Elsevier

Journal of Biomechanics

Volume 31, Issue 1, 14 November 1997, Pages 1-9
Journal of Biomechanics

Common protective movements govern unexpected falls from standing height

https://doi.org/10.1016/S0021-9290(97)00114-0Get rights and content

Abstract

Simple energy considerations suggest that any fall from standing height has the potential to cause hip fracture. However, only 1–2% of falls among the elderly actually result in hip fracture, and less than 10% cause serious injury. This suggests that highly effective movement strategies exist for preventing injury during a fall. To determine the nature of these, we measured body segment movements as subjects (aged 22–35 yr) stood upon a gymnasium mattress and attempted to prevent themselves from falling after the mattress was made to translate abruptly. Subjects were more than twice as likely to fall after anterior translations of the feet, when compared to posterior or lateral translations. In falls which resulted in impact to the pelvis, a complex sequence of upper extremity movements allowed subjects to impact their wrist at nearly the same instant as the pelvis (average time interval between contacts=38 ms), suggesting a sharing of contact energy between the two body parts. Finally, marked trunk rotation was exhibited in falls due to lateral (but not anterior or posterior) perturbations, resulting in the avoidance of impact to the lateral aspect of the hip. These results suggest that body segment movements during falls, rather than being random and unpredictable, involve a repeatable series of responses which facilitate safe landing.

Introduction

Falls are a major source of death and morbidity among the elderly, including 90% of hip fractures (Grisso et al., 1991; Spaite et al., 1990). In 1988, approximately 250,000 hip fractures occurred in the United States, at an estimated cost of $8.7 billion (Praemar et al., 1992). Given the aging of the population and the fact that hip fracture incidence increases exponentially with age, some have projected a growth in these numbers to over 600,000 annual cases by the year 2050 (Cooper et al., 1992).

Considerable evidence suggests that hip fracture risk depends at least as strongly on the mechanics of the fall as it does on the bone strength of the faller. Of particular importance is the occurrence of impact to the hip. For example, Nevitt and Cummings (1993)found in a case-control study of elderly fallers that impacting on or near the hip increased fracture risk by over 30-fold. Similarly, Greenspan et al. (1994)found that fracture risk increased nearly 6-fold by falling sideways, which has been associated with impact to the hip (Hayes et al., 1993). In comparison, a decline in femoral bone density of one-standard deviation increased fracture risk 2 to 3-fold. Biomechanical considerations also suggest that fall mechanics influence fracture risk, since during a fall from standing, both the energy available (Lotz and Hayes, 1990) and force generated during impact to the hip (Robinovitch et al., 1991) exceed values required to fracture young or elderly cadaveric femora (Courtney et al., 1994).

The question then arises why hip fractures occur so rarely among young individuals (even among athletes who regularly fall onto hard surfaces), and in only 1–2% of falls in the elderly (Gryfe et al., 1977; Tinetti et al., 1988)? Furthermore, why do fall-related wrist fractures far outnumber hip fractures in the young, while this trend is reversed in the very old (Owen et al., 1982)? Cummings and Nevitt (1989)speculated that the answers to such questions lie in the existence of characteristic protective responses which allow for ‘safe landing’ during a fall. Better understanding of such responses, and how they change with age, should enhance our ability to identify at-risk individuals and design effective hip fracture prevention strategies (such as strength and balance conditioning, the design of safe living spaces, or selection of fall-resistant footwear).

However, few experimental studies have examined movement strategies during falling, and these have focused on self-initiated rather than unexpected falls. For example, van den Kroonenberg et al. (1996)measured body movements after instructing subjects ‘to launch themselves and subsequently to fall as naturally as possible’ onto a gymnasium mattress, and Henderson et al. (1993)examined body movements during parachutist landings. Given that the subjects’ task in these studies was to execute falls, observed motions were likely governed by motor plans selected well before fall initiation, chosen to satisfy both the experimental instructions and desire for safe landing. Considerably different movement strategies may arise during an un- expected loss-of-balance (e.g. slip or trip), where the sudden nature of the event leaves little time to ponder alternative landing strategies, and the initial focus is likely to prevent a fall rather than land safely.

In the present study, our aim was to assess whether, in the event of an unexpected slip, young individuals utilize common movement strategies to both prevent falls and achieve safe landings during a fall. Three hypotheses were tested, each of which proposes specific biomechanical mechanisms underlying the low incidence of fall-related hip fractures among the young. First, based on evidence that falling sideways increases hip fracture risk, we hypothesized that young subjects are less likely to fall after sideways perturbations to balance than after forward or backward perturbations. Second, based on the higher frequency of fall-related upper extremity fractures than hip fractures in the young, we hypothesized that when falls do occur, young subjects impact their wrists before their pelvis. Finally, based on evidence that hip fracture risk increases dramatically when contact occurs to the hip region during a fall, we hypothesized that in the event of a fall, regardless of the direction of the perturbation, young subjects avoid impact to the lateral aspect of the hip.

Section snippets

Subjects

Three males and three females participated (ranging in age from 22 to 35 yr, body mass from 54 to 90 kg, and height from 152 to 177 cm), none of which had a history of impaired balance, unexplained falls, neurological disease, or uncorrected visual deficit. None had training in balance or safe falling techniques (e.g. martial arts), but all claimed to exercise regularly (>3 h/week). The protocol was approved by the Committee on Human Research of the University of California, San Francisco, and

Results

Perturbation direction strongly influenced subjects’ ability to recover balance, with all subjects being more effective at recovering balance after anterior and lateral perturbations, in comparison to posterior perturbations (Fig. 3a). Grouping all trials together, fall avoidance was observed in 78% of anterior and 72% of lateral trials, but only 37% of posterior trials (Fig. 3bFig. 3cFig. 3d). Stepping was the predominant balance recovery technique for all subjects: only one trial involved

Discussion

While previous studies have linked hip fracture to sideways falls (Greenspan et al., 1994; Nevitt and Cummings, 1993), few falls actually cause hip fracture. We hypothesized that, at least among young individuals, this is in part explained by a reduced likelihood for falling when perturbations are applied in the lateral, as opposed to anterior or posterior directions. We found that all subjects had lowest stability during posteriorly directed perturbations, being, on average, about half as

Acknowledgements

This study was supported by grants from the Whitaker Foundation and the Academic Senate of the University of California, San Francisco. We wish to express our gratitude to Michel Kearns, Vlad Frenk, and Qi Liu for assistance in data analysis.

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