Biomechanical simulations of forward fall arrests: effects of upper extremity arrest strategy, gender and aging-related declines in muscle strength

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Abstract

Computer simulation was used to predict the extent to which age-related muscle atrophy may adversely affect the safe arrest of a forward fall onto the arms. The biomechanical factors affecting the separate risks for wrist fracture or head impact were examined using a two-dimensional, 5-link, forward dynamic model. The hypothesis was tested in older females that age-related loss in muscular strength renders the use of the arms ineffective in arresting a forward fall without either a torso impact exceeding 0.5 m/s or distal forearm loads sufficient to fracture the wrist. The results demonstrate that typical age-related decline in arm muscle strength substantially reduces the ability to arrest a forward fall without the elbows buckling and, therefore, a risk of torso and/or head impact. The model predicted that older women with below-average bone strength risk a Colles fracture when arresting typical falls, particularly with an extended arm.

Introduction

Fall-related injuries are likely to cost this country $85 billion by the year 2020 (Englander et al., 1996). In older adults over 65 yr 60% of falls occur in a forward direction (O’Neill et al., 1994). A common strategy to arrest such falls is to use one or both upper extremities to protect the head and trunk (Hsiao and Robinovitch, 1998). Epidemiological evidence that the arms do bear the brunt of the impact comes from the fact that between 45% to 85% of distal forearm fractures are the result of falls (Oskam et al., 1998). Moreover, in those over 45 yr of age, forearm fractures cost the US approximately $385 million in total direct medical costs in 1995 (Ray et al., 1997). Clearly, insights that reduce these injuries can have significant socioeconomic ramifications.

With advancing age, healthy striated muscle exhibits an age-related atrophy and loss of strength (Evans, 1995). Based upon the inverse correlation between triceps strength and hip fracture risk, it has been postulated that one factor that may increase the risk of hip fracture is the ineffective use of the arm(s) to arrest a fall (Nevitt and Cummings, 1993). Further evidence for ineffective arm use is that the head is a frequent site of major injury in elderly fallers (Luukinen et al., 1995). Our primary objective in this study was to test the hypothesis that loss in upper extremity strength can preclude the safe arrest of a forward fall to the ground. An unsafe arrest was defined to be one in which either the torso impacted the ground with a velocity above a set threshold or the load applied to the distal forearm exceeded a minimum limit.

We have studied the ability of young subjects to volitionally reduce the peak impact force applied to their distal forearm during mild falls (DeGoede and Ashton-Miller, 2002). The conduct of such experiments in older subjects, however, may pose an unacceptable level of risk, especially in frail elderly. Through the use of a biomechanical model, validated using the above-mentioned experimental data, we explore in this paper the strength requirement necessary to arrest a fall with the arms and the peak force applied to the distal forearm.

Section snippets

Methods

A half-body, sagittally symmetric, 5-link (leg, torso (including head and neck), upper arm, forearm, and hand) closed-chain dynamic model of a human arresting a forward fall with the arms (Fig. 1) was implemented using ADAMS® dynamic simulation software (Mechanical Dynamics Incorporated). The fingertips and the ankle were pinned to the ground with fixed-axis, frictionless, pin joints. The wrist, elbow, and hip joints were also assumed to be frictionless planar pin joints between the respective

Results

Comparison of the simulated and measured time histories of the GRF during a fall arrest with given initial impact conditions showed reasonable agreement (Fig. 2a and b), especially for F1 and the initial time history of the elbow angle. Moreover, this agreement persisted over all five test conditions as long as parameter values were altered to reflect the behavioral differences observed in each test condition (Table 3). For reference, the deflection of the center of rotation of the wrist into

Discussion

The results support the primary hypothesis that an age-related reduction in upper-extremity extensor strength can significantly reduce the ability of older women to use their arms to safely arrest a fall. There are two reasons for this. Firstly, the results show that inadequate arm extensor muscle strength can lead the slightly flexed elbow to buckle under the force of hand impact, thereby raising the specter of rib fracture or head injury. When arm muscle strength levels were reduced below 70%

Acknowledgements

We are grateful for the support of PHS grants P01 AG10542 and P30 AG 08808.

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    Present address. Elizabethtown College, Elizabethtown, PA.

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