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Common protective movements govern unexpected falls from standing height
Journal of Biomechanics 31 (1998) 1—9
Common protective movements govern unexpected falls from standing height E.T. Hsiao, S.N. Robinovitch* Biomechanics Laboratory, Department of Orthopaedic Surgery, San Francisco General Hospital, University of California, San Francisco, San Francisco, CA 94110, U.S.A. Received in final form 16 October 1997
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. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: Falls; Balance; Hip fracture; Wrist fracture; Protective responses
1. 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 casecontrol study of elderly fallers that impacting on or near the hip increased fracture risk by over 30-fold. Similarly,
* Corresponding author. 0021-9290/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII S0021-9290(97)00114-0
Greenspan and co-workers (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
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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 and co-workers (1996) measured body movements after instructing subjects ‘to launch themselves and subsequently to fall as naturally as possible’ onto a gymnasium mattress, and Henderson and co-workers (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 unexpected 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.
2. Materials and methods 2.1. 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 informed consent was provided by each subject. 2.2. Methods During the experiments, subjects stood barefoot on a large, vinyl-covered gymnasium mattress of the type used in athletic high jumping (dimensions 2.4 m] 1.2 m]0.3 m), with their feet shoulder-width apart and arms at their sides. Without warning, the mattress was then made to translate abruptly by means of a springactuated platform (Fig. 1). We consider such a perturbation to simulate a ‘slip,’ since it suddenly displaces the feet away from the body’s center of gravity. Throughout the testing session, we randomly varied (through the use of random number tables) both the direction of the perturbation (by having the subject stand forward, backward, or sideways to the perturbation direction), and the strength of the perturbation (4 acceleration levels, detailed in Table 1). To evoke ‘natural’ responses, no practice trials were allowed, and subjects were only instructed that (1) in the event of platform movement, they should ‘try to prevent themselves from falling,’ and (2) prior to platform movement, they should maintain their gaze directed forward and at eye level (wording in the subject consent form was ‘‘the platform will move2this motion may make me fall onto the gym mat, but I will try to
Fig. 1. Experimental setup for simulated slipping experiments. Subjects stood barefoot on a large gymnasium mattress, with their feet shoulderwidth apart and their arms at their sides. The mattress rested on a low-friction platform, which was held fixed at one end by an electromagnetic brake (model DCA-600-1101, Automatic Equipment Corp., Cincinnati, OH), and attached at the other end to a spring (of spring constant 1280 N m~1) held under tension. Without warning, the electromagnet was released manually by depressing a button on the brake controller (model MR-110-150, Magnetool, Inc., Troy, MI), causing horizontal translation of the platform. Throughout the sessions, we randomly varied both the strength of the perturbation (via the initial tension in the spring) and the direction of the perturbation (by having the subject stand anterior, posterior, or lateral to the direction of platform movement) Subjects were instructed to ‘try to prevent themselves from falling’.
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Table 1 Perturbation characteristics Spring deflection (cm)*
Peak horizontal acceleration (m s~2)!
Time to peak acceleration (ms)!
Peak horizontal velocity (cm s~1)
Time to peak velocity (ms)!
Displacement at time of peak velocity (cm)!
81
4.2$0)4 (3.6—4.9)
89$75 (16—199)
91$23 (53—109)
329$48 (250—383)
21$6 (13—31)
94
6.4$0.3 (5.0—6.9)
50$18 (16—83)
163$19 (125—182)
342$33 (266—383)
34$5 (28—43)
107
8.6$0.4 (8.0—9.1)
47$15 (33—67)
226$14 (202—243)
354$72 (283—533)
51$12 (51—83)
113
9.7$0.5 (8.9—10.4)
59$16 (33—83)
250$16 (226—276)
342$27 (300—383)
52$4 (46—60)
*Spring defleciton is stretch of the spring before release of the electromagnetic brake. !Table cells provide parameter values associated with spring deflections shown on the far left of each row. The first line in each cell displays the mean value of the parameter over all trials$one standard deviation, while the second line provides the observed range.
prevent myself from falling’’). We will use the term ‘posterior-directed perturbation,’ or simply ‘posterior trial,’ to refer to trials in which the platform and the feet initially moved anterior to the trunk (since, in the absence of protective responses, these would cause backwards falls), the term ‘anterior-directed perturbation’ to refer to trials in which the platform moved posterior to the trunk, and the term ‘lateral-directed perturbation’ to refer to trials in which the platform moved to the subject’s left. Four of the six subjects were randomly cycled twice through the 12 different perturbation conditions (3 directions]4 accelerations), for a total of 24 measurements. Subjects M2 and M3 underwent only 20 and 21 trials, respectively, since repeat trials were not conducted at 9.7 m/s2 in any direction (and, for M3, at 8.6 m/s2 in the posterior direction). The reason for this was concern that the subject might step off the mattress and suffer injury. Two additional trials, involving posterior perturbations to F3, were lost due to software malfunction. The total number of trials was therefore 46 in the anterior direction, 46 in the lateral direction, and 43 in the posterior direction. In each trial, a 6-camera, 60 H motion measurement 3 system (MacReflex, Qualisys Inc., Glastonbury, CT) was used to acquire the 3-dimensional positions of 20 (soft foam) markers located at the third metatarsals, lateral malleoli, midpoint of the shins, lateral femoral epicondyles, midpoint of the thighs, anterior superior iliac spines (ASIS), junction between vertebrae L5 and S1, acromium processes, lateral radial epicondyles, dorsal aspect of the wrists, and top of the head. Data acquisition began approximately 1 s before the onset of the perturbation, and lasted 4 s. Music was played to minimize aural cues of brake release.
2.3. Data analysis Based on inspection of data by two independent investigators, trials were categorized in terms of the following balance recovery (Maki et al., 1996; Wolfson et al., 1986) or falling strategies: (1) balance recovery through sway; (2) balance recovery through a single step; (3) balance recovery through two or more steps; (4) ‘partial fall’, involving contact to one or both knees and/or hands, but not trunk or pelvis; and (5) ‘complete fall’, involving contact to the trunk and/or pelvis. The latter two categories separate falls into those that might involve risk for hip fracture (complete falls) from those that would not (partial falls). Complete falls were further analyzed to determine the time of contact and vertical impact velocity (an index of contact severity) of wrist and pelvic markers. To derive these parameters, custom routines (MATLAB, The MathWorks, Natick, MA) were used to filter (recursive low-pass Butterworth, 6 Hz cut-off justified by power spectral analysis) and differentiate position data. Ground contact to a given body part was defined by passage of the corresponding body marker below a horizontal plane located 100 mm above the gym mattress (based on a sequence of static measures of subjects lying on a rigid platform). Pelvic orientations at contact were described by three-dimensional (Cardan) rotations occurring between the onset of the perturbation and the instant of pelvic contact (Fig. 2). In five posterior and one lateral complete fall, the pelvic marker closest to the mattress was lost from camera view before reaching a level we defined as contact; in these cases pelvic rotations could not be calculated, and contact velocities were based on the next-lowest pelvic marker.
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Fig. 2. Convention for defining pelvis rotations. The pelvis is defined by the nearly transverse plane formed by the left anterior superior iliac spine (L), right anterior superior iliac spine (R), and sacral marker at the junction between the L5 and S1 vertebrae (S). The origin of a body-fixed reference frame uvw is at S. Successive (Cardan) rotations about the w, v, and u axes define anterior—posterior pelvic tilt, mediolateral pelvic tilt, and pelvic rotation, respectively. Angles are calculated between the onset of the platform perturbation (where all angles have zero magnitude), and the moment of pelvic contact (in ‘fall’ trials) or wrist contact (in ‘partial fall’ trials).
3. 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. 3b—d). Stepping was the predominant balance recovery technique for all subjects: only one trial involved stabilization of posture through sway. In all but one fall trial, a failed attempt at balance recovery by stepping was observed, with the foot striking the mattress at an average time of 454$86 (S.D.) ms after perturbation onset (Fig. 4). Perturbation direction also influenced fall type (Fig. 3b—d): of the 27 posterior falls, 25 of these (92% of cases) were complete falls involving pelvic impact, while of the 13 lateral falls, only three (23%) involved pelvic impact. None of the 10 anterior falls involved pelvic impact. Wrist contact was observed in all complete and partial falls. In complete falls, impact was nearly as likely to occur to the pelvis before the wrist (43% of trials) as vice versa. However, the time difference between wrist and pelvic impacts was typically small, equaling less than 50 ms in 72% of falls (Figs. 5a and c). On average, pelvic contact occurred 715$160 (S.D.) ms after the onset of mat movement, while wrist contact occurred at 680$116 ms, or 38$97 ms (range: !66 to 317 ms) earlier (Fig. 5a). A consistent sequence of upper extremity movements appeared central in allowing this nearsimultaneous sequence of collisions. This involved an immediate upward movement of the wrist (perhaps reflecting a startle response), followed by a rapid downward movement, and a second upward acceleration just prior to impact (Fig. 6). This last deceleration substan-
Fig. 3. Balance recovery and falling responses. (a) For all subjects (labeled ‘F1’—‘M3’), falls were more common after posterior perturbations (labeled ‘P’) than after anterior or lateral perturbations (labeled ‘A’ and ‘L’). In the event of an anterior or lateral perturbation, loss-ofbalance was more likely to lead to a ‘partial fall’ (defined as contact to one or both knees and/or wrists) than a ‘complete fall’ (defined as contact to the pelvis and/or trunk). The reverse was true for posterior perturbations. (b)—(d) Of the total of 176 trials, 77% involved balance recovery with one or more steps. With increasing perturbation strength, the frequency of single-step recoveries declined, and fall incidence increased.
tially decreased wrist contact velocity, which averaged !2.64$0.66 m s~1 (Fig. 5b), or 66$19% of its peak downward velocity during descent. Impact to both wrists was observed in 26 of the 28 complete falls, with an average interval of 69.8$62.3 ms between contacts (range: 0—233 ms). Head impact was observed in only five falls, three of which involved the same subject. In contrast to the upper extremities, the pelvis typically displayed steady downward movement during complete falls (Fig. 6), contacting with an average velocity of !2.55$0.85 m s~1 (Fig. 5b), or 83$21% of its peak downward velocity during descent. A weak but significant correlation was observed between pelvic contact velocity and the time interval between wrist and pelvic contacts (Fig. 5c): as this interval increased, the contact velocity of the pelvis decreased (r2"0.29; p"0.003). This moderate dependency suggests that mechanisms other than wrist impact influenced pelvic contact velocity, such as initial impact to the knee and/or thigh (commonly observed), and energy absorption in lower extremity muscles during the descent stage of the fall (e.g. eccentric contraction of quadriceps).
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Fig. 4. Stick-figure (oblique view) image of a typical complete fall from a posterior perturbation (subject: M1). Note the failed attempt to recover balance by stepping at approximately t"0.45 s, the initial upward and then downward movement of the upper extremity, the small degree of trunk rotation during descent, and the near-simultaneous impact to the wrist and pelvis at approximately t"0.75 s.
In complete falls resulting from laterally-directed perturbations, subjects clearly avoided impact to the lateral aspect of the hip. This is best observed by the magnitudes of pelvic rotation during descent about the local w axis (Table 2), which reflect trunk rotation about an inferiorsuperior axis. In a posterior fall, a value of 90° in this parameter would reflect hip impact, while in a lateral fall, a value of zero degrees would reflect hip impact. Average (absolute) values among the 20 posterior falls involving no pelvic marker dropout were 16.3°$12.3° (SD) (range: 0°—41°). In contrast, values for the two lateral falls not involving dropout were 62° and 37°. Review of stickfigure animations for these trials revealed that trunk rotation commenced late in the descent stage of the fall, resulting in impact with the body anteriorly facing the contact surface (Fig. 7). This, in turn, allowed subjects to contact the impact surface with both right and left upper extremities, and avoid impact to the lateral aspect of the hip. To supplement the small number of lateral complete fall trials, we also examined partial falls resulting from lateral perturbations (nine analyzed; one unavailable due to marker drop-out). Large rotations were again observed, averaging 44.6°$15.3° (range: 18°—69°) between the onset of the perturbation and the time of wrist contact.
4. Discussion While previous studies have linked hip fracture to sideways falls (Greenspan et al., 1994; Nevitt and Cum-
mings, 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 likely to fall during lateral perturbations and one-third as likely to fall during anterior perturbations. If a random distribution of perturbation directions arose during everyday activities, our results suggest that falls in the posterior direction (and their related injuries) should be most common. This result, however, contrasts with the finding by Nevitt and Cummings (1993) that falls in the elderly most often occur in an anterior direction; a discrepancy likely due to the fact that about 50% of falls they examined occurred during walking (an activity that future fall mechanics studies should incorporate). Previous multidirectional balance studies have not identified a directional dependency to fall susceptibility, since postural limits have not been stressed to the point that falls ensue. However, our observations represent reasonable extensions of previous work showing a lower sway-to-stepping threshold for posterior as opposed to anterior-directed perturbations (Maki et al., 1996). While we cannot, at present, explain why perturbation direction influences stability, we suspect it relates to an inability to visualize the environment posterior to the body, or achieve large step sizes during posterior stepping (due, for example to limits on hip extension).
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Fig. 6. Temporal variation in the position (top panel) and velocity (bottom panel, where negative velocity implies downward movement) of wrist and pelvic markers during a typical fall due to a posterior perturbation. Mat movement begins at t"0 s. The pelvis moves steadily downward, impacting at t"0.79 s with a velocity of !2.8 m s~1 (p). The wrist, in contrast, undergoes two reversals in the direction of downward acceleration before impacting at t"0.70 s, with a velocity of !2.0 m s~1 (w).
Fig. 5. Descent and impact kinematics for the 28 trials resulting in complete falls (impact to the trunk and/or pelvis). (a) Descent times, defined as the interval between onset of the perturbation (brake release) and the moment the pelvis (open circles) or wrist (filled circles) impacted the ground. Lines join data points from a given trial. Subjects exhibited similar ranges in descent times, averaging 715$16 ms for pelvic contact, and 680$116 ms for wrist contact. Subjects were nearly as likely to impact their pelvis before their wrist (43% of trials) as vice-versa. However, the time interval between pelvic and wrist impact was less than 50 ms in 72% of trials, suggesting a sharing of contact energy between the pelvis and upper extremity. (b) Contact velocities, defined as the vertical velocity of the pelvis (open circles) or wrist (filled circles) at the instant the body part impacted the ground. Negative velocities reflect downward movement. Lines join data points from a given trial. Subjects exhibited similar ranges in contact velocities, averaging !2.64$0.66 m s~1 for the wrist and !2.55$0.85 m s~1 for the pelvis. (c) A weak but significant association was observed between the time interval between pelvic and wrist impacts and the contact velocity of the pelvis (r2"0.29).
We also hypothesized that, in the event of a fall, young individuals contact the ground with the upper extremity before the trunk or pelvis. We found that, while wrist impact occurred during all falls, perturbation direction affected its functional significance. In particular, during lateral and anterior trials, wrist impact allowed for near-complete avoidance of pelvic and trunk impact (which, almost certainly, would have otherwise occurred).
In contrast, during posterior perturbations, impact to the pelvis was common, and subjects were nearly as likely to first impact their pelvis as they were their wrist. However, perhaps of greater significance than impact order is the time difference between contacts, which was less than 50 ms in nearly 75% of the cases. Since previous studies indicate that approximately 50 ms is required to reach peak force after contact to either the hip (Robinovitch et al., 1991) or wrist (Chiu and Robinovitch, 1996; Kim et al., 1997) during a fall, impact configurations during posterior falls are apparently chosen to allow sharing of impact energy between the upper extremities and pelvis. Indeed, it may be that injury risk is greatest for falls in which this time interval exceeds 50 ms (i.e. data points outside the gray band in Fig. 5c). In coordinating nearsimultaneous contact between wrist and pelvis, a chief control parameter appears to be wrist position, which (in contrast to the steady downward movement of the pelvis), undergoes two reversals in downward acceleration during descent, the latter of which allows not only for near-simultaneous pelvic and wrist impact, but also for a reduction in the contact energy of the upper extremity, and the related risk for wrist fracture. Finally, we hypothesized that young individuals avoid impact to the lateral aspect of the hip during a fall, thereby further reducing hip fracture risk. Our major observation in this regard was the occurrence of substantial trunk rotation during the descent phase of lateral (but not anterior or posterior) falls. This movement, in
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Table 2 Pelvic orientations at impact Subject
Perturbation type
w rotation*
v rotation*
M1 F2 F2 F2 F2 F2 F2 F3 F3 F3 F3 F3 F1 F1 F1 F1 F1 F1 M3 M3 F2 F3 M1 F2 F2 F2 F3 F3 F3 F3 F3
Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Lateral Lateral Lateral (partial) Lateral (partial) Lateral (partial Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial)
15.000 1.9000 10.900 1.8000 4.0000 4.8000 !19.800 !40.900 !29.600 !35.500 !31.100 !35.900 !15.500 10.300 0.0000 13.100 10.400 15.300 11.700 !17.800 !61.600 !37.100 !31.900 !17.700 !60.600 !69.400 !47.900 !39.900 !50.100 !37.200 !46.900
43.200 34.600 10.200 33.900 32.600 24.700 10.800 3.0000 23.100 9.6000 11.400 !2.5000 20.200 18.800 23.600 16.200 29.000 19.700 45.400 47.400 !53.300 !15.500 !35.600 !14.500 1.0700 !74.700 !32.300 !36.300 !26.100 3.3000 !35.900
u rotation* 20.000 2.5000 20.500 1.4000 14.400 10.500 !10.300 !76.200 !41.20 !76.400 51.200 !66.600 !26.700 21.100 13.400 13.100 1.4500 35.200 28.100 22.100 17.200 !4.9000 !5.8000 !23.000 !12.800 !28.000 !21.600 !10.500 !6.4000 11.100 !18.300
*Values in columns 3—5 of the table represent pelvic orientations at impact, in terms of successive rotations about the local w, v, and u of the pelvis (as defined in Fig. 2).
combination with upper extremity impact, allowed subjects to either completely avoid pelvic impact, or contact the anterior—lateral aspect of the pelvis rather than the hip. Of course, the ‘goal’ underlying trunk rotation may not have been avoidance of hip impact (which could have been accomplished also by a posterior rotation to land on the buttocks); perhaps as likely was the desire to better visualize the impact surface and facilitate braking of the fall with the outstretched hands. As in any laboratory study involving postural perturbations, questions arise regarding how accurately our experiments simulate real-life causes of imbalance. The perturbations we used simulate a slip, which is often cited as a chief cause for falls (Brocklehurst et al., 1978; Cumming and Klineberg, 1994; Nevitt et al., 1989; Overstall et al., 1977). However, as previously mentioned, real-life slips often occur during ambulation, which we did not simulate. Also, the compliance of the mattress surface may have caused impairment in subjects’ ability to recover balance. Despite randomization of the perturbation strength and direction, learning effects may have occurred, although the interspersing of falls
throughout the testing sessions suggests these were minimal. Furthermore, subjects may have been less fearful of falling onto a soft mattress than onto a rigid surface, even in some cases electing to fall due, for example to fear of stumbling off the mattress. We believe that such effects were minimized, however, by having subjects concentrate before the onset of the perturbation on balance recovery rather than on falling. The consistent observation of a ‘failed’ stepping response in fall trials suggests that this goal persisted until the subject realized balance recovery was futile, and an ensuing fall imminent. At this point, subjects had likely already commenced movements directed at safe landing, and we can think of few reasons why these should differ from those which govern real-life falls. While our sample size prevents meaningful assessment of the effects on fall mechanics of gender and anthropomorphics, we foresee no reason why additional subjects would invalidate conclusions specific to our hypotheses. The reason for this is that, while subjects differed in the number of trials which resulted in falls (with females being more prone to falls than males), they displayed strong similarity in the outcome parameters of interest to
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Fig. 7. Stick figure image of a sideways complete fall (subject: F2). As was typical of such trials, trunk rotation about an inferior—superior axis allowed for avoidance of impact to the lateral aspect of the hip. In this case, impact occurs instead to the anterior—lateral aspect of the pelvis, at nearly the same instant that contact occurs to the wrist.
this study (effect of perturbation direction on stability and risk for pelvic impact during a fall, timing of wrist and pelvic contacts, contact velocities of the wrist and pelvis, and trunk rotations during descent). Of course, a larger sample size might reveal gender differences in these parameters. However, based on our current results, we have little reason to think that, among young, healthy individuals, these would be large and therefore clinically relevant. Considerable differences exist in the movement strategies observed in our study and that of van den Kroonenberg and co-workers (1996), who explored body movements during voluntary (self-initiated) sideways falls onto a gymnasium mat. They observed little trunk rotation, found that subjects impacted their hips during the fall (with a similar range in pelvic contact velocity (2.1—4.8 m s~1) to that observed in the present study), and noted general avoidance of impact to the outstretched hand. These differences likely arise from the different methods involved in the studies: they measured how subjects fall when asked to do so, while we measured landing strategies after subjects experienced unexpected perturbations to posture, and attempted to avoid falling. In summary, we observed three mechanisms which likely reduce young individuals’ risk for injury (and, in particular, hip fracture), following loss-of-balance: (1) a directional-dependency to postural stability which reduces risk for sideways falls; (2) the occurrence of
trunk rotation to avoid hip impact during a sideways fall; and (3) the sharing of impact energy through nearsimultaneous contacts to the upper extremity and pelvis. Due to safety concerns, elderly subjects were not included in this study, and whether they exhibit similar falling patterns is therefore unknown. On the one hand, the fact that fewer than 10% of falls among the elderly result in serious injury suggests that fall-related protective responses persist through much of one’s life span. However, the observation that fall-related hip fractures increase exponentially with age, while wrist fractures level or decrease (Melton et al., 1988), suggests that age-related changes do occur in falling behavior. By employing safer techniques (such as use of an impact restraint harness), future studies might explore whether these involve specific impairments to lateral stability, or alterations in protective reflexes governing upper extremity and trunk movements during descent.
Acknowledgments 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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References Brocklehurst, J.C., Exton-Smith, A.N., Lempert-Barber, S.M., Hunt, L.P., Palmer, M.K., 1978. Fracture of the femur in old age: a two-centre study of associated clinical factors and the cause of the fall. Age and Ageing 7, 7—15. Chiu, J., Robinovitch, S.N., 1996. Transient impact response of the body during a fall on the outstretched hand. ASME International Mechanical Engineering Congress and Exposition, Altanta, GA, BED-Vol. 33, pp. 269—270. Cooper, C., Campion, G., Melton, L.J. III, 1992. Hip fractures in the elderly: a world-wide projection. Osteoporosis International 2, 285—289. Courtney, A.C., Wachtel, E.F., Myers, E.R., Hayes, W.C., 1994. Effects of loading rate on strength of the proximal femur. Calcified Tissue International 55, 53—58. Cumming, R.G., Klineberg, R.J., 1994. Fall frequency and characteristics and the risk of hip fractures. Journal of the American Geriatrics Society 42, 774—778. Cummings, S.R., Nevitt, M.C., 1989. A hypothesis: the cause of hip fractures. Journal of Gerontology 44, 107—111. Greenspan, S.L., Myers, E.R., Maitland, L.A., Resnick, N.M., Hayes, W.C., 1994. Fall severity and bone mineral density as risk factors for hip fracture in ambulatory elderly. Journal of the American Medical Association 271, 128—133. Grisso, J.A., Kelsey, J.L., Strom, B.L. et al., 1991. Risk factors for falls as a cause of hip fracture in women. New England Journal of Medicine 324, 1326—1331. Gryfe, C.I., Amies, A., Ashley, M.J., 1977. A longitudinal study of falls in an elderly population, I: incidence and morbidity. Age and Ageing 6, 201—210. Hayes, W.C., Myers, E.R., Morris, J.N., Gerhart, T.N., Yett, H.S., Lipsitz, L.A., 1993. Impact near the hip dominates fracture risk in elderly nursing home residents who fall. Calcified Tissue International 52, 192—198. Henderson, J.M., Hunter, S.C., Berry, W.J., 1993. The biomechanics of the knee during the parachute landing fall. Military Medicine 158, 810—816. Kim, K.J., Schultz, A.B., Ashton-Miller, J.A., Alexander, N.B., 1997. Impact characteristics of an outstretched arm when arresting a forward fall. 1997, ASME Summer Bioengineering Conference, Sunriver, OR, BED-Vol. 35, pp. 331—332.
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Lotz, J.C., Hayes, W.C., 1990. The use of quantitative computed tomography to estimate risk of fracture of the hip from falls. Journal of Bone and Joint Surgery 72-A, 689—700. Maki, B.E., McIlroy, W.E., Perry, S.D., 1996. Influence of lateral destabilization on compensatory stepping responses. Journal of Biomechanics 29, 343—353. Melton, L.J. III, Chao, E.Y.S., Lane, J., 1988. Biomechanical aspects of fractures. In: Riggs, B.L., Melton, L.J., (Eds.), Osteoporosis: Etiology, Diagnosis, and Management, Raven Press, New York, pp. 111—131. Nevitt, M.C., Cummings, S.R., Kidd, S., Black, D., 1989. Risk factors for recurrent nonsyncopal falls: a prospective study. Journal of the American Medical Association 261, 2663—2668. Nevitt, M.C., Cummings, S.R., 1993. Type of fall and risk of hip and wrist fractures: the study of osteoporotic fractures. Journal of the American Geriatrics Society 41, 1226—1234. Overstall, P.W., Exton-Smith, A.N., Imms, F.J., Johnson, A.L., 1977. Falls in the elderly related to postual imbalance. British Medical Journal 1, 261—264. Owen, R.A., Melton, L.J. III, Johnson, K.A., Ilstrup, D.M., Riggs, B.L., 1982. Incidence of Colles’ fracture in a North American community. American Journal of Public Health 72, 605—607. Praemar, A., Furner, S., Rice, D.P., 1992. Costs of musculoskeletal conditions. In: Musculoskeletal conditions on the United States, American Academy of Orthopaedic Surgeons, Park Ridge, IL, pp. 143—170. Robinovitch, S.N., Hayes, W.C., McMahon, T.A., 1991. Prediction of femoral impact forces in falls on the hip. Journal of Biomechanical Engineering 113, 366—374. Spaite, D.W., Criss, E.A., Valenzuela, T.D., Meislin, H.W., Ross, J., 1990. Geriatric injury: an analysis of prehospital demographics, mechanisms, and patterns. Annals of Emergency Medicine 19, 1418—1421. Tinetti, M.E., Speechley, M., Ginter, S.F., 1988. Risk factors for falls among elderly persons living in the community. New England Journal of Medicine 319, 1701—1707. van den Kroonenberg, A., Hayes, W.C., McMahon, T.A., 1996. Hip impact velocities and body configurations for experimental falls from standing height. Journal of Biomechanics 29, 807—811. Wolfson, L.I., Whipple, R., Amerman, P., Kleinberg, A., 1986. Stressing the postural response: a quantitative method for testing balance. Journal of the American Geriatrics Society 34, 845—850.
Common protective movements govern unexpected falls from standing height E.T. Hsiao, S.N. Robinovitch* Biomechanics Laboratory, Department of Orthopaedic Surgery, San Francisco General Hospital, University of California, San Francisco, San Francisco, CA 94110, U.S.A. Received in final form 16 October 1997
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. ( 1998 Elsevier Science Ltd. All rights reserved. Keywords: Falls; Balance; Hip fracture; Wrist fracture; Protective responses
1. 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 casecontrol study of elderly fallers that impacting on or near the hip increased fracture risk by over 30-fold. Similarly,
* Corresponding author. 0021-9290/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII S0021-9290(97)00114-0
Greenspan and co-workers (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
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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 and co-workers (1996) measured body movements after instructing subjects ‘to launch themselves and subsequently to fall as naturally as possible’ onto a gymnasium mattress, and Henderson and co-workers (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 unexpected 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.
2. Materials and methods 2.1. 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 informed consent was provided by each subject. 2.2. Methods During the experiments, subjects stood barefoot on a large, vinyl-covered gymnasium mattress of the type used in athletic high jumping (dimensions 2.4 m] 1.2 m]0.3 m), with their feet shoulder-width apart and arms at their sides. Without warning, the mattress was then made to translate abruptly by means of a springactuated platform (Fig. 1). We consider such a perturbation to simulate a ‘slip,’ since it suddenly displaces the feet away from the body’s center of gravity. Throughout the testing session, we randomly varied (through the use of random number tables) both the direction of the perturbation (by having the subject stand forward, backward, or sideways to the perturbation direction), and the strength of the perturbation (4 acceleration levels, detailed in Table 1). To evoke ‘natural’ responses, no practice trials were allowed, and subjects were only instructed that (1) in the event of platform movement, they should ‘try to prevent themselves from falling,’ and (2) prior to platform movement, they should maintain their gaze directed forward and at eye level (wording in the subject consent form was ‘‘the platform will move2this motion may make me fall onto the gym mat, but I will try to
Fig. 1. Experimental setup for simulated slipping experiments. Subjects stood barefoot on a large gymnasium mattress, with their feet shoulderwidth apart and their arms at their sides. The mattress rested on a low-friction platform, which was held fixed at one end by an electromagnetic brake (model DCA-600-1101, Automatic Equipment Corp., Cincinnati, OH), and attached at the other end to a spring (of spring constant 1280 N m~1) held under tension. Without warning, the electromagnet was released manually by depressing a button on the brake controller (model MR-110-150, Magnetool, Inc., Troy, MI), causing horizontal translation of the platform. Throughout the sessions, we randomly varied both the strength of the perturbation (via the initial tension in the spring) and the direction of the perturbation (by having the subject stand anterior, posterior, or lateral to the direction of platform movement) Subjects were instructed to ‘try to prevent themselves from falling’.
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3
Table 1 Perturbation characteristics Spring deflection (cm)*
Peak horizontal acceleration (m s~2)!
Time to peak acceleration (ms)!
Peak horizontal velocity (cm s~1)
Time to peak velocity (ms)!
Displacement at time of peak velocity (cm)!
81
4.2$0)4 (3.6—4.9)
89$75 (16—199)
91$23 (53—109)
329$48 (250—383)
21$6 (13—31)
94
6.4$0.3 (5.0—6.9)
50$18 (16—83)
163$19 (125—182)
342$33 (266—383)
34$5 (28—43)
107
8.6$0.4 (8.0—9.1)
47$15 (33—67)
226$14 (202—243)
354$72 (283—533)
51$12 (51—83)
113
9.7$0.5 (8.9—10.4)
59$16 (33—83)
250$16 (226—276)
342$27 (300—383)
52$4 (46—60)
*Spring defleciton is stretch of the spring before release of the electromagnetic brake. !Table cells provide parameter values associated with spring deflections shown on the far left of each row. The first line in each cell displays the mean value of the parameter over all trials$one standard deviation, while the second line provides the observed range.
prevent myself from falling’’). We will use the term ‘posterior-directed perturbation,’ or simply ‘posterior trial,’ to refer to trials in which the platform and the feet initially moved anterior to the trunk (since, in the absence of protective responses, these would cause backwards falls), the term ‘anterior-directed perturbation’ to refer to trials in which the platform moved posterior to the trunk, and the term ‘lateral-directed perturbation’ to refer to trials in which the platform moved to the subject’s left. Four of the six subjects were randomly cycled twice through the 12 different perturbation conditions (3 directions]4 accelerations), for a total of 24 measurements. Subjects M2 and M3 underwent only 20 and 21 trials, respectively, since repeat trials were not conducted at 9.7 m/s2 in any direction (and, for M3, at 8.6 m/s2 in the posterior direction). The reason for this was concern that the subject might step off the mattress and suffer injury. Two additional trials, involving posterior perturbations to F3, were lost due to software malfunction. The total number of trials was therefore 46 in the anterior direction, 46 in the lateral direction, and 43 in the posterior direction. In each trial, a 6-camera, 60 H motion measurement 3 system (MacReflex, Qualisys Inc., Glastonbury, CT) was used to acquire the 3-dimensional positions of 20 (soft foam) markers located at the third metatarsals, lateral malleoli, midpoint of the shins, lateral femoral epicondyles, midpoint of the thighs, anterior superior iliac spines (ASIS), junction between vertebrae L5 and S1, acromium processes, lateral radial epicondyles, dorsal aspect of the wrists, and top of the head. Data acquisition began approximately 1 s before the onset of the perturbation, and lasted 4 s. Music was played to minimize aural cues of brake release.
2.3. Data analysis Based on inspection of data by two independent investigators, trials were categorized in terms of the following balance recovery (Maki et al., 1996; Wolfson et al., 1986) or falling strategies: (1) balance recovery through sway; (2) balance recovery through a single step; (3) balance recovery through two or more steps; (4) ‘partial fall’, involving contact to one or both knees and/or hands, but not trunk or pelvis; and (5) ‘complete fall’, involving contact to the trunk and/or pelvis. The latter two categories separate falls into those that might involve risk for hip fracture (complete falls) from those that would not (partial falls). Complete falls were further analyzed to determine the time of contact and vertical impact velocity (an index of contact severity) of wrist and pelvic markers. To derive these parameters, custom routines (MATLAB, The MathWorks, Natick, MA) were used to filter (recursive low-pass Butterworth, 6 Hz cut-off justified by power spectral analysis) and differentiate position data. Ground contact to a given body part was defined by passage of the corresponding body marker below a horizontal plane located 100 mm above the gym mattress (based on a sequence of static measures of subjects lying on a rigid platform). Pelvic orientations at contact were described by three-dimensional (Cardan) rotations occurring between the onset of the perturbation and the instant of pelvic contact (Fig. 2). In five posterior and one lateral complete fall, the pelvic marker closest to the mattress was lost from camera view before reaching a level we defined as contact; in these cases pelvic rotations could not be calculated, and contact velocities were based on the next-lowest pelvic marker.
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Fig. 2. Convention for defining pelvis rotations. The pelvis is defined by the nearly transverse plane formed by the left anterior superior iliac spine (L), right anterior superior iliac spine (R), and sacral marker at the junction between the L5 and S1 vertebrae (S). The origin of a body-fixed reference frame uvw is at S. Successive (Cardan) rotations about the w, v, and u axes define anterior—posterior pelvic tilt, mediolateral pelvic tilt, and pelvic rotation, respectively. Angles are calculated between the onset of the platform perturbation (where all angles have zero magnitude), and the moment of pelvic contact (in ‘fall’ trials) or wrist contact (in ‘partial fall’ trials).
3. 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. 3b—d). Stepping was the predominant balance recovery technique for all subjects: only one trial involved stabilization of posture through sway. In all but one fall trial, a failed attempt at balance recovery by stepping was observed, with the foot striking the mattress at an average time of 454$86 (S.D.) ms after perturbation onset (Fig. 4). Perturbation direction also influenced fall type (Fig. 3b—d): of the 27 posterior falls, 25 of these (92% of cases) were complete falls involving pelvic impact, while of the 13 lateral falls, only three (23%) involved pelvic impact. None of the 10 anterior falls involved pelvic impact. Wrist contact was observed in all complete and partial falls. In complete falls, impact was nearly as likely to occur to the pelvis before the wrist (43% of trials) as vice versa. However, the time difference between wrist and pelvic impacts was typically small, equaling less than 50 ms in 72% of falls (Figs. 5a and c). On average, pelvic contact occurred 715$160 (S.D.) ms after the onset of mat movement, while wrist contact occurred at 680$116 ms, or 38$97 ms (range: !66 to 317 ms) earlier (Fig. 5a). A consistent sequence of upper extremity movements appeared central in allowing this nearsimultaneous sequence of collisions. This involved an immediate upward movement of the wrist (perhaps reflecting a startle response), followed by a rapid downward movement, and a second upward acceleration just prior to impact (Fig. 6). This last deceleration substan-
Fig. 3. Balance recovery and falling responses. (a) For all subjects (labeled ‘F1’—‘M3’), falls were more common after posterior perturbations (labeled ‘P’) than after anterior or lateral perturbations (labeled ‘A’ and ‘L’). In the event of an anterior or lateral perturbation, loss-ofbalance was more likely to lead to a ‘partial fall’ (defined as contact to one or both knees and/or wrists) than a ‘complete fall’ (defined as contact to the pelvis and/or trunk). The reverse was true for posterior perturbations. (b)—(d) Of the total of 176 trials, 77% involved balance recovery with one or more steps. With increasing perturbation strength, the frequency of single-step recoveries declined, and fall incidence increased.
tially decreased wrist contact velocity, which averaged !2.64$0.66 m s~1 (Fig. 5b), or 66$19% of its peak downward velocity during descent. Impact to both wrists was observed in 26 of the 28 complete falls, with an average interval of 69.8$62.3 ms between contacts (range: 0—233 ms). Head impact was observed in only five falls, three of which involved the same subject. In contrast to the upper extremities, the pelvis typically displayed steady downward movement during complete falls (Fig. 6), contacting with an average velocity of !2.55$0.85 m s~1 (Fig. 5b), or 83$21% of its peak downward velocity during descent. A weak but significant correlation was observed between pelvic contact velocity and the time interval between wrist and pelvic contacts (Fig. 5c): as this interval increased, the contact velocity of the pelvis decreased (r2"0.29; p"0.003). This moderate dependency suggests that mechanisms other than wrist impact influenced pelvic contact velocity, such as initial impact to the knee and/or thigh (commonly observed), and energy absorption in lower extremity muscles during the descent stage of the fall (e.g. eccentric contraction of quadriceps).
E.T. Hsiao, S.N. Robinovitch / Journal of Biomechanics 31 (1998) 1—9
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Fig. 4. Stick-figure (oblique view) image of a typical complete fall from a posterior perturbation (subject: M1). Note the failed attempt to recover balance by stepping at approximately t"0.45 s, the initial upward and then downward movement of the upper extremity, the small degree of trunk rotation during descent, and the near-simultaneous impact to the wrist and pelvis at approximately t"0.75 s.
In complete falls resulting from laterally-directed perturbations, subjects clearly avoided impact to the lateral aspect of the hip. This is best observed by the magnitudes of pelvic rotation during descent about the local w axis (Table 2), which reflect trunk rotation about an inferiorsuperior axis. In a posterior fall, a value of 90° in this parameter would reflect hip impact, while in a lateral fall, a value of zero degrees would reflect hip impact. Average (absolute) values among the 20 posterior falls involving no pelvic marker dropout were 16.3°$12.3° (SD) (range: 0°—41°). In contrast, values for the two lateral falls not involving dropout were 62° and 37°. Review of stickfigure animations for these trials revealed that trunk rotation commenced late in the descent stage of the fall, resulting in impact with the body anteriorly facing the contact surface (Fig. 7). This, in turn, allowed subjects to contact the impact surface with both right and left upper extremities, and avoid impact to the lateral aspect of the hip. To supplement the small number of lateral complete fall trials, we also examined partial falls resulting from lateral perturbations (nine analyzed; one unavailable due to marker drop-out). Large rotations were again observed, averaging 44.6°$15.3° (range: 18°—69°) between the onset of the perturbation and the time of wrist contact.
4. Discussion While previous studies have linked hip fracture to sideways falls (Greenspan et al., 1994; Nevitt and Cum-
mings, 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 likely to fall during lateral perturbations and one-third as likely to fall during anterior perturbations. If a random distribution of perturbation directions arose during everyday activities, our results suggest that falls in the posterior direction (and their related injuries) should be most common. This result, however, contrasts with the finding by Nevitt and Cummings (1993) that falls in the elderly most often occur in an anterior direction; a discrepancy likely due to the fact that about 50% of falls they examined occurred during walking (an activity that future fall mechanics studies should incorporate). Previous multidirectional balance studies have not identified a directional dependency to fall susceptibility, since postural limits have not been stressed to the point that falls ensue. However, our observations represent reasonable extensions of previous work showing a lower sway-to-stepping threshold for posterior as opposed to anterior-directed perturbations (Maki et al., 1996). While we cannot, at present, explain why perturbation direction influences stability, we suspect it relates to an inability to visualize the environment posterior to the body, or achieve large step sizes during posterior stepping (due, for example to limits on hip extension).
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Fig. 6. Temporal variation in the position (top panel) and velocity (bottom panel, where negative velocity implies downward movement) of wrist and pelvic markers during a typical fall due to a posterior perturbation. Mat movement begins at t"0 s. The pelvis moves steadily downward, impacting at t"0.79 s with a velocity of !2.8 m s~1 (p). The wrist, in contrast, undergoes two reversals in the direction of downward acceleration before impacting at t"0.70 s, with a velocity of !2.0 m s~1 (w).
Fig. 5. Descent and impact kinematics for the 28 trials resulting in complete falls (impact to the trunk and/or pelvis). (a) Descent times, defined as the interval between onset of the perturbation (brake release) and the moment the pelvis (open circles) or wrist (filled circles) impacted the ground. Lines join data points from a given trial. Subjects exhibited similar ranges in descent times, averaging 715$16 ms for pelvic contact, and 680$116 ms for wrist contact. Subjects were nearly as likely to impact their pelvis before their wrist (43% of trials) as vice-versa. However, the time interval between pelvic and wrist impact was less than 50 ms in 72% of trials, suggesting a sharing of contact energy between the pelvis and upper extremity. (b) Contact velocities, defined as the vertical velocity of the pelvis (open circles) or wrist (filled circles) at the instant the body part impacted the ground. Negative velocities reflect downward movement. Lines join data points from a given trial. Subjects exhibited similar ranges in contact velocities, averaging !2.64$0.66 m s~1 for the wrist and !2.55$0.85 m s~1 for the pelvis. (c) A weak but significant association was observed between the time interval between pelvic and wrist impacts and the contact velocity of the pelvis (r2"0.29).
We also hypothesized that, in the event of a fall, young individuals contact the ground with the upper extremity before the trunk or pelvis. We found that, while wrist impact occurred during all falls, perturbation direction affected its functional significance. In particular, during lateral and anterior trials, wrist impact allowed for near-complete avoidance of pelvic and trunk impact (which, almost certainly, would have otherwise occurred).
In contrast, during posterior perturbations, impact to the pelvis was common, and subjects were nearly as likely to first impact their pelvis as they were their wrist. However, perhaps of greater significance than impact order is the time difference between contacts, which was less than 50 ms in nearly 75% of the cases. Since previous studies indicate that approximately 50 ms is required to reach peak force after contact to either the hip (Robinovitch et al., 1991) or wrist (Chiu and Robinovitch, 1996; Kim et al., 1997) during a fall, impact configurations during posterior falls are apparently chosen to allow sharing of impact energy between the upper extremities and pelvis. Indeed, it may be that injury risk is greatest for falls in which this time interval exceeds 50 ms (i.e. data points outside the gray band in Fig. 5c). In coordinating nearsimultaneous contact between wrist and pelvis, a chief control parameter appears to be wrist position, which (in contrast to the steady downward movement of the pelvis), undergoes two reversals in downward acceleration during descent, the latter of which allows not only for near-simultaneous pelvic and wrist impact, but also for a reduction in the contact energy of the upper extremity, and the related risk for wrist fracture. Finally, we hypothesized that young individuals avoid impact to the lateral aspect of the hip during a fall, thereby further reducing hip fracture risk. Our major observation in this regard was the occurrence of substantial trunk rotation during the descent phase of lateral (but not anterior or posterior) falls. This movement, in
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Table 2 Pelvic orientations at impact Subject
Perturbation type
w rotation*
v rotation*
M1 F2 F2 F2 F2 F2 F2 F3 F3 F3 F3 F3 F1 F1 F1 F1 F1 F1 M3 M3 F2 F3 M1 F2 F2 F2 F3 F3 F3 F3 F3
Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Posterior Lateral Lateral Lateral (partial) Lateral (partial) Lateral (partial Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial) Lateral (partial)
15.000 1.9000 10.900 1.8000 4.0000 4.8000 !19.800 !40.900 !29.600 !35.500 !31.100 !35.900 !15.500 10.300 0.0000 13.100 10.400 15.300 11.700 !17.800 !61.600 !37.100 !31.900 !17.700 !60.600 !69.400 !47.900 !39.900 !50.100 !37.200 !46.900
43.200 34.600 10.200 33.900 32.600 24.700 10.800 3.0000 23.100 9.6000 11.400 !2.5000 20.200 18.800 23.600 16.200 29.000 19.700 45.400 47.400 !53.300 !15.500 !35.600 !14.500 1.0700 !74.700 !32.300 !36.300 !26.100 3.3000 !35.900
u rotation* 20.000 2.5000 20.500 1.4000 14.400 10.500 !10.300 !76.200 !41.20 !76.400 51.200 !66.600 !26.700 21.100 13.400 13.100 1.4500 35.200 28.100 22.100 17.200 !4.9000 !5.8000 !23.000 !12.800 !28.000 !21.600 !10.500 !6.4000 11.100 !18.300
*Values in columns 3—5 of the table represent pelvic orientations at impact, in terms of successive rotations about the local w, v, and u of the pelvis (as defined in Fig. 2).
combination with upper extremity impact, allowed subjects to either completely avoid pelvic impact, or contact the anterior—lateral aspect of the pelvis rather than the hip. Of course, the ‘goal’ underlying trunk rotation may not have been avoidance of hip impact (which could have been accomplished also by a posterior rotation to land on the buttocks); perhaps as likely was the desire to better visualize the impact surface and facilitate braking of the fall with the outstretched hands. As in any laboratory study involving postural perturbations, questions arise regarding how accurately our experiments simulate real-life causes of imbalance. The perturbations we used simulate a slip, which is often cited as a chief cause for falls (Brocklehurst et al., 1978; Cumming and Klineberg, 1994; Nevitt et al., 1989; Overstall et al., 1977). However, as previously mentioned, real-life slips often occur during ambulation, which we did not simulate. Also, the compliance of the mattress surface may have caused impairment in subjects’ ability to recover balance. Despite randomization of the perturbation strength and direction, learning effects may have occurred, although the interspersing of falls
throughout the testing sessions suggests these were minimal. Furthermore, subjects may have been less fearful of falling onto a soft mattress than onto a rigid surface, even in some cases electing to fall due, for example to fear of stumbling off the mattress. We believe that such effects were minimized, however, by having subjects concentrate before the onset of the perturbation on balance recovery rather than on falling. The consistent observation of a ‘failed’ stepping response in fall trials suggests that this goal persisted until the subject realized balance recovery was futile, and an ensuing fall imminent. At this point, subjects had likely already commenced movements directed at safe landing, and we can think of few reasons why these should differ from those which govern real-life falls. While our sample size prevents meaningful assessment of the effects on fall mechanics of gender and anthropomorphics, we foresee no reason why additional subjects would invalidate conclusions specific to our hypotheses. The reason for this is that, while subjects differed in the number of trials which resulted in falls (with females being more prone to falls than males), they displayed strong similarity in the outcome parameters of interest to
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Fig. 7. Stick figure image of a sideways complete fall (subject: F2). As was typical of such trials, trunk rotation about an inferior—superior axis allowed for avoidance of impact to the lateral aspect of the hip. In this case, impact occurs instead to the anterior—lateral aspect of the pelvis, at nearly the same instant that contact occurs to the wrist.
this study (effect of perturbation direction on stability and risk for pelvic impact during a fall, timing of wrist and pelvic contacts, contact velocities of the wrist and pelvis, and trunk rotations during descent). Of course, a larger sample size might reveal gender differences in these parameters. However, based on our current results, we have little reason to think that, among young, healthy individuals, these would be large and therefore clinically relevant. Considerable differences exist in the movement strategies observed in our study and that of van den Kroonenberg and co-workers (1996), who explored body movements during voluntary (self-initiated) sideways falls onto a gymnasium mat. They observed little trunk rotation, found that subjects impacted their hips during the fall (with a similar range in pelvic contact velocity (2.1—4.8 m s~1) to that observed in the present study), and noted general avoidance of impact to the outstretched hand. These differences likely arise from the different methods involved in the studies: they measured how subjects fall when asked to do so, while we measured landing strategies after subjects experienced unexpected perturbations to posture, and attempted to avoid falling. In summary, we observed three mechanisms which likely reduce young individuals’ risk for injury (and, in particular, hip fracture), following loss-of-balance: (1) a directional-dependency to postural stability which reduces risk for sideways falls; (2) the occurrence of
trunk rotation to avoid hip impact during a sideways fall; and (3) the sharing of impact energy through nearsimultaneous contacts to the upper extremity and pelvis. Due to safety concerns, elderly subjects were not included in this study, and whether they exhibit similar falling patterns is therefore unknown. On the one hand, the fact that fewer than 10% of falls among the elderly result in serious injury suggests that fall-related protective responses persist through much of one’s life span. However, the observation that fall-related hip fractures increase exponentially with age, while wrist fractures level or decrease (Melton et al., 1988), suggests that age-related changes do occur in falling behavior. By employing safer techniques (such as use of an impact restraint harness), future studies might explore whether these involve specific impairments to lateral stability, or alterations in protective reflexes governing upper extremity and trunk movements during descent.
Acknowledgments 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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