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Research Papers

# Effect of Upper and Lower Extremity Control Strategies on Predicted Injury Risk During Simulated Forward Falls: A Study in Healthy Young Adults

[+] Author and Article Information
JiaHsuan Lo

Department of Biomedical Engineering, Biomechanics Research Laboratory, GGB 3208, University of Michigan, Ann Arbor, MI 48109-2125joshualo@umich.edu

James A. Ashton-Miller

Department of Biomedical Engineering, Biomechanics Research Laboratory, GGB 3208, University of Michigan, Ann Arbor, MI 48109-2125jaam@umich.edu

J Biomech Eng 130(4), 041015 (Jun 20, 2008) (8 pages) doi:10.1115/1.2947275 History: Received October 24, 2006; Revised May 15, 2008; Published June 20, 2008

## Abstract

Fall-related wrist fractures are common at any age. We used a seven-link, sagittally symmetric, biomechanical model to test the hypothesis that systematically alterations in the configuration of the body during a forward fall from standing height can significantly influence the impact force on the wrists. Movement of each joint was accomplished by a pair of agonist and antagonist joint muscle torque actuators with assigned torque-angle, torque-velocity, and neuromuscular latency properties. Proportional-derivative joint controllers were used to achieve desired target body segment configurations in the pre- and∕or postground contact phases of the fall. Outcome measures included wrist impact forces and whole-body kinetic energy at impact in the best, and worst, case impact injury risk scenarios. The results showed that peak wrist impact force ranged from less than $1kN$ to more than $2.5kN$, reflecting a fourfold difference in whole-body kinetic energy at impact (from less than $40J$ to more than $160J$) over the range of precontact hip and knee joint angles used at impact. A reduction in the whole-body kinetic energy at impact was primarily associated with increasing negative work associated with hip flexion. Altering upper extremity configuration prior to impact significantly reduced the peak wrist impact force by up to 58% (from $919Nto2212N$). Increased peak wrist impact forces associated greater shoulder flexion and less elbow flexion. Increasing postcontact arm retraction can reduce the peak wrist impact force by 28% (from $1491Nto1078N$), but postcontact hip and knee rotations had a relatively small effect on the peak wrist impact force (8% reduction; from $1411Nto1303N$). In summary, the choice of the joint control strategy during a forward fall can significantly affect the risk of wrist injury. The most effective strategy was to increase the negative work during hip flexion in order to dissipate kinetic energy thereby reducing the loss in potential energy prior to first impact. Extended hip or elbow configurations should be avoided in order to reduce forearm impact forces.

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## Figures

Figure 1

Block diagram showing interactions among the components of the muscle joint model

Figure 2

Illustration showing the two phases in a forward fall. Secondary impacts (i.e., knee impact in this illustration) can only occur during the postcontact phase.

Figure 3

Contours of first-impact whole-body kinetic energy (in Joules). Horizontal and vertical axes represent the knee and hip flexion angles at the instant of first impact, respectively.

Figure 4

Contours of peak impact wrist (left) and knee (right) forces (in Newtons). The horizontal and vertical axes represent the knee and hip flexion angles at the instant of first impact, respectively.

Figure 5

Energy dissipation contours by knee joint work (left, in Joules) and hip joint work (right, in Joules). The horizontal and vertical axes represent the knee and hip flexion angles at the instant of first impact, respectively.

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