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Research Papers

Effect of Finger Posture on the Tendon Force Distribution Within the Finger Extensor Mechanism

[+] Author and Article Information
Sang Wook Lee1

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, 345 East Superior Street, Suite 1406, Chicago, IL 60611sanglee2@northwestern.eduDepartment of Biomedical Engineering, Illinois Institute of Technology, 10 West 32nd Street, Chicago, IL 60616sanglee2@northwestern.edu

Hua Chen

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, 345 East Superior Street, Suite 1406, Chicago, IL 60611Department of Biomedical Engineering, Illinois Institute of Technology, 10 West 32nd Street, Chicago, IL 60616

Joseph D. Towles

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, 345 East Superior Street, Suite 1406, Chicago, IL 60611

Derek G. Kamper

Sensory Motor Performance Program, Rehabilitation Institute of Chicago, 345 East Superior Street, Suite 1406, Chicago, IL 60611; Department of Biomedical Engineering, Illinois Institute of Technology, 10 West 32nd Street, Chicago, IL 60616

1

Corresponding author.

J Biomech Eng 130(5), 051014 (Sep 11, 2008) (9 pages) doi:10.1115/1.2978983 History: Received March 12, 2008; Revised August 13, 2008; Published September 11, 2008

Understanding the transformation of tendon forces into joint torques would greatly aid in the investigation of the complex temporal and spatial coordination of multiple muscles in finger movements. In this study, the effects of the finger posture on the tendon force transmission within the finger extensor apparatus were investigated. In five cadaver specimens, a constant force was applied sequentially to the two extrinsic extensor tendons in the index finger, extensor digitorum communis and extensor indicis proprius. The responses to this loading, i.e., fingertip force/moment and regional strains of the extensor apparatus, were measured and analyzed to estimate the tendon force transmission into the terminal and central slips of the extensor hood. Repeated measures analysis of variance revealed that the amount of tendon force transmitted to each tendon slip was significantly affected by finger posture, specifically by the interphalangeal (IP) joint angles (p<0.01). Tendon force transmitted to each of the tendon slips was found to decrease with the IP flexion. The main effect of the metacarpophalangeal (MCP) joint angle was not as consistent as the IP angle, but there was a strong interaction effect for which MCP flexion led to large decreases in the slip forces (>30%) when the IP joints were extended. The ratio of terminal slip force:central slip force remained relatively constant across postures at approximately 1.7:1. Force dissipation into surrounding structures was found to be largely responsible for the observed force-posture relationship. Due to the significance of posture in the force transmission to the tendon slips, the impact of finger posture should be carefully considered when studying finger motor control or examining injury mechanisms in the extensor apparatus.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Marker placement on the finger extensor apparatus for the strain estimation (Specimen No. 5). Small squares around markers denote their locations as recognized by the motion capture system. Two subsets of markers, STS and SCS, were employed to estimate the longitudinal strain values at the terminal and central slips, respectively.

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Figure 2

Experimental setup for the measurement of fingertip force/moment and strains in the cadaveric specimen

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Figure 3

Representative plot of fingertip force and moment vectors in response to the EDC tendon force measured in nine postures (Specimen No. 1: right hand): (a) fingertip force and (b) fingertip moment. The force and moment vectors are projected onto the sagittal plane. The effects of the IP posture on the fingertip force and moment vectors appear to be more significant than that of the MCP posture.

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Figure 4

Mean and standard deviation values of the estimated terminal and central slip force magnitudes across all specimens: (a) EDC and (b) EIP. Magnitudes of both slip forces decreased with the IP flexion.

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Figure 5

Strain measurement. (a) Measured marker movements under EDC tendon loading (Specimen No. 2, Posture 1: all joint angles=0 deg, tendon: EDC, tendon force (fT)=11.8 N). (b) Longitudinal and (c) lateral strain distributions at maximum loading condition (fT) estimated from the recorded marker movements.

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Figure 6

Mean and standard deviation values of the estimated terminal and central slip strain values across all specimens: (a) EDC and (b) EIP. As observed in the calculated slip forces, the magnitudes of the both slip forces decreased with the IP flexion.

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Figure 7

Free-body diagram of the four-segment finger model. Only extensor tendon force is applied in this model. All force and directional vectors are projected to the sagittal plane.

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