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Mechanisms Leading to a Fall From an Induced Trip, Czasopisma i artukuły, angielskie, o upadkach

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Journal of Gerontology:
MEDICAL SCIENCES
Copyright 2001 by The Gerontological Society of America
2001, Vol. 56A, No. 7, M428–M437
Mechanisms Leading to a Fall From an Induced Trip
in Healthy Older Adults
Michael J. Pavol,
1,2
Tammy M. Owings,
2
Kevin T. Foley,
3
and Mark D. Grabiner
2
1
Biomedical Engineering Center, Ohio State University, Columbus.
2
Department of Biomedical Engineering, Lerner Research Institute, and
3
Section of Geriatric Medicine, The Cleveland Clinic
Foundation, Ohio.
Tripping is a leading cause of falls in older adults, often resulting in serious injury. Although the re-
quirements for recovery from a trip are well characterized, the mechanisms whereby trips by older adults actually result
in falls are not known. This study sought to identify such mechanisms.
Background.
Trips were induced during gait in 79 healthy, community-dwelling, safety-harnessed, older adults (50
women) using a concealed, mechanical obstacle. Kinematic and kinetic variables describing the recovery attempts were
compared between those who fell and those who recovered. Subjects were analyzed according to the recovery strategy
employed (lowering vs elevating) and the time of the “fall” (during step vs after step).
Methods.
Three apparent mechanisms of falling were identified. For a lowering strategy, during-step falls were asso-
ciated with a faster walking speed at the time of the trip (91%
Results.
8% vs 68%
11% body height [bh] per second;
p
.001) and delayed support limb loading (267
49 milliseconds vs 160
39 milliseconds;
p
.001). After-step falls
were associated with a more anterior head-arms-torso center of mass at the time of the trip (6.2
1.3 degrees vs 0.2
.01), followed by excessive lumbar flexion and buckling of the recovery limb. The elevating strategy
fall was associated with a faster walking speed (93% vs 68%
p
11% bh per second;
p
.001) followed by excessive
lumbar flexion.
Walking quickly may be the greatest cause of falling following a trip in healthy older adults. An ante-
rior body mass carriage, accompanied by back and knee extensor weakness, may also lead to falls following a trip. Defi-
cient stepping responses did not contribute to the falls.
Conclusions.
RIPPING is a leading cause of falls in community-
dwelling older adults, responsible for up to 53% of falls
in this population (1). These falls have serious conse-
quences. Eleven percent of all falls by older adults result in
serious injury (2), and falls are the leading cause of uninten-
tional-injury death in older adults in the United States (3).
Trip-related falls are specifically responsible for 12% to
22% of the hip fractures suffered by older adults (4,5). Even
noninjurious falls can bring about decreased quality of life
through the fear of falling and, in turn, the restriction of ac-
tivities (2,6). There is, therefore, a need for effective inter-
ventions for reducing the incidence of trip-related falls in
the older adult population.
Studies of fall epidemiology have identified the charac-
teristics of older adults who are most likely to suffer a fall
(7), but these studies have not considered the biomechanics
of falling. Therefore, factors that are directly related to the
ability to prevent a fall have not been differentiated from
factors that simply covary with the likelihood of falling.
Such a differentiation is needed to appropriately target in-
terventions for fall prevention at the former versus the latter
factors.
To better understand the factors directly involved in re-
storing balance following a trip, studies have characterized
the kinematic, kinetic, and neuromotor responses associated
with recovering from an induced trip or stumble (8–14). The
following three common strategies for recovery have been
identified (9). In a lowering strategy, the tripped foot is im-
mediately lowered to the ground on the near side of the ob-
stacle. The tripped limb then acts as the support limb as the
contralateral recovery limb executes the initial recovery
step across the obstacle. In an elevating strategy or in a
reaching strategy, the tripped limb is used as the recovery
limb as the tripped foot is lifted over the obstacle in a con-
tinuation of the original step. The contralateral stance limb
acts as the support limb during the recovery step. Elevating
and reaching strategies are differentiated based on whether
recovery limb flexion occurs at multiple joints or primarily
at the hip, respectively.
Independent of the strategy employed, successful recov-
ery from a trip has been associated, conceptually, with the
ability to react rapidly with an appropriate response (15,16)
to control the forward rotation of the trunk (12,13) and exe-
cute a recovery step of sufficient length to establish a new,
functional, base of support (12,17). Also important is the ef-
fective use of the support limb in slowing the fall of the
head, arms, and torso (HAT) during the stepping phase; that
is, from the time of the trip until the recovery foot ground
contact (8,9). Finally, recovery requires that the recovery
limb provide sufficient hip height during stance for the sup-
port limb to execute an effective follow-through step.
Although the requirements for recovery from a trip are
fairly well characterized, the primary mechanisms whereby
trips by older adults actually result in falls are not known.
M428
4.4 degrees;
T
MECHANISMS OF FALLING FROM A TRIP
M429
To date, the biomechanics of a failed recovery from a trip or
stumble have not been reported. Therefore, any fall-preven-
tion efforts aimed at reducing an older adult’s likelihood of
falling following a trip can, at present, only have been based
on theories as to why these falls occur. Since one may hy-
pothesize any number of manners in which the recovery
process may fail, many or most of which may rarely be ob-
served in practice, an experimental validation of these theo-
ries is clearly needed.
This study attempted to identify the mechanisms whereby
selected healthy older adults fell following an induced trip.
Ten possible contributing factors were considered. It was
hypothesized that, in comparison to those who recovered,
fallers (i) were walking faster at the time of the trip, (ii) had
a more forward-oriented HAT center of mass at the time of
the trip, (iii) fell faster initially, (iv) selected an inappropri-
ate recovery strategy, (v) were slower in initiating the
phases of their recovery, (vi) were less effective at slowing
their fall through their stepping phase motor response, (vii)
took a shorter recovery step, (viii) took a slower recovery
step, (ix) experienced buckling of the recovery limb after
ground contact, and (x) experienced greater lumbar flexion.
The rope provided a visible hazard, the purpose of which
was to mislead the subject as to the time, location, and
mechanism of the trip.
Subjects were informed that a trip would take place dur-
ing an upcoming, but unspecified, trial. Instructions were to
walk at a self-selected, “normal” speed from a designated
starting point to a point approximately 7 m distant, looking
straight ahead. If tripped, subjects were to recover and con-
tinue walking. On a subsequent trial, the obstacle was trig-
gered and a trip induced. Only one attempt was made to trip
each subject.
The kinematics of the trip and subsequent recovery at-
tempt were recorded using a six-camera motion capture sys-
tem (Motion Analysis, Santa Rosa, CA). The cameras, oper-
ating at 60 Hz, recorded the motion of 18 hemispherical
passive reflective markers applied over selected anatomical
landmarks of the bilateral upper and lower limbs, torso, and
head. In addition, ground reaction forces and moments were
measured by two forceplates (AMTI, Newton, MA), located
immediately preceding the obstacle and in the expected re-
gion of recovery foot ground contact, respectively. Force-
plate and safety harness load cell data were sampled at 1000
Hz in synchrony with the kinematic data.
M
ETHODS
Data Analysis
Subjects
Each trip outcome was classified as either a recovery,
fall, rope assist, or miss (18). Falls corresponded to the sub-
ject being fully supported by the safety harness. Recoveries
and rope assists were differentiated based on the integral,
over the 1 second following the triggering of the obstacle, of
the filtered, load cell, rope force signal from the safety har-
ness. Trip outcomes with less than 5% body weight · second
exerted on the ropes were classified as recoveries. Out-
comes with larger integrated forces were considered rope
assists. Misses resulted when impact with the obstacle did
not occur as intended.
The recovery attempts of subjects who were successfully
tripped were classified as a lowering, an elevating, or an
“other” strategy. Classifications were based on the earlier
descriptions of these strategies, except that elevating and
reaching strategies were grouped together since they differ
only slightly in the mechanics of their recovery step. Two
subjects employing an “other” strategy, in which the tripped
foot was lowered onto the obstacle, were excluded from
analysis.
Six events were of interest in the analysis. The time of the
trip was registered as a high-frequency impact artifact in the
data of the forceplate in front of the obstacle. For subjects
who employed a lowering strategy, the start and end of the
large mediolateral shift in the center of pressure on this
same forceplate (computed after recursive, fourth-order,
Butterworth low-pass filtering of the data at 50 Hz) identi-
fied the times of support limb loading and recovery foot toe-
off, respectively. Recovery foot ground contact and the fol-
low-through step toe-off were registered by the underlying
forceplates. Where initial recovery foot ground contact oc-
curred beyond the forceplates, this event was identified
from the foot-marker paths in the kinematic data. Finally,
for those trips that resulted in a fall, the “fall” was defined to
occur when 50% of the subject’s body weight was sup-
Fifty women and 29 men (age, 72
5 years; height, 1.64
14.0 kg), all healthy, community-
dwelling, and at least 65 years of age, provided written in-
formed consent to participate in this experiment, which was
part of a larger study of falling in these older adults. Each
subject was screened by a geriatrician for exclusionary fac-
tors that included neurological, musculoskeletal, cardiovas-
cular, pulmonary, and cognitive disorders, as well as a his-
tory of repeated falling. Five subjects reported having fallen
once in the past year because of an external disturbance. A
minimum bone mineral density of the femoral neck, as-
sessed by dual-energy x-ray absorptiometry (Hologic QDR
1000, Waltham, MA), of 0.65 g·cm
was also required.
Subjects were paid for their participation.
-2
Experimental Protocol
Subjects were placed in a safety harness and tripped dur-
ing gait. This previously described protocol (18) is summa-
rized here.
Subjects wore a full-body safety harness that was at-
tached by a pair of dynamic ropes to a bearing on a ceiling-
mounted track. Rope lengths were adjusted such that the
wrists and knees could not touch the floor. A calibrated load
cell (Omega Engineering, Stamford, CT), in series with the
dynamic ropes, measured the force exerted on the ropes by
the subject. The safety harness did not introduce any mean-
ingful changes in gait (18).
Trips were induced using a concealed, pneumatically
driven, metal obstacle. This obstacle would rise 5.1 cm from
the floor in approximately 170 milliseconds when manually
triggered by the investigator, inducing a trip by obstructing
the toe of the shoe of the swing foot during mid-to-late
swing. For the trip, a decoy “tripping rope” was also laid
across the gait path, 1.5 m before the mechanical obstacle.
0.09 m; mass, 76.0
M430
PAVOL ET AL.
ported by the safety harness ropes, as indicated by the fil-
tered load cell rope force signal. All event times were vali-
dated against the kinematic data.
The analysis of the recovery attempts was based on a sim-
plified two-link model of the body in the sagittal plane (Fig-
ure 1), supplemented by measures of trunk kinematics. The
lower limb of primary interest was represented as an elastic
link from the ankle to the midpoint of the bilateral hip joint
centers. The HAT was represented as a single link from the
bilateral hip joint centers to the HAT center of mass. De-
scriptors of the model included the ankle and hip positions,
the hip-to-ankle distance (measured three-dimensionally
from the ipsilateral hip joint center), the moment arm of the
HAT weight about the ankle, the link orientations with re-
spect to vertical, and the rate of change of each measure.
Descriptors were computed from the kinematic data after
recursive, fourth-order, Butterworth low-pass filtering of
the reflective marker paths. Marker-specific cutoff frequen-
cies, determined by a residual analysis (19), ranged from 5.5
to 7.5 Hz. Ankle positions were represented by the markers
on the lateral malleoli. The locations of the hip joint centers
and the positions and spatial orientations of the pelvis,
trunk, head, upper arm, and forearm segments were com-
puted from the three-dimensional paths of the reflective
markers affixed to the greater trochanters and HAT. The
computations employed transformations derived from an-
thropometric measurements and from kinematic data col-
lected during a static initialization trial. Anthropometric
measurements were also used to derive subject-specific esti-
mates of body segment mass and center of mass locations
(20). These and the body segment kinematics determined
the position of the HAT center of mass. All distances were
normalized to body height (bh).
In analyzing trunk kinematics, lumbar flexion was com-
puted as the forward rotation of the trunk segment, with re-
spect to the orientation of the pelvis, about an axis at the
level of L
with the shoulder joint centers 6.8 cm posterior to the hip
joint centers, based on the mode of the observed distribution
in our subjects. The forward inclination of the trunk with re-
spect to vertical was computed in the same basic manner,
with the vertical orientation of the trunk defined as de-
scribed previously.
From all possible kinematic variables defined by the pre-
viously described events and analytic models, a set of 36 de-
scriptors of the recovery attempts (Tables 1–5) was selected
to allow evaluation of the factors hypothesized as contribut-
ing to a fall following a trip. One or more variables were
uniquely associated with each hypothesized factor (Table
6). Two kinetic variables were also used as gross indicators
of the motor responses employed to control the fall during
the stepping phase. The support impulse was computed as
the integral, from the time of the trip until follow-through
step toe-off, of the filtered vertical reaction force measured
by the forceplate preceding the obstacle. A propulsive im-
pulse was also computed as the corresponding integral of the
filtered anterior-posterior shear force measured by the for-
ceplate, with the direction of the vector formed by the sup-
port and propulsive impulses defining the angle of the net
support force.
Finally, the phase of gait in which each trip occurred was
determined as the perpendicular distance to the obstacle
from the static location of the obstructed toe during the pre-
ceding stance phase. This distance was expressed as a per-
centage of the length of the contralateral stride preceding
the trip.
Statistics
and parallel to the pelvis mediolateral axis.
The Cardan angle approach of Grood and Suntay (21) was
employed. Lumbar flexion was defined to be zero for the
relative segment orientation observed during quiet standing
3
L
4
The kinematic and kinetic variables that differed between
those who successfully recovered and those who fell were
determined. Subjects were grouped, for this analysis, by the
recovery strategy employed. In addition, within this group-
ing, two distinct groups of fallers who employed a lowering
strategy emerged. “During-step” fallers fell within 80 milli-
seconds of recovery foot ground contact (range, –25 to 77
ms), whereas “after-step” fallers did not fall until after tak-
ing a follow-through step (range, 471 to 785 ms after recov-
ery foot ground contact). Because this large difference
could reflect differing falling mechanisms, during-step and
after-step fallers were analyzed as separate groups. The
Mann-Whitney test was used to compare the during-step
fallers and the after-step fallers with those who successfully
recovered using a lowering strategy. Because only one sub-
ject who employed an elevating strategy fell, one-sample
t
tests were employed to determine whether those who recov-
ered using an elevating strategy differed from the elevating
faller.
Results of these comparisons were used to determine
which of the hypothesized factors contributed to the falls by
each of the three groups of fallers analyzed. A factor was
considered to have contributed to the falls of a group if any
of the kinematic or kinetic variables associated with the fac-
tor (Table 6) differed significantly between the fallers and
those who successfully recovered using a similar strategy.
Logistic regression analysis was used to determine whether
the recovery strategy employed was related to the forward ve-
locity of the hips or the phase of gait at the time of the trip, or
Figure 1. Simplified two-link model of the body in the sagittal
plane. The lower limb is modeled as an elastic link from the ankle to
the midpoint of the bilateral hip joint centers. The head-arms-torso
(HAT) is modeled as a link from the bilateral hip joint centers to the
HAT center of mass (COM). All angles (
) and distances shown are
positive, including those for the net support force (F
support
).
MECHANISMS OF FALLING FROM A TRIP
M431
Table 1. Kinematics of the Trip and Initial Fall in Each Group of Subjects
Lowering Strategy
Elevating Strategy
Recovery
(
During-Step Fall
(
After-Step Fall
(
Recovery
(
Fall
Variable
n
26)
n
5)
n
3)
n
11)
n
1)
0. Hip horizontal velocity at time of trip
†
(m/s)
1.13
0.19
1.45
0.12
1.29
0.09
1.12
0.20
1.40
1. Hip horizontal velocity at time of trip (%bh/s)
68.2
11.1
91.3
7.8****
79.4
6.5
68.0
10.6
92.5****
2. Hip-HAT COM angle at time of trip (degrees)
0.2
4.4
1.0
2.5
6.2
1.3**
0.6
4.4
1.0
3. Trunk inclination at time of trip (degrees)
9.1
5.7
7.5
3.5
18.8
8.3*
8.7
7.2
14.3
4. Hip horizontal velocity 100 ms posttrip (%bh/s)
72.9
11.0
94.5
5.0****
82.2
13.3
67.4
9.4
86.5****
5. Hip vertical velocity 100 ms posttrip (%bh/s)
9.8
5.3
11.8
6.8
7.2
6.8
9.3
5.6
8.1
6. Hip-HAT COM velocity 100 ms posttrip (degrees)
15.9
13.6
22.4
13.9
22.4
13.6
19.5
9.5
42.8****
Notes
: Values are mean
SD
. Velocities 100 ms posttrip reflect the initial rate of falling and preceded support limb loading in all who employed a lowering strat-
egy. bh
body height; HAT
head-arms-torso; COM
center of mass.
Variable displayed for informational purposes only; no statistical analyses were performed on or using this variable.
*
p
.05; **
p
.01; ****
p
.001 (vs recovery group for the corresponding strategy).
to the speed or height of the swing ankle 17 milliseconds
prior to the trip. The pooled data of the recovery, rope-assist,
and fall groups were analyzed. A backward, stepwise, multi-
variable, logistic regression analysis was subsequently per-
formed. All variables possessing a significant univariate rela-
tionship to the recovery strategy were included in the initial
model, and the likelihood ratio test with a cutoff probability
of 0.1 was used in variable elimination. Outliers (standard-
ized residual greater than 2.0) in the logistic relationship ob-
tained from the stepwise analysis were defined as having em-
ployed an inappropriate recovery strategy.
Analyses were performed using SPSS for Windows Re-
lease 7.0 (SPSS, Inc., Chicago, IL). A significance level of
.05 was used in all analyses except the
were 26 recoveries, 5 during-step falls, 3 after-step falls,
and 9 rope assists. In those employing an elevating strategy,
there were 11 recoveries, 1 fall, and 3 rope assists. Data for
one other subject who fell were unavailable, as his trip kine-
matics failed to record.
The recovery strategy employed was significantly related
to the phase of gait in which the trip occurred, with the odds
of employing a lowering strategy increasing by a factor of
1.31 (95% confidence interval [CI] 1.08–1.59) for each 1%
stride length increase in the phase of gait (Figure 2;
R
.28,
58). In addition, the odds of employing a
lowering strategy increased by a factor of 2.05 (95% CI
1.16–3.59) for each 1% bh decrease in the swing ankle
height at the time of the trip (
.007;
n
tests involving the
elevating faller, where a significance level of .001 was used.
t
57).
However, the choice of recovery strategy was unrelated to
the forward hip velocity (
R
–.26,
p
.013;
n
R
0,
p
.241;
n
56) or the
R
ESULTS
57) just prior to
the trip. Respective odds ratios were 1.03 (95% CI 0.98–
1.08) and 1.01 (95% CI 0.99–1.03) for a 1% bh per second
increase in speed. The logistic model obtained through
backwards stepwise analysis was identical to the previously
described model that included the phase of gait of the trip as
the only predictor of recovery strategy. According to our
R
0,
p
.209;
n
Sixty-one subjects were successfully tripped. Forty-three
subjects employed a lowering strategy in their recovery at-
tempt, 15 subjects employed an elevating strategy, and 2
subjects (both successful recoveries) lowered the tripped
foot onto the obstacle in an “other” strategy. In those who
employed a lowering strategy, the outcomes of the trips
Table 2. Kinematics and Kinetics of the Support Limb and Head-Arms-Torso During the Stepping Phase in Each Group of Subjects
Lowering Strategy
Elevating Strategy
Variable
Recovery
(
26)
During-Step Fall
(
n
5)
After-Step Fall
(
n
3)
Recovery
(
n
11)
Fall
(
n
1)
7. Time from trip to support limb loading (ms)
160
39
267
49****
144
35
0
0
8. Time from trip to follow-through toe-off (ms)
498
71
490
52
517
75
450
38
400****
†
9. Ankle-hip angle at time of loading (degrees)
9.8
4.2
23.6
8.5****
9.1
3.0
8.9
1.8
11.9****
10. Hip-HAT COM angle at time of loading (degrees)
4.4
5.5
16.9
7.8****
10.4
2.6*
0.6
4.4
1.0
11. Moment arm of HAT weight at loading (%bh)
8.7
4.0
23.2
9.5****
9.7
2.5
7.9
1.4
10.2****
12. Lumbar flexion at time of loading (degrees)
6.1
8.9
6.4
4.5
15.4
6.8
6.7
10.9
17.2
13. Support impulse (%bw/s)
44.1
4.5
35.0
6.5***
42.5
3.8
38.3
2.4
31.8****
14. Angle of net support force (degrees)
10.9
2.1
14.1
2.3**
12.1
2.0
10.6
0.8
13.4****
15. Maximum hip upward velocity
‡
(%bh/s)
21.4
9.2
3.3
2.7****
5.6
6.5*
14.8
7.2
2.1****
Notes
: Values are mean
SD
. For those who employed an elevating strategy, the time of support limb loading corresponded to the time of the trip. bh
body
height; bw
body weight; HAT
head-arms-torso; COM
center of mass; loading
support limb loading.
†
Difference is in direction other than hypothesized.
‡
Determined over the period from the time of trip until recovery foot ground contact.
*
p
.05; **
p
.01; ***
p
.005; ****
p
.001 (vs recovery group for the corresponding strategy).
(
†
p
swing ankle speed (
n
M432
PAVOL ET AL.
Table 3. Kinematics of the Recovery Step in Each Group of Subjects
Lowering Strategy
Elevating Strategy
Variable
Recovery
(
n
26)
During-Step Fall
(
n
5)
After-Step Fall
(
n
3)
Recovery
(
n
11)
Fall
(
n
1)
16. Recovery step length
‡
(%bh)
49.4
5.7
36.9
8.3****
49.1
8.0
49.8
5.6
51.8
17. Recovery stride length
‡
(%bh)
59.9
6.2
51.4
7.2*
61.7
7.4
89.7
5.5
93.2
18. Obstacle-ankle distance at ground contact (%bh)
39.6
5.9
32.0
7.0*
40.0
5.5
32.2
6.8
32.6
19. Minimum hip-ankle distance (%bh)
33.0
3.7
31.8
3.7
28.8
1.4
34.5
2.7
31.0
20. Maximum ankle ground clearance (%bh)
24.7
3.9
23.8
4.9
25.8
2.5
22.1
3.8
24.0
21. Time from trip to recovery foot toe-off (ms)
257
27
280
28
244
10
—
—
22. Time from trip-to-ground contact (ms)
523
44
493
25
505
54
447
46
400
23. Recovery step duration (ms)
265
36
213
43*
†
261
63
—
—
24. Maximum horizontal ankle velocity (%bh/s)
263
34
227
50
264
10
225
22
203
25. Average horizontal ankle velocity (%bh/s)
115
15
109
17
117
16
54
9
56
26. Maximum rate of hip-ankle distance decrease (%bh/s)
144
29
138
18
140
23
80
18
71
27. Maximum rate of hip-ankle distance increase (%bh/s)
171
30
86
50***
177
8
152
30
134
SD
) were computed for the recovery (i.e., stepping) limb between the times of toe-off and ground contact for a low-
ering strategy, and between the time of the trip and ground contact for an elevating strategy. bh
body height.
†
Difference is in direction other than hypothesized.
‡
Computed between the appropriate static positions of the ankles during stance.
*
p
.05; **
p
.005; ****
p
.001 (vs recovery group for the corresponding strategy).
definition, no subject who fell following the trip employed
an inappropriate recovery strategy.
As indicated by the kinematic and kinetic descriptors of
the recovery attempts, each group of fallers differed signifi-
cantly in selected aspects from the corresponding group of
subjects who recovered (Tables 1–5). These differences
were apparent across the entire period from the time of the
trip through the time of the fall. Based on the observed dif-
ferences, 8 of the 10 hypothesized factors were found to
play a role in the falls of at least one of the three groups of
fallers (Table 6). However, the sets of factors identified as
contributing to the falls differed between groups of fallers.
For example, a slowed recovery phase initiation contributed
only to the during-step falls, as only this group showed a
significant difference in one of the three variables associ-
ated with this factor in Table 6: the time from the trip to
support limb loading (variable 7, shown in Table 2).
that each group reflected a different mechanism of falling.
During-step fallers responded to the trip with a lowering
strategy and essentially fell before completing their recov-
ery step. After-step fallers responded using a lowering strat-
egy and were able to successfully execute a recovery step,
but proceeded to fall after the subsequent follow-through
step. The final faller responded to the trip using an elevating
strategy and successfully executed several steps after the re-
covery step before finally falling.
During-Step Falls
The during-step fallers were walking significantly faster at
the time of the trip than those who recovered and, as a result,
fell forward faster initially. The during-step fallers also took
significantly longer to lower and begin loading the support
limb. The combined effect of these factors was that, at the
time of support limb loading, the hips and HAT center of
mass of the during-step fallers were significantly more for-
ward than in those who recovered (Figure 3A,B). The impli-
cations of this difference are seen by considering the direction
of the net support force, which is an indicator of the net effect
of the joint moments generated over the stepping phase.
D
ISCUSSION
We have identified three distinct groups of older adults
who fell following an induced trip, with observed differ-
ences in the factors contributing to these falls suggesting
Table 4. Kinematics of the Recovery Limb and Head-Arms-Torso at the Time of Recovery Foot Ground Contact in Each Group of Subjects
Lowering Strategy
Elevating Strategy
Variable
Recovery
(
n
26)
During-Step Fall
(
n
5)
After-Step Fall
(
n
3)
Recovery
(
n
11)
Fall
(
n
1)
28. Hip height (%bh)
54.5
2.3
47.2
4.5***
50.9
2.8
54.5
1.5
51.1****
29. Ankle-hip angle (degrees)
10.1
3.8
12.7
5.9****
7.8
6.4
7.6
6.0
0.28
30. Moment arm of HAT weight (%bh)
2.0
4.8
18.5
4.8****
3.9
4.9*
0.4
6.5
10.8
31. Hip-HAT COM angle (degrees)
25.6
11.9
41.5
5.2***
38.5
3.1
26.6
8.3
36.7
32. Hip-HAT COM velocity (degrees)
43.0
52.8
81.6
41.4****
36.1
40.9*
13.8
57.9
97.3****
33. Trunk inclination from vertical (degrees)
36.0
12.6
48.3
4.7*
55.2
11.4*
37.3
11.1
58.5****
34. Lumbar flexion (degrees)
23.5
10.0
22.1
8.5
38.7
10.9*
23.1
13.3
45.2****
Notes
: Values are mean
SD
. bh
body height; HAT
head-arms-torso; COM
center of mass.
*
p
.05; ***
p
.005; ****
p
.001 (vs recovery group for the corresponding strategy).
Notes
: Unless otherwise noted, values (mean
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