Mechanisms Leading to a Fall From an Induced Trip.pdf

Journal of Gerontology:

MEDICAL SCIENCES

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,

Kevin T. Foley,

and Mark D. Grabiner

Biomedical Engineering Center, Ohio State University, Columbus.

Department of Biomedical Engineering, Lerner Research Institute, and

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;

.001) and delayed support limb loading (267

49 milliseconds vs 160

39 milliseconds;

.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%

11% bh per second;

.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;

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.

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

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

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

26)

11)

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

. 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.

.05; **

.01; ****

.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;

.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;

tests involving the

elevating faller, where a significance level of .001 was used.

57).

However, the choice of recovery strategy was unrelated to

the forward hip velocity (

–.26,

.013;

.241;

56) or the

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

.209;

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

(

After-Step Fall

(

Recovery

( n

11)

Fall

( n

7. Time from trip to support limb loading (ms)

160

267

49****

144

8. Time from trip to follow-through toe-off (ms)

498

490

517

450

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).

(

†

swing ankle speed (

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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

After-Step Fall

( n

Recovery

( n

11)

Fall

( n

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

280

244

—

22. Time from trip-to-ground contact (ms)

523

493

505

447

400

23. Recovery step duration (ms)

265

213

43* †

261

—

24. Maximum horizontal ankle velocity (%bh/s)

263

227

264

225

203

25. Average horizontal ankle velocity (%bh/s)

115

109

117

26. Maximum rate of hip-ankle distance decrease (%bh/s)

144

138

140

27. Maximum rate of hip-ankle distance increase (%bh/s)

171

50***

177

152

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

After-Step Fall

( n

Recovery

( n

11)

Fall

( n

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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