Journal of Biomechanics 31 (1998) 685 — 691
Placing the trailing foot closer to an obstacle reduces flexion of the hip,
knee, and ankle to increase the risk of tripping
Li-Shan Chou1, Louis F. Draganich*
Motion Analysis Laboratory, Section of Orthopaedic Surgery and Rehabilitation Medicine, Department of Surgery, The University of Chicago,
MC 3079, 581 South Maryland Avenue, Chicago, IL 60637, U.S.A.
Received in final form 27 May 1998
Abstract
This study was performed to test the hypothesis that reducing the horizontal distance between the trailing foot (foot crossing the
obstacle last) and obstacle, during stance just prior to stepping over the obstacle, would reduce flexion of the hip, knee, and ankle
joints of the trailing limb when the toe is over the obstacle to reduce the vertical toe-obstacle clearance and increase the risk of
tripping. Fourteen healthy young adults stepped over an obstacle of 51, 102, 153, and 204 mm height in a self-selected manner (i.e.,
toe-obstacle distance was not controlled) and for toe-obstacle distance targets of 10, 20, 30, and 40% of their step lengths measured
during unobstructed gait. The reductions in toe-obstacle distance resulted in linear decreases in flexion of the hip, knee, and ankle
when the toe was over the obstacle. Toe-obstacle clearance of the trailing limb decreased significantly as toe-obstacle distance
decreased. The reductions in toe-obstacle distance led to contact of the trailing (but not the leading) foot with the obstacle, the closer
the obstacle the greater the number of contacts. The reductions also resulted in linear decreases in swing time of the trailing limb from
toe-off to when the toe was over the obstacle. The height of the hip was not affected by toe-obstacle distance. Angular velocity of knee
flexion was found to increase linearly as toe-obstacle distance decreased and appears to be of primary importance in avoiding obstacle
contact. ( 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Gait; Obstacle; Toe-obstacle distance; Toe-obstacle clearance; Tripping
1. Introduction
Falls are a serious problem among the elderly, frequently resulting in physical injury and significant costs
for health care (Gryfe et al., 1977; Prudham and Evans,
1981; Tinetti et al., 1988) and tripping over obstacles is
the most frequently mentioned cause of falls in the elderly
(Blake et al., 1988; Campbell et al., 1990; Overstall et al.,
1977; Prudham and Evans, 1981; Tinetti and Speechley,
1989). To understand tripping and develop methods for
avoiding tripping, it is important to identify and quantify
those variables that are responsible for elevating the foot
to clear the obstacle. Obstacle location affects maximum
flexion of the hip and ankle (McFadyen et al., 1993),
* Corresponding author. Fax: (773) 702-0076; e-mail: ldragani@
surgery.bsd.uchicago.edu
1 Current address: Orthopedic Biomechanics Laboratory, Mayo
Clinic / Mayo Foundation, Rochester, MN 55905, USA.
0021-9290/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved.
PII S0021-9290(98)00081-5
mechanical power at the hip and knee (McFadyen et al.,
1993), and muscle activity (Patla et al., 1991) of the limb
crossing the obstacle. Toe-obstacle clearance is relatively
constant for either limb, ranging from 100 to 150 mm,
when stepping over obstacles between 25 and 268 mm
high in a self-selected manner (Chen et al., 1991; Chou
and Draganich, 1997; Patla and Rietdyk, 1993), and
flexion of the knee is of primary importance in elevating
the foot to clear the obstacle (Chou and Draganich, 1997;
McFadyen and Winter, 1991). Also, reducing the available reaction time increases the risk of tripping when
stepping over a virtual obstacle (light flashed on floor;
Chen et al., 1994) or over obstacles 20 or 80 mm high
(Patla et al., 1991) in a self-selected manner.
We have been unable to find literature documenting
that tripping of the leading or trailing foot results in falls.
However, the trailing foot is closer to the obstacle than
the leading foot during stance just prior to stepping over
the obstacle. Furthermore, visual contact with the obstacle is normally absent when stepping over the obstacle
686
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
with the trailing foot. Thus, tripping over obstacles with
the trailing foot might be expected to occur more frequently than with the leading foot. Recovery from tripping with the trailing foot is expected to be easier than
with the leading foot. Nevertheless, tripping with the
trailing foot may lead to falling in the elderly because of
their slower reaction time (Morgan et al., 1994) and
decreased muscle strength (Whipple et al., 1987).
When stepping over obstacles of various heights in
a self-selected manner both young and older subjects
consistently place their trailing feet at the same distance
from the obstacle just before stepping over it (Chen et al.,
1991; Chou and Draganich, 1996). This suggests that
placement of the trailing foot during stance just prior to
stepping over the obstacle is precisely controlled by the
central nervous system to allow enough time to flex
the joints of the trailing limb, elevate the foot, and clear
the obstacle. Reducing the horizontal toe-obstacle distance below that of the self-selected distance would reduce the time available for reaching the obstacle and
alter the geometrical configuration of the lower extremities. We hypothesized that such a reduction would reduce
flexion of the hip, knee, and ankle joints of the trailing
limb when the toe is over the obstacle to reduce the
vertical toe-obstacle clearance and increase the risk of
tripping.
2. Methods
Gait analysis was performed on fourteen healthy
young adults (7 males, 7 females) having a mean age of
23 yr (range, 19—32 yr). Their average height was 171 cm
(range, 158—184 cm), and their average weight was
694 N (range, 516 N to 953 N). All but one of the subjects
were right-hand dominant. The protocol for this
study was approved by the Institutional Review Board
of The University of Chicago. The experimental procedures for the study were explained to the subjects
and informed consents were obtained prior to the experiments.
Clusters of six or eight infrared light-emitting diodes
were attached to the foot, shank, and thigh of the left
lower limb (trailing limb) and pelvis of the subject with
elastic straps. Kinematic parameters were collected with
the WATSMART (Northern Digital, Inc., Waterloo, Ontario, Canada) optoelectronic, 3-D digitizing system.
Kinematic parameters were sampled at a rate of 100 Hz.
The overall accuracy of the system was better than 5 mm
for a volume 2 m long, 1.5 m high, and 0.7 m wide. Two
force-sensing resistors (Interlink Electronics, Carpinteria,
California) were attached to the sole of the left shoe with
self-adhesive to register the times of heel-contact and
toe-off. The use of this instrumentation in our laboratory
has been reported (Chou et al., 1995, 1997; Chou and
Draganich, 1997; Draganich et al., 1995).
Three sets of experiments were performed. In Experiment 1, the subjects were asked to walk on the walkway,
without encountering obstacles, in their self-selected
manner. Each subject’s average step length over three to
six steps was measured. Before performing Experiments
2 and 3 we discovered the subject’s final starting position
and base-line location for the obstacle by adjusting both
the subject’s starting position and obstacle location until
the toe of the subject’s trailing foot landed consistently
within 5 cm of a target on the force plate. The subject was
unaware of the target and was asked to walk and step
over the obstacle comfortably. Thus, we discovered
a final starting position and base-line location for the
obstacle that allowed the subject to step within the target
area and then over the obstacle with the trailing foot, all
in a self-selected manner. The obstacle consisted of
a white elastic band 6 mm wide and 1 mm thick stretched
across an aluminum frame at heights of 51, 102, 153, or
204 mm. The aluminum frame was 91.5 cm wide and
placed across the walkway, requiring the subjects to step
over the obstacle with both limbs. In Experiment 2, the
subject was asked to begin walking comfortably from the
final starting position, step over the obstacle placed at
the base-line location, and continue walking. This was
repeated for each of the four obstacle heights in a random
order. In Experiment 3, the obstacle of each height was
randomly selected and randomly placed at a distance of
10, 20, 30 or 40% of the subject’s step length measured
during unobstructed walking from the target. The subject
was instructed to begin walking from the final starting
position, maintain the same step lengths used in Experiment 2 until stance of the trailing foot just prior to
stepping over the obstacle, and then step over the obstacle and continue walking. Subjects were asked to wear
their own low-heel shoes and performed several practice
trials to become accustomed to the instrumentation and
environment.
The kinematic data were analyzed for the trailing limb
from heel-contact just before crossing the obstacle to the
next heel-contact just after crossing the obstacle. The
average angular velocities of the hip, knee, and ankle of
the trailing limb from toe-off to when the toe was over the
obstacle were computed by dividing the angular range
traversed by the time required. Detailed descriptions of
the methodologies used in our laboratory for measuring the temporal-distance variables, three-dimensional
motion of the joints of the lower limb, and the coordinates of the end of the great toe were reported previously
(Chou and Draganich, 1997).
Only trials were accepted for which the toe of the
subject’s trailing foot landed within 5 cm of the target,
defined as a successful foot placement. The trial was
repeated if foot placement was not successful until three
successful trials were collected for each task (i.e., for each
height and for each height at each distance). A total of 63
trials were collected, including those for tripping and for
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
unobstructed level walking. Foot-obstacle contact was
detected visually. The data from the non-tripping trials
were averaged and the means used to perform the statistical analyses.
When stepping over obstacles of different heights the
effects of obstacle location on crossing speed, swing time
from toe-off to when the toe was over the obstacle,
vertical toe-obstacle clearance, height of the hip joint,
joint angles when the toe was over the obstacle, and
average angular velocities of the hip, knee, and ankle
joints of the trailing limb in the flexion-extension direction were tested using two-way ANOVA with repeated
measures (SYSTAT, Inc., Evanston, IL). The Greenhouse-Geiser adjustment to the degrees of freedom was
used in assessing significance levels. An a"0.01 level
of significance was used to account for multiple comparisons. If a significant difference was detected, the
polynomial test was performed at the a"0.05 level of
significance to determine the trend (linear, quadratic, or
cubic). Logistic regression was used to access the odds
687
ratio to determine if the occurrence of foot-obstacle contacts were associated with obstacle height and toe-obstacle distance at the 0.05 level of significance.
3. Results
Reducing the horizontal toe-obstacle distance resulted
in less flexion of the hip, knee, and ankle joints when the
toe of the trailing foot was directly over the obstacle
(Table 1). Flexion of the knee and hip decreased linearly
(p(0.0001) as toe-obstacle distance decreased and increased linearly (p(0.0001 and p"0.0004, respectively)
as obstacle height increased. Abduction and external
rotation of the hip increased linearly (p"0.002) as
toe-obstacle distance decreased. Flexion of the ankle
changed linearly (p"0.0001) from dorsiflexion to plantarflexion as toe-obstacle distance decreased. There was
also a significant height by distance interaction effect
(p"0.0004).
Table 1
Joint angles (deg) of the trailing limb when the toe was directly over the obstacle
Obstacle height
Obstacle location
51 mm
10%
20%
30%
40%
Internal(#)/external(!)
rotation
Knee joint
Flexion(#)/extension(!)
Ankle joint
Dorsi(#)/plantar(!) flexion
10.5
$5.3
!6.6
$3.6
!3.1
$4.2
82.8
$6.6
!9.2
$6.9
11.5
$3.4
!5.6
$3.5
!3.0
$4.2
84.5
$4.7
!3.6
$6.9
14.2
$3.9
!5.4
$2.9
!2.7
$4.0
89.8
$5.2
0.5
$5.7
15.5
$3.2
!5.1
$3.1
!1.6
$4.7
89.0
$5.3
2.0
$5.1
Hip joint
Flexion (#)/extension(!)
153 mm
13.7
16.4
$5.7 $4.8
17.4
$3.5
Adduction(#)/abduction(!)
!6.0
$2.7
!5.0
$3.0
Internal(#)/external(!)
Rotation
!1.9
$5.0
Knee joint
Flexion(#)/extension(!)
93.1
$8.8
Ankle joint
Dorsi(#)/plantar(!) flexion
Hip joint
Flexion (#)/extension(!)
Adduction(#)/abduction(!)
p-values"
102 mm
SelfSelected
10%
20%
30%
40%
SelfSelected!
20.0
$4.3
!4.4
$3.1
!1.3
$4.7
95.5
$6.9
5.3
$5.8
12.2
$6.5
!6.2
$3.1
!3.4
$4.2
85.7
$8.0
!8.0
$10.1
14.2
$4.2
!5.7
$3.3
!2.1
$4.4
91.6
$5.2
!5.1
$8.6
16.0
$3.3
!5.0
$3.1
!2.4
$4.5
93.4
$4.7
1.8
$7.1
18.6
$4.5
!4.3
$3.1
!1.5
$4.1
97.6
$8.1
2.3
$6.8
22.7
$5.3
!4.1
$2.9
!0.9
$4.5
103.4
$7.1
7.1
$7.2
20.1
$3.8
24.7
$5.9
204 mm
17.3
17.7
$9.5 $5.6
19.8
$6.0
21.8
$6.2
25.2
$6.8
!4.9
$2.8
!4.4
$2.9
!4.5
$3.4
!5.9
$2.7
!5.6
$3.3
!5.5
$3.3
!5.1
$3.1
!5.3
$3.8
!2.0
$4.4
!1.5
$4.5
!1.0
$4.7
!0.9
$4.6
!2.3
$5.0
!1.4
$4.7
!1.8
$4.2
!0.9
$4.0
!1.0
$4.7
94.8
$5.5
97.4
$5.4
102.1
$5.6
109.7
$7.4
101.5
103.0
$11.4 $6.6
103.6
$7.6
106.5
$6.9
113.6
$7.8
!1.2 !1.2
$10.7 $6.3
0.8
$8.2
2.7
$6.7
6.5
$7.9
0.7
0.2
$10.4 $8.1
0.6
$7.5
3.9
$6.3
7.7
$8.4
p "0.00026
H
p (0.00001
L
p "0.48
HL
p "0.18
H
p "0.0045
L
p "0.35
HL
p "0.068
H
p "0.0005
L
p "0.52
HL
p (0.00001
H
p (0.00001
L
p "0.099
HL
p "0.0087
H
p (0.00001
L
p "0.00043
HL
! The self-selected distance ranged from 42.1 to 44.0% of step length for all obstacle heights.
" p-values were for the repeated measures ANOVA. p : p-value for the effect of obstacle height; p : p-value for the effect of toe-obstacle distance;
H
L
p : p-value for the interaction between obstacle height and toe-obstacle distance.
HL
688
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
Fig. 1. The vertical toe-obstacle clearance of the trailing limb decreased linearly as toe-obstacle distance decreased when stepping over
the obstacle of 51 and 102 mm heights and decreased quadratically as
toe-obstacle distance decreased when stepping over the obstacle of 153
and 204 mm heights. In this and the following figures error bars
represent 1 standard deviation.
Fig. 2. Reducing the toe-obstacle distance of the trailing limb resulted
in contact of the trailing foot with the obstacle of 153 and 204 mm
heights. The total number of obstacle contacts increased with obstacle
height and with decreasing toe-obstacle distance. Numbers given at the
data points represent the total number of contacts for all subjects and
for the number of subjects contacting the obstacle. For example,
16(N"12) indicates that there were a total of 16 obstacle contacts for
the group of subjects with 12 of the subjects contacting the obstacle of
204 mm height at a distance of 10%.
Vertical toe-obstacle clearance of the trailing foot decreased significantly (p"0.0084) as horizontal toe-obstacle distance decreased for any of the obstacle heights
(Fig. 1). The clearance decreased linearly (p)0.041)
when stepping over the obstacle of 51 and 102 mm
heights and decreased quadratically (p)0.015) when
stepping over the obstacle of 153 and 204 mm heights as
toe-obstacle distance decreased.
Reducing the toe-obstacle distance resulted in toeobstacle contact of the trailing (but not the leading) foot,
the closer the obstacle, the greater the number of footobstacle contacts (Fig. 2). Contact of the trailing foot
Fig. 3. When the toe was directly over the obstacle the average height
of the hip of the trailing limb increased linearly as obstacle height
increased.
occurred when stepping over the obstacle of 153 mm
height for locations of 10, 20, and 30% of step length and
when stepping over the obstacle of 204 mm height for
locations of 10, 20, 30, and 40% of step length. A total of
10 obstacle contacts occurred for the first trial, 19 for the
second trial, and 11 for the third trial. Both obstacle
height and toe-obstacle distance were found to be significantly associated with the number of foot-obstacle contacts (p)0.0001).
Swing time of the trailing limb from toe-off to when the
toe was over the obstacle decreased linearly (p(0.0001)
as toe-obstacle distance decreased and increased linearly
(p(0.0001) as obstacle height increased (Table 2).
Crossing speed was not affected by toe-obstacle distance,
but decreased linearly (p(0.0001) as obstacle height
increased.
The angular velocity of knee flexion increased linearly
(p(0.0001) as toe-obstacle distance decreased and increased linearly (p"0.008) as obstacle height increased
(Table 3). The angular velocity of hip flexion decreased
quadratically (p"0.0001) as toe-obstacle distance decreased. The angular velocity of ankle flexion changed
linearly (p"0.0001) from dorsiflexion to plantar flexion
as toe-obstacle distance decreased.
When the toe of the trailing foot was directly over the
obstacle the height of the hip was not affected by toeobstacle distance, but increased linearly (p"0.0001)
with obstacle height (Fig. 3).
The mean toe-obstacle distances measured, 13.8, 20.9,
27.4, and 35.0% of the step length, were different from
those desired, 10, 20, 30, and 40%, respectively. However,
the measured distances were significantly different from
each other (p(0.0001). When stepping over an obstacle
in a self-selected manner the mean toe-obstacle distance
for the obstacle heights ranged from 42.1 to 44.0% of the
step length found for unobstructed level walking. This
distance was not affected by obstacle height.
Each of the subjects reported not being fatigued at the
end of testing.
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
689
Table 2
Temporal distance measurements of gait when stepping over the obstacle of each height at each toe-obstacle distance
Obstacle height
Obstacle location
Swing Time
(toe-off to obstacle)(s)
Crossing speed (m s~1)
51 mm
10%
p-values!
102 mm
20%
30%
40%
Self-Selected
0.098
0.107
0.117
0.127
0.155
$0.023 $0.018 $0.017 $0.016 $0.027
1.19
1.21
1.22
1.22
1.21
$0.13 $0.12 $0.12 $0.13 $0.13
10%
20%
30%
40%
Self-Selected
0.111
0.121
0.129
0.146
0.175
$0.023 $0.027 $0.020 $0.022 $0.027
1.16
1.18
1.18
1.19
1.17
$0.16 $0.13 $0.12 $0.13 $0.12
153 mm
204 mm
Swing Time
(toe-off to obstacle)(s)
0.139
0.137
0.141
0.154
0.186
$0.034 $0.028 $0.024 $0.019 $0.028
0.159
0.156
0.159
0.168
0.202
$0.036 $0.028 $0.023 $0.028 $0.030
Crossing speed (m s~1)
1.12
1.13
1.16
1.17
1.16
$0.14 $0.14 $0.13 $0.13 $0.11
1.08
1.12
1.13
1.14
1.11
$0.14 $0.11 $0.14 $0.13 $0.11
p (0.00001
H
p (0.00001
L
p "0.183
HL
p (0.00001
H
p "0.014
L
p "0.46
HL
! p-values were for the repeated measures ANOVA. p : p-value for the effect of obstacle height; p : p-value for the effect of toe-obstacle distance; p :
H
L
HL
p-value for the interaction between obstacle height and toe-obstacle distance.
Table 3
Average angular velocities (deg/sec) of flexion of the joints of the trailing limb from toe-off to when the toe was directly over the obstacle
Obstacle height
Obstacle location
51 mm
10%
p-values!
102 mm
20%
30%
40%
SelfSelected
10%
20%
30%
40%
SelfSelected
Hip joint
Flexion (#)/Extension(!)
155.3
168.4
173.2
183.5
180.9
$37.5 $31.4 $26.5 $28.2 $22.8
156.0
165.0
178.5
173.1
171.0
$39.2 $38.1 $30.9 $21.9 $31.5
Knee Joint
Flexion(#)/extension(!)
418.0
383.3
382.7
351.2
337.9
$53.4 $41.7 $35.4 $45.0 $34.2
415.9
412.4
397.8
380.5
358.3
$53.3 $45.1 $39.4 $39.0 $41.4
Ankle Joint
Dorsi(#)/plantar(!) flexion
!69.3 !32.1
30.3
38.1
58.8
$87.0 $42.6 $32.8 $33.0 $31.7
!61.5 !34.0
18.2
37.8
60.2
$73.9 $50.5 $37.1 $36.1 $31.6
153 mm
204 mm
Hip Joint
Flexion (#)/extension(!)
141.0
163.9
173.3
180.2
167.5
$32.4 $36.5 $33.6 $35.1 $29.8
135.3
149.7
162.4
165.5
158.3
$42.4 $36.1 $35.6 $32.9 $27.1
Knee Joint
Flexion(#)/extension(!)
416.0
395.7
410.1
397.0
381.3
$50.0 $72.1 $43.7 $45.9 $48.6
434.0
428.9
410.7
402.3
377.3
$62.8 $56.5 $59.1 $47.6 $45.3
Ankle Joint
Dorsi(#)/plantar(!) flexion
!26.8 !10.8
14.3
35.4
53.1
$65.4 $45.2 $44.2 $25.0 $33.5
!9.5
7.9
32.3
42.9
53.6
$57.0 $40.6 $39.6 $34.9 $36.7
p "0.01
H
p (0.00001
L
p "0.586
HL
p "0.006
H
p (0.00001
L
p "0.043
HL
p "0.016
H
p (0.00001
L
p "0.032
HL
! p-values were for the repeated measures ANOVA. p : p-value for the effect of obstacle height; p : p-value for the effect of toe-obstacle distance; p :
H
L
HL
p-value for the interaction between obstacle height and toe-obstacle distance.
4. Discussion
Falls are a serious problem among the elderly and
tripping over obstacles is the most frequently mentioned
cause of falls. We conducted this study to test the hypothesis that reducing the horizontal distance between the
trailing foot and obstacle, during stance just prior to
stepping over the obstacle, would reduce flexion of the
hip, knee, and ankle joints of the trailing limb when the
toe is over the obstacle to reduce vertical toe-obstacle
clearance and increase the risk of tripping. We discovered
that reducing the toe-obstacle distance resulted in linear
decreases in flexion of the trailing hip, knee, and ankle
when the trailing foot reached the obstacle, decreases in
690
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
toe-obstacle clearance of the trailing limb, and in contact
of the trailing (but not the leading) foot with the obstacle,
the shorter the distance, the greater the risk of contact
with the obstacle.
Although much care was taken, our methodology included several sources of error. These consisted of identifying the axes of the body-embedded coordinate systems
(i.e., of the thigh, shank, and foot) using bony landmarks,
visually aligning the axes of the IRED clusters with those
of the thigh, shank, and foot, and the error in our optoelectronic system, approximately 5 mm. In support of
the validity of our results, the three-dimensional joint
angles reported from our laboratory using this methodology (Draganich et al., 1995; Chou, 1995) were in good
agreement with those reported from other laboratories
(Kadaba et al., 1990; LaFortune et al., 1992). There were
also discrepancies between the desired toe-obstacle distances and those measured. This was because we allowed
the toe of the trailing foot to land within of $5 cm of
the target before stepping over the obstacle. However, the
means of the distances measured were within 5% of the
desired distances and were significantly different from
each other. Moreover, obstacle location was found to
have significant effects on ten of the variables that we
studied, the discrepancies in distance notwithstanding.
Toe-obstacle clearance decreased linearly when stepping over the obstacle of 51 and 102 mm heights and
quadratically when stepping over the obstacle of 153 and
204 mm heights as toe-obstacle distance decreased. Although a linear trend in clearance was found for the
lowest height, there was some variation from linearity.
An explanation for this variation is that this height represented little, if any, challenge, obscuring or moderating
the subjects’ responses. An explanation for the change in
trends from linear to quadratic is that avoiding contact
with the obstacle of 153 mm height required a more
vertical trajectory of the trailing foot for a toe-obstacle
distance of 10% than did the obstacle of 102 mm height.
As the height increased to 204 mm a more vertical trajectory was also required to avoid contact with the obstacle for a toe-obstacle distance of 20%.
The distribution of obstacle contacts found for the
three trials (10 for the first trial, 19 for the second, and 11
for the third) did not indicate a trend toward increased
obstacle contact with time. This suggests that fatigue was
not a factor in these young active adults.
Reducing the toe-obstacle distance below that selfselected led to reduced flexion of the joints of the trailing
limb when the toe was over the obstacle. For example,
stepping over the obstacle of any height for a toe-obstacle
distance of 10% of step length resulted in average reductions of 10, 15, and 11° in flexion of the trailing hip, knee,
and ankle joints, respectively, compared with those found
for the self-selected distance. This is important because
the preliminary results of a parallel study revealed trends
towards significant differences in hip, knee, and ankle
flexion of the trailing limb between successful and failed
attempts to cross over obstacles (Draganich and Chou,
1997).
The height of the trailing hip was not found to be
significantly affected by toe-obstacle distance. Thus, reductions in flexion of the joints are responsible for the
reductions in toe-obstacle clearance for the trailing limb
when toe-obstacle distance is reduced.
Decreases in toe-obstacle distance affected joint
motion. When the toe of the trailing limb was directly
over the obstacle hip flexion and angular velocity of hip
flexion were found to decrease as toe-obstacle distance
decreased. An explanation for this is that during gait
flexion of the hip elevates the swing foot and moves it
anteriorly. Thus, when stepping over an obstacle that is
close to the trailing foot this motion would be expected to
result in contact of the trailing foot with the obstacle
unless adjustments to hip flexion are made. Furthermore,
toe-obstacle clearance and flexion of the knee decreased
and angular velocity of knee flexion increased. An explanation for this is that as toe-obstacle distance decreased, the time available for the knee to flex after toe-off
decreased. The reduced time available to clear the obstacle limited flexion of the knee, reducing the clearance.
To flex the knee enough to clear the obstacle required an
increase in angular velocity of knee flexion. Therefore,
greater angular velocity of knee flexion appears to be of
primary importance in avoiding obstacle contact when
stepping over an obstacle with a shorter toe-obstacle
distance. Finally, during gait the ankle joint plantar
flexes during toe-off, reaching maximum plantar flexion
just after toe-off and then plantar flexion begins to decrease (Kadaba et al., 1990). When the toe was over the
obstacle plantar flexion and angular velocity of plantar
flexion of the ankle was found to increase as toe-obstacle
distance decreased. This can also be explained by the
reduced time available. As toe-obstacle distance decreased, the toe of the trailing limb was over the obstacle
earlier in swing phase and, therefore, the degree of plantar flexion was greater.
Elderly adults were reported to require approximately
20% longer simple reaction time than young adults (Welford, 1984 and 1988) and to exhibit greater hesitancy and
more submovements during a line drawing task (Morgan
et al., 1994). Furthermore, reductions in available response time as small as 50 or 100 ms decreased the rate of
success in both young and elderly adults when crossing
a virtual obstacle with the leading limb (Chen et al.,
1994). In the present study, the swing time for the trailing
limb from toe-off to when the toe was over the obstacle
averaged 53 ms less for the 10% distance, for all four
obstacle heights, than for crossing an obstacle in a selfselected manner. The reductions in swing time led to
reductions in flexion of the joints and, therefore, to reductions in toe-obstacle clearance. Moreover, stepping over
an obstacle with a shorter toe-obstacle distance required
L-S. Chou, L.F. Draganich / Journal of Biomechanics 31 (1998) 685—691
a faster response at the knee joint and more precise
control at the hip joint of the trailing limb compared to
stepping over an obstacle in a self-selected manner. Thus,
control of hip and knee flexion may be important to
tripping in the elderly.
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