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Research Papers

Variable Thumb Moment Arm Modeling and Thumb-Tip Force Production of a Human-Like Robotic Hand

[+] Author and Article Information
Taylor D. Niehues

Department of Mechanical Engineering,
The University of Texas at Austin,
Austin, TX 78712
e-mail: taylor.niehues@utexas.edu

Ashish D. Deshpande

Mem. ASME
Department of Mechanical Engineering,
The University of Texas at Austin,
Austin, TX 78712
e-mail: ashish@austin.utexas.edu

Manuscript received November 14, 2016; final manuscript received July 12, 2017; published online August 16, 2017. Assoc. Editor: Zong-Ming Li.

J Biomech Eng 139(10), 101005 (Aug 16, 2017) (6 pages) Paper No: BIO-16-1450; doi: 10.1115/1.4037402 History: Received November 14, 2016; Revised July 12, 2017

The anatomically correct testbed (ACT) hand mechanically simulates the musculoskeletal structure of the fingers and thumb of the human hand. In this work, we analyze the muscle moment arms (MAs) and thumb-tip force vectors in the ACT thumb in order to compare the ACT thumb's mechanical structure to the human thumb. Motion data are used to determine joint angle-dependent MA models, and thumb-tip three-dimensional (3D) force vectors are experimentally analyzed when forces are applied to individual muscles. Results are presented for both a nominal ACT thumb model designed to match human MAs and an adjusted model that more closely replicates human-like thumb-tip forces. The results confirm that the ACT thumb is capable of faithfully representing human musculoskeletal structure and muscle functionality. Using the ACT hand as a physical simulation platform allows us to gain a better understanding of the underlying biomechanical and neuromuscular properties of the human hand to ultimately inform the design and control of robotic and prosthetic hands.

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Figures

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Fig. 1

Musculoskeletal structure of the ACT thumb. (a) The joint axes for the ACT thumb's five anatomical degrees-of-freedom including flexion/extension and abduction/adduction of the CMC joint, flexion/extension and abduction/adduction of the metacarpophalangeal (MCP) joint, and flexion/extension of the interphalangeal (IP) joint. (b) Dorsal view of the ACT thumb showing the adductor pollicis longus (ADP), extensor pollicis longus (EPL), abductor pollicis longus (APL), extensor pollicis brevis (EPB) tendons. (c) Palmar view showing the flexor pollicis longus (FPL), flexor pollicis brevis (FPB), opponens (OPP), and abductor pollicis brevis (APB) tendons.

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Fig. 2

ACT thumb MAs for (a) CMC flexion/extension and (b) CMC abduction/adduction (ab/ad) for the nominal model designed to match human MAs (solid lines) and the adjusted model designed to match thumb-tip forces (dashed lines, where modification was necessary), compared with experimental human data collected by Smutz et al. [1] (dotted lines with error bars, mean ± 1SD). Positive angles and MAs for CMC ab/ad correspond to thumb adduction (toward the palm).

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Fig. 3

ACT thumb MAs for (a) MCP abduction/adduction (ab/ad), (b) MCP flexion/extension, and (c) IP flexion/extension for the nominal model designed to match human MAs (solid lines) and the adjusted model designed to match thumb-tip forces (dashed lines, where modification was necessary), compared with experimental human data collected by Smutz et al. [1] (dotted lines with error bars, mean ± 1SD). Positive angles and MAs for MCP ab/ad correspond to thumb adduction (toward the palm).

Grahic Jump Location
Fig. 4

Thumb-tip force data from the ACT thumb when forces are applied to each thumb muscle individually. All data are rotated to apply to a right hand. Muscle force values are identical to the maximum forces from cadaveric experiments in Ref. [2]. Results are shown for the nominal ACT thumb model, designed to match cadaveric MA measurements from Ref. [1], in (a) key pinch and (c) opposition pinch postures. For comparison, we show results from the adjusted model that was redesigned to better match thumb-tip forces from Ref. [2] in (b) key pinch and (d) opposition pinch postures. Solid lines represent experimental force vectors from the ACT thumb, and dashed arcs represent the magnitude (mean) and angle (mean ± SD) of cadaveric data reported by Pearlman et al. [2]. The force vector is said to show a good directional match if it falls within the corresponding dashed arc. Actual values are reported in Table 1.

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