Biomech Eng 134(11), 111005 (Oct 26, 2012) (10 pages) doi:10.1115/1.4007824 History: Received April 18, 2012; Revised September 13, 2012; Posted October 10, 2012; Published October 26, 2012; Online October 26, 2012

Unilateral below-knee amputees develop abnormal gait characteristics that include bilateral asymmetries and an elevated metabolic cost relative to non-amputees. In addition, long-term prosthesis use has been linked to an increased prevalence of joint pain and osteoarthritis in the intact leg knee. To improve amputee mobility, prosthetic feet that utilize elastic energy storage and return (ESAR) have been designed, which perform important biomechanical functions such as providing body support and forward propulsion. However, the prescription of appropriate design characteristics (e.g., stiffness) is not well-defined since its influence on foot function and important in vivo biomechanical quantities such as metabolic cost and joint loading remain unclear. The design of feet that improve these quantities could provide considerable advancements in amputee care. Therefore, the purpose of this study was to couple design optimization with dynamic simulations of amputee walking to identify the optimal foot stiffness that minimizes metabolic cost and intact knee joint loading. A musculoskeletal model and distributed stiffness ESAR prosthetic foot model were developed to generate muscle-actuated forward dynamics simulations of amputee walking. Dynamic optimization was used to solve for the optimal muscle excitation patterns and foot stiffness profile that produced simulations that tracked experimental amputee walking data while minimizing metabolic cost and intact leg internal knee contact forces. Muscle and foot function were evaluated by calculating their contributions to the important walking subtasks of body support, forward propulsion and leg swing. The analyses showed that altering a nominal prosthetic foot stiffness distribution by stiffening the toe and mid-foot while making the ankle and heel less stiff improved ESAR foot performance by offloading the intact knee during early to mid-stance of the intact leg and reducing metabolic cost. The optimal design also provided moderate braking and body support during the first half of residual leg stance, while increasing the prosthesis contributions to forward propulsion and body support during the second half of residual leg stance. Future work will be directed at experimentally validating these results, which have important implications for future designs of prosthetic feet that could significantly improve amputee care.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 2

Schematic of the prosthetic foot model consisting of 22 rigid segments connected in series. The prosthetic foot model had 13 keel (KR1–KR13,

) and 5 heel (HR1–HR5, ) rotational degrees-of-freedom. Foot stiffness was modeled using viscoelastic elements at each rotational degree-of-freedom.

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Figure 3

Comparison of simulation excitation timing (plotted below each x-axis) with group average experimental EMG data (+1 SD) of amputee subjects walking with the SLS ESAR foot plotted with respect to the residual leg gait cycle (only those muscles in which EMG data were collected are shown)

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Figure 4

Intact and residual knee and hip contact forces for each walking simulation plotted with respect to the residual leg gait cycle. For each joint, forces are expressed in the distal segment reference frame and represent the force of the proximal segment on the distal segment. For example, the intact knee axial force is expressed in the intact leg tibia reference frame and represents the force of the intact femur on the intact tibia. Segment reference frames are defined to be positive in the vertical axial direction and positive in the anterior horizontal direction. The total body weight of the model was 715 N.

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Figure 5

Metabolic cost profiles of the intact and residual leg muscle groups with the largest contributions plotted with respect to the residual leg gait cycle. VASL and VASM values were combined (VAS). All others had minimal values.

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Figure 6

Mean contributions of the prosthetic keel and heel, muscles (residual and intact legs), and gravity to residual leg A/P and vertical ground reaction forces during the first (left column) and second (right column) halves of residual leg stance

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Figure 7

Mean residual leg muscle contributions to A/P and vertical GRFs of the residual leg. Data were averaged during the first (left column) and second (right column) halves of residual leg stance.

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Figure 8

Mean intact leg muscle contributions to A/P and vertical GRFs of the intact leg. Data were averaged during the first (left column) and second (right column) halves of intact leg stance. Contributions of VASL and VASM were combined (VAS) for presentation of these data.

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Figure 9

Mean prosthetic foot and residual leg muscle contributions to total power delivered to the residual leg. Data were averaged during the first (left column) and second (middle column) halves of residual leg stance and swing (right column).

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Figure 1

Amputee musculoskeletal model. The intact leg was actuated by 25 individual Hill-type musculotendon actuators, which were grouped into 14 muscle groups based on anatomical classification with muscles in each group receiving the same excitation pattern. The 14 muscle groups consisted of GMED (anterior and posterior compartments of the gluteus medius), GMAX (gluteus maximus, adductor magnus), HAM (biceps femoris long head, medial hamstrings), BFsh (biceps femoris short head), IL (psoas, iliacus), RF (rectus femoris), VASL (vastus lateralis, vastus intermedius), VASM (vastus medialis), GAS (medial and lateral gastrocnemius), SOL (soleus, tibialis posterior), TA (tibialis anterior, peroneus tertius), PR (peroneus longus, peroneus brevis), FLXDG (flexor digitorum longus, flexor hallucis longus), and EXTDG (extensor digitorum longus, extensor hallucis longus). To improve model visualization, the smaller muscle groups that actuated the foot (PR, FLXDG, and EXTDG) are not shown. In the amputee residual leg, the same muscle groups were included except for those crossing the ankle joint (GAS, SOL, TA, PR, FLXDG, and EXTDG).

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