Biomechanical Efficiency of Wrist Guards as a Shock Isolator

[+] Author and Article Information
Il-Kyu Hwang

Department of Factory Automation, Dongyang Technical College, Seoul, Korea

Kyu-Jung Kim1

Mechanical Engineering Department, California State Polytechnic University–Pomona, Pomona, CA 91768kyujungkim@csupomona.edu

Kenton R. Kaufman, William P. Cooney, Kai-Nan An

Orthopedic Biomechanics Laboratory, Mayo Clinic/Mayo Foundation, Rochester, MN 55905


Corresponding author.

J Biomech Eng 128(2), 229-234 (Oct 19, 2005) (6 pages) doi:10.1115/1.2165695 History: Received October 10, 2002; Revised October 19, 2005

Despite the use of wrist guards during skate- and snowboard activities, fractures still occur at the wrist or at further proximal locations of the forearm. The main objectives of this study were to conduct a human subject testing under simulated falling conditions for measurement of the impact force on the hand, to model wrist guards as a shock isolator, to construct a linear mass-spring-damper model for quantification of the impact force attenuation (Q-ratio) and energy absorption (S-ratio), and to determine whether wrist guards play a role of an efficient shock isolator. While the falling direction (forward and backward) significantly influenced the impact responses, use of wrist guards provided minimal improvements in the Q- and S-ratios. It was suggested based on the results under the submaximal loading conditions that protective functions of the common wrist guard design could be enhanced with substantial increase in the damping ratio so as to maximize the energy absorption. This would bring forth minor deterioration in the impact force attenuation but significant increase in the energy absorption by 19%, which would help better protection against fall-related injuries of the upper extremity.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

A cable-released fall testing setup. The subject wearing a safety harness leaned forward or backward with hands at sides into the lean control cable at 10deg from the vertical. After a random time delay the cable was released, and the subject bimanually arrested the fall onto the wall-mounted force plate.

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Figure 2

Wrist guards used for testing, having two semirigid nylon splints on the dorsal and volar sides of the hand (dosal splint not shown) and with Velcrose wrapping around the wrist

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Figure 3

(a) Bimodal shape of the measured ground reaction force on the hand during fall arrests (solid), having two force components—impact (dotted) and braking (dashed) and showing two characteristic peaks, impact force peak (Fimp) and braking force peak (Fbrk) for each force component, respectively. (b) Power spectrum of the measured ground reaction force using Welch’s averaged, modified periodogram method with Hanning window, demonstrating the bimodality as well.

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Figure 4

A Voigt solid model of the impact force component subject to an impulsive loading Foδ(t). (m represents the effective mass of the whole body, while c and k the damping coefficient and spring constant of the upper extremity alone or combined with wrist guards, respectively.)

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Figure 5

Theoretical Q-ratio (broken) and reciprocal of S-ratio (solid) as a function of damping ratio from Eqs. 4,5. The shaded area denotes the 95% confidence interval of the damping ratio for all the trials with wrist guards (0.09–0.32, mean=0.20).

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Figure 6

Comparisons between the measured (dashed) and simulated impact force components (solid) using a Voigt solid vibration model for a subject with body mass of 89kg. (a) A forward fall without wrist guards (damped natural frequency ωd=87.9rad∕s, damping ratio ζ=0.20, initial impulse Fo=0.059Ns∕BW; correlation coefficient ρ=0.92). (b) A forward fall with wrist guards (ωd=80.1rad∕s, ζ=0.23, Fo=0.078Ns∕BW; ρ=0.92). (c) A backward fall without wrist guards (ωd=78.5rad∕s, ζ=0.24, Fo=0.038Ns∕BW; ρ=0.88). (d) A backward fall with wrist guards (ωd=103.0rad∕s, ζ=0.21, Fo=0.032Ns∕BW; ρ=0.91). The correlation coefficients between the two curves were estimated over the time between 0–400ms.

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Figure 7

Maximum force transmission ratio (Q-ratio) for all the subjects. Over 86% of the Q-ratios obtained using the estimated damping ratios are distributed to within one standard deviation along the theoretical curve (broken line) using Eq. 4.




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