Copyright 1998 by The Gemntological Society of America
Journal of Gerontology: MEDICAL SCIENCES
1998, Vol. 53A, No. 4, M32O-M326
The Relation Between Age-Related Changes
in Neuromusculoskeletal System and Dynamic
Postural Responses to Balance Disturbance
GeWu
Department of Physical Therapy, University of Vermont, Burlington.
Background. The purpose of this study was to identify some critical factors whose functional changes with age in
the neuromusculoskeletal systems would potentially relate to the maintenance of standing balance.
Methods. A total of 38 healthy subjects were tested (age range, 21—77 years). A postural disturbance at the foot was
provided, and the range of movement of the head, trunk, thigh, shank, and foot was examined. Three supporting surfaces were tested: hard, soft, and reduced. The functions evaluated in each subject included general health, touch, and
cutaneous vibratory perception threshold at four different locations, ankle strength, and range of motion. The functions
were then correlated with the maximum magnitude of the body movement.
Results. There were significant age-related changes in cutaneous vibratory perception threshold at all four locations
of the foot, in plantarflexor strength, and in touch sensation at the heel region. Age had a positive correlation with head
movement regardless of the supporting surface and the movement direction of the platform. Plantarflexor strength was
also correlated with head movement but only when the platform moved in the backward direction. When standing on a
large supporting base the change in cutaneous vibratory perception threshold at the 5th metatarsal head with respect to
age showed a larger, although weak, effect on the head movement than the cutaneous vibratory perception threshold at
other regions.
Conclusions. The stabilization of an upright balance, especially the head, was related to age, plantarflexor strength,
and vibratory perception at the foot. The laboratory test in this study identified such correlations.
F
ALLING is a serious problem in the aged population.
Not only does the frequency of falls increase with age
(1), but the consequences of elderly adults falling are associated with significant morbidity and mortality (2). It is
predicted that the scope of this problem will continue to
expand in the immediate future because of the increasing
proportion of elderly individuals in our population (2).
Maintaining a balanced upright posture is a complicated
task in humans. It involves a delicate control of muscular
activities that not only balance the multiple body segments
against gravity but also resist the external disturbance that
can potentially destabilize the upright balance. There are
many factors, both internal and external, that can affect
postural control. For example, some of the internal factors
are anthropometric characteristics of individual body segments, neurological properties, sensory and motor functions, and mental status. However, many questions remain
to be answered. How would the postural control strategy be
modified when one or more of these factors are changed?
To what degree would changes in these factors, or the combination of these factors, result in insufficient control of upright balance?
The exploration of these questions becomes relevant and
important when we try to understand why elderly individuals are at higher risks for falls than young individuals. An
earlier study found that postural responses to external postural disturbances are significantly different in healthy old
adults than in healthy young adults (3). Moreover, it is
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known that there are a number of functional changes in
sensory, neurological, musculoskeletal, and physiological
aspects accompanying the normal aging process. Would
these age-related changes be significant enough to alter
individuals' postural control strategies and result in frequent falls among older adults? Which of these changes
would be more responsible for the frequent falls than others? Unfortunately, there have not been consistent answers
to these questions (4).
For example, it is well accepted that peripheral sensory
systems such as visual, vestibular, and somatosensory systems play important roles in postural control and postural
balance (5-7). As people age, the functions of these sensory systems often deteriorate (8-11). Studies have shown
that these functional changes will indeed increase the incidence of falls in the elderly population (12). In some studies, variables such as poor vision, low vibration sense, and
poor proprioception have been identified, among others, as
the key factors to predict the recurrence of falls (13). However, Era et al. (13) indicated that these factors can help to
explain only 11-13% of the variance in balance.
Furthermore, muscular strength has also been considered
to be an important component in the ability to maintain a
balanced upright posture (14). For example, Whipple et al.
(15) reported that the number of falls in the elderly population is related to weak ankle dorsiflexion strength. In a
recent study by Wolfson et al. (16), a significant correlation
(correlation coefficient = -.37) was found between lower
AGE-RELATED PHYSIOLOGICAL CHANGES AND POSTURE
extremity strength and the occurrence of loss of balance
during dynamic postural tasks among nursing home residents with a history of falls. These studies have suggested a
positive effect of muscular strength on postural balance.
However, this relation has not been demonstrated successfully in laboratory tests using common biomechanical
measures. For example, Brown et al. (17) reported weak
and nonsignificant correlations between lower limb strength
and functional activities as measured by gait speed and timing of the activity. Moreover, many strength trainings designed to improve individuals' postural stability have shown
little effect on postural balance as quantified by postural
sway (18). Although these findings are supported by the
notion that the joint torques required to maintain a balanced
upright posture and to perform many daily life activities are
considerably less than the available strength in most older
adults (4), it is yet possible that the biomechanical measures used to quantify postural balance are not appropriate.
The primary goal of the present study was to address this
question: Do age-related changes in neuromusculoskeletal
systems relate to the postural responses to externally imposed postural perturbations? A total of 38 healthy subjects
were tested; their ages ranged from 21 years to 77 years.
Postural perturbations were provided to the subjects, and
their postural responses to the perturbations were examined. In this study, the postural responses to the perturbations were quantitatively characterized by the range of
movement of each of the five articulated body segments.
The supporting surface was altered to vary the difficulty of
the balance tasks. The sensory and muscular functions of
all the subjects were quantitatively assessed and then correlated with the postural responses. The magnitude of the perturbation was controlled so that all of the subjects (across a
wide range of age) could regain balance without altering
foot position. Although it can be argued that these testing
conditions do not necessarily represent a real fall situation,
they do, however, represent more common situations encountered in daily life activities when postural balance is
disturbed, such as a slight slip on the wet surface or a forward or backward shift of the upper body during reaching
and other movements. It is hoped that the results of this
study can shed light on ways of identifying people who are
more at risk for losing their balance than others.
METHOD
Subjects
A total of 47 subjects were recruited for this study. They
ranged in age from 22 to 77 years. All subjects were
screened before participating in the experiment; before testing, each subject was asked to read and sign a consent form.
The screening test assessed, among other characteristics, the
subject's general health, touch, and cutaneous vibratory perception threshold at hallux; 1st and 5th metatarsal heads;
heel, dorsiflexor, and plantarflexor strength; and dorsiflexion
and plantarflexion range of motion with both knees flexed
and extended. Table 1 summarizes all the testing items.
Based on these tests, 7 subjects were excluded according to
the preset exclusion criteria (e.g., taking centrally active
medications, drugs, or alcohol on a daily basis; having a his-
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Tablel. Subject Screening Test
Test
Blood pressure
Vestibular testing
Visual acuity
Body segment parameters
Monofilaments"
Vibration"
Range of motion0
Strength testing"1
Deep tendon reflexes
Health history questionaire
Measurement
Gaze fixation with head rest and moving
Romberg test, touch nose, tremor
Body mass, height, foot length
Circumference of head, waist, upper arm,
forearm, leg, ankle, ball of foot
Height of suprasternale, trochanterion,
tibiale
Breadth between shoulder joints, and knee
At hallux, 1st and 5th metatarsal heads,
heel
At hallux, 1st and 5th metatarsal heads,
heel
Plantarflexion and dorsiflexion (knee ext)
Plantarflexor and dorsiflexor
Patellar and archilles tendons
Taking medications on a regular basis?
Disabilities affecting walking?
Had broken bones, surgery, or injury to
lower extremity?
Arthritis?
History of neurological diseases?
Back problems?
Balance problems?
Drug/smoke/alcohol/caffeine dependence?
Heart problems?
Hypertension?
Nephropathy?
Lung problems?
Retinopathy?
Tinnitus?
Claudication?
History of falls?
Activity level?
'The monofilament test was done with the subject in the supine position
with the knee bent and the foot and leg supported. The plantar aspect of
the foot was parallel to the floor. The monofilament was applied perpendicular to the skin surface and pushed to obtain a C-shaped deformation
(first order bending). Testing was done at random sites (the hallux, 1st
metatarsal, 5th metatarsal, and heel) and frequency and a forced choice
method was used. Three Semmes-Weinstein monofilaments were used:
4.17, 5.07, and 6.10. Subjects were required to answer three of five repetitions correctly at each site in order to be assigned that number.
"The vibration perception threshold was assessed by using a fixed-frequency (60 Hz) variable amplitude vibrometer (Biothesiometer, Bio-Medical Instrument, Newbury, OH) by the method of limits protocol, with an
upper limit value of 50 V. The vibration was increased until the subject
felt the vibration; after passing this threshold, the stimulus was then
decreased until the perception of vibration disappeared. Upward and
downward limits were defined after the process was repeated three times
in random order at each of four sites: the hallux, 1st and 5th metatarsals,
and heel.
c
The dorsiflexion/plantarflexion range of motion of the ankle joint was
done with the subject in a prone position with the knee extended and the
foot over the edge of the table. The fulcrum of the goniometer was positioned over the lateral distal aspect of the lateral malleolus with the proximal arm aligned with the lateral midline of the fibula and the distal arm
aligned parallel to and above the lateral midline of the 5th metatarsal. The
joint was brought by the examiner into full dorsiflexion/plantarflexion
after instructing the subject to actively dorsiflex/plantarflex the foot.
The dorsiflexor/plantarflexor strength of the ankle joint was tested with
a hand-held dynamometer (Nicholas MMT, Lafayette Instrument, Lafayette, IN). The examiner resisted motion of the subject, with the resistance being applied to the dorsal/plantar aspect of the foot, proximal to
the metatarsophalangeal joints, while the subject was seated. The measurements were converted into torque values by multiplying the force
applied (in kg) and the distance between the point of application and the
center of rotation of the joint (in cm).
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M322
tory of musculoskeletal and neurological problems likely to
affect posture; significant visual impairment; abnormal clinical evaluation of vestibular function; significant peripheral
neuropathy; overweight; and any other characteristic^] of
the potential subject, such as mental disorders, communication problems, acute diseases, that might interfere with the
subject's correct participation in the study). Also, 2 subjects
did not complete the experiment. Detailed subject information is summarized in Table 2.
Protocol
The general experimental protocol is similar to one
described elsewhere (3). Therefore, it is only summarized
briefly in this section. During the experiment, the subject
stood quietly on a movable platform with arms folded
across the chest. The platform moved in the anterior-posterior direction of the subject with a maximum acceleration
of 8 m/s2, speed of 30 crn/s, and total displacement of 4 cm.
An example of the time trajectories of the platform acceleration, velocity, and displacement is shown in Figure 1. In
response to the platform movement, the subject was
instructed to try to maintain upright balance by moving any
part of the body except the arms. A total of 12 conditions
was tested for each subject: vision/no vision, platform mov-
Table 2. Selected Subject Information
Age
O
(year)
22
22
23
24
24
25
25
26
28
38
39
40
40
44
45
45
51
52
60
61
62
62
63
67
69
69
70
71
71
72
72
73
73
73
74
75
76
77
77
No.
Sex
2
6
4
3
11
5
20
7
19
35
34
40
46
29
27
37
32
14
23
25
22
30
45
10
16
47
39
9
12
17
26
24
31
41
28
13
21
18
36
M
F
cc
M
M
M
F
F
M
M
F
M
F
F
M
M
M
M
F
F
M
M
M
F
F
F
F
F
F
M
F
M
M
M
F
F
F
M
F
M
Mass
(kg)
Height
(cm)
Foot
(cm)
72.2
53.0
68.4
74.5
72.0
68.7
60.9
66.8
94.4
73.2
91.4
68.4
61.5
81.3
84.5
93.0
80.0
65.2
72.9
78.1
71.9
73.0
79.5
62.4
73.2
87.1
86.1
48.3
80.0
84.0
74.1
68.3
70.7
85.4
59.1
76.8
65.6
70.1
83.3
170
157
172
175
170
175
169
171
189
168
185
168
161
177
183
171
170
165
182
174
161
158
172
155
160
178
170
153
172
165
181
153
167
179
157
167
165
167
175
26.5
22.6
25.6
26.9
25.6
24.0
25.3
26.2
28.4
25.6
28.3
25.0
25.0
28.0
25/4
25.8
26.4
25.7
27.9
27.5
23.7
24.5
24.7
23.0
24.6
28.0
27.4
21.7
26.6
26.3
27.0
24.2
26.6
26.4
25.3
27.0
24.9
27.7
27.5
.09
-.26
.05
Vibration
Perception Threshold
(voltage)
Monofilament
(3 types)
Range of
Motion
(deg)
Strength
(kgXcm)
R3
R4
Rl
R2
R3
R4
DF
PF
PF
DF
1
1
2
1
2
2
1
1
1
1
2
1
2
2
1
1
1
1
2
1
2
1
2
2
1
1
2
1
2
2
2
1
1
1
2
2
2
2
1
1
1
2
1
4
4
4
5
6
4
5
5
6
7
9
5
5
8
19
10
7
7
49
13
21
9
4
5
14
15
18
6
7
35
22
17
33
39
18
11
15
24
9
3
4
3
4
4
3
4
3
4
3
4
3
4
5
7
6
7
4
5
8
21
6
4
10
9
15
13
10
8
40
13
24
26
36
13
10
18
11
40
2
3
3
3
4
4
4
3
4
3
5
3
3
5
4
8
5
4
38
5
19
5
5
5
7
9
10
5
8
19
8
21
20
25
14
6
10
8
48
2
3
3
3
4
4
4
4
4
2
4
2
4
5
8
5
6
4
50
7
10
5
7
4
5
6
6
8
13
37
18
22
32
39
10
5
6
9
5
12
10
15
12
20
10
10
19
8
15
12
20
10
12
14
15
12
12
12
11
9
10
9
14
8
10
15
15
8
10
10
12
13
15
15
13
13
12
12
42
35
56
48
32
44
48
47
48
33
51
38
45
48
40
35
48
38
48
58
30
43
40
38
48
60
30
40
45
51
40
45
50
32
50
52
40
42
41
255
177
192
280
220
209
207
203
218
243
258
261
215
212
189
197
156
204
223
161
198
117
247
213
153
222
155
76
96
118
238
104
99
162
99
114
175
139
95
283
147
198
225
265
120
259
211
227
143
272
288
192
228
219
224
126
191
232
241
193
134
176
131
207
158
189
129
191
290
187
204
142
203
101
250
232
220
135
.19
.19
.33
.45
.54*
.64*
.51*
.43*
-.18
.01
-.64*
-.26
Rl
R2
1
1
1
;>
:I
1
1
1
2
1
1
1
1
1
1
I
2
1
2
1
Notes: Rl, R2, R3, and R4 are hallux, 1st metatarsal head, 5th metatarsal head, and heel, respectively. PF and DF, plantarflex and dorsiflex, respectively,
cc, Correlation coefficient with age.
•Significant correlation with age atp < .05.
AGE-RELATED PHYSIOLOGICAL CHANGES AND POSTURE
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regions. However, only half of the subjects older than 60
years of age increased the sensation threshold, and none of
these subjects exceeded the no. 2 filament. The most significant changes were observed in the cutaneous vibratory perception threshold at the plantar surface of the foot and
ankle strength. With an increase in age, the cutaneous
vibratory perception thresholds were increased (correlation
coefficient [cc] = .46-.60), especially for those subjects
older than 70 years of age, and the ankle strengths were
reduced, especially for the plantarflexors (cc = -.64).
ace.
[m/s/s]
vel.
[m/s]
disp.
[m]
Time [x 1/120 sec]
Figure 1. Illustration of time trajectories of platform acceleration,
velocity, and displacement. Time zero is the onset time of the platform
movement.
ing forward/backward, and standing on hard (or normal)/
soft (or foam)/reduced (or half-shoe-sized wooden block
located under the heel) surfaces. All these conditions were
provided randomly in a 4 X 3 block design; that is, 4
blocks of randomized vision and movement direction conditions for each of the 3 blocks of randomized surfaces.
The response of each subject to the platform movement
was quantified by the rotation of the foot, shank, thigh,
trunk (with arms fixed to the trunk), and head in the sagittal
plane, respectively. These rotations were derived based on
the direct measurement of segmental velocity and acceleration by a set of integrated kinematic sensors (IKS). Each
IKS included one angular rate sensor (Watson Industries,
Inc., Eau Claire, WI) and two linear accelerometers (Kistler
Instrument Co., Amherst, NY). The dynamic range of the
IKS was larger than 100 Hz, which allowed the capture of
the relatively small but rapidly changing movement of the
body during the dynamic postural task. The outputs from
the IKSs were digitized at 120 Hz for 3 s.
The maximum peak-to-peak magnitude of each body
segmental movement was first computed for each trial. The
mean values of the last three trials for each of the 12 conditions were obtained. Previous analysis indicated nonsignificant difference between two visual conditions. Therefore,
the results of these two conditions were combined. These
mean values were then correlated with the following variables: age, ankle strength, ankle range of motion, and cutaneous vibratory perception threshold, respectively.
RESULTS
Age-Related Changes in Neuromusculoskeletal Systems
As seen from the data in Table 2, there were no significant age-related changes (p > .05) in whole body mass,
body height, foot length, ankle range of motions, and
monofilament test at hallux and 1st and 5th metatarsal
heads. The monofilament sensation at the heel region
showed a more noticeable change with age than the other
Postural Responses
Because the results in Table 2 revealed significant agerelated changes in cutaneous vibratory perception threshold
and ankle strength only, the maximum peak-to-peak magnitudes of body movement were correlated only to those variables as well as age. The correlation coefficients are listed
in Table 3.
First, age had a significant positive effect on the head
movement in all conditions. That is, with an increase in
age, the head tended to move in a larger range. Comparing
the postural responses between two movement directions of
the platform, it seemed that the backward movement was
more affected by age (cc = .48, .42, and .43 on normal, soft,
and reduced surfaces, respectively) than the forward movement (cc = .42, .32, and .31, respectively). A scattered plot
of head movement with respect to age is shown in Figure 2.
In addition, when standing on a normal surface, trunk movement also showed a positive correlation with age (cc = .43
and .32 for forward and backward movement, respectively).
Second, ankle strength also showed a significant influence on head movement. In particular, head movement was
mostly correlated with the plantarflexor strength when the
platform movement was in the backward direction (cc =
-.50, -.44, and -.44 on normal, soft, and reduced surfaces,
respectively). Surprisingly, when the platform moved in the
forward direction, this relation was much poorer (cc = -.34,
-.17, and -.24, respectively). Nevertheless, the weaker the
plantarflexor strength, the larger the head movement. Figure 3 illustrates a set of scattered plots of head movement
with respect to plantarflexor strength. Moreover, when
standing on a reduced surface and moving backward, foot
movement showed a significant correlation with plantarflexor strength (cc = -.33), and head movement was also
correlated with dorsiflexor strength (cc = -.32).
Third, although there was a significant age-related change
in the cutaneous vibratory perception threshold in the foot,
its effect on body movement was weak. The only significant effect was found from the cutaneous vibratory perception threshold at the 5th metatarsal head. With an increase
in the cutaneous vibratory perception threshold at the 5 th
metatarsal head, head movement increased when standing
on either normal or soft surfaces with the platform moving
in the backward direction (cc = .34 and .32, respectively),
and thigh movement was increased when standing on a
reduced surface only (cc = .38).
Finally, regardless of the surfaces on which the subjects
stood and the movement directions of the platform, the
range of movement of the trunk, thigh, shank, and foot had,
in general, very poor correlation with age, cutaneous vibra-
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M324
Table 3. Correlation Coefficient of Maximum Peak to Peak Movement of Head (Hd), Trunk (Tk), Thigh (Th),
Shank (Sk), and Foot (Ft), With Age, Vibration Perception Threshold and Strength
Forward Platform Movement
Surface
Variable
Hd
Tk
Th
Sk
Ft
Hd
Tk
Th
Sk
Ft
iNormal
Age
Vib@Rl
Vib@R2
Vib@R3
Vib@R4
PFstr
DFstr
.42*
.13
.16
.19
.11
-.34*
-.05
.43*
.17
.13
.14
-.02
-.18
-.01
-.07
.21
.20
.29
.22
.05
.02
-.09
.13
.07
.12
.08
.11
-.07
.16
-.02
.08
.14
-.07
-.02
.08
.48*
.22
.25
.34*
.25
-.50*
-.18
.32*
.09
.17
.18
.09
-.30*
-.04
-.17
.09
.09
.19
.22
.02
.12
.06
.17
.18
.29
.21
-.04
-.09
.14
-.13
-.01
.11
.00
-.23
-.04
Soft
Age
Vib@Rl
Vib@R2
Vib@R3
Vib@R4
PFstr
DFstr
.32*
.16
.16
.17
-.11
.11
.09
.16
.20
-.08
-.02
-.09
-.13
-.07
-.22
.24
.07
.15
.14
-.18
-.04
-.01
.07
.13
.05
-.44*
-.14
-.19
-.09
.22
.05
-.10
-.13
-.07
-.04
-.02
.18
-.03
.17
-.19
-.05
-.04
.26
.05
-.17
-.01
-.09
-.02
-.07
.23
.04
.42*
.24
.25
.32*
-.17
-.05
.25
.03
.07
.06
-.09
.05
-.06
-.17
-.27
Age
Vib@Rl
Vib@R2
Vib@R3
Vib@R4
PFstr
DFstr
.31*
.00
.03
.05
.04
-.24
-.18
.00
-.15
-.16
-.15
-.24
.04
-.17
.04
.20
.26
.38*
.20
.02
-.13
-.01
.08
.06
.13
.02
.14
-.10
.02
-.07
-.07
.03
-.15
-.20
.12
.43*
.14
.19
.21
.24
-.44*
-.32*
.13
-.04
.00
-.06
-.02
-.17
-.14
.01
.17
.26
.25
.28
-.14
-.01
.17
.05
.16
.12
.08
-.20
-.08
.10
.05
.22
.23
.16
-.33*
.17
Reduced
/
Backward Platform Movement
.10
.11
.25
.00
t Notes: Vib@Rl, jjibration perception threshold at Rl (as defined in Table 2); PF/DF str, plantarflexor/dorsiflexor strength.
*Significant correlation at p < .05.
NF
cc = .42
u.o
o
NB
cc = .48
0°
0
0
0
c
°oo
o
o
0
-%—£~"
0
5 c . oo
• c'
o
%
0
0
Sr
0.6
0
cc = .32
oo
<b c?
_fl_ — & o
ooo
o
ft°
0
oo
•
_ -
0
RF
0.6
KB
„ d?
o
"Co"—-8—
n
o
0
0
o °
0
cc = .43
cc=.31
0
v°80
-
20
o
0
0
o
o
%
0
o
0
0
8 o
80
Age [year]
300
50
Plantarflexor strength [kgcm]
Figure 2. A set of scattered plots of head movement with respect to age
for standing on normal (N), soft (S), and reduced (R) surfaces and the
platform moving in either forward (F) or backward (B) directions.
Figure 3. A set of scattered plots of head movement with respect to the
plantarflexor strength for standing on normal (N), soft (S), and reduced
(R) surfaces and the platform moving in either forward (F) or backward
(B) directions.
tory perception threshold, and ankle strength (except for a
few cases).
include pathologies, gait speed, peripheral sensation, and
strength. However, most of the laboratory-based studies
have not been able to demonstrate a significant effect of
muscular strength on postural balance (17,18). The reason
might be partly due to the biomechanical measures used to
define postural balance such as static posturography and
muscular activities from a limited number of muscles.
DISCUSSION
There have been many studies in the past that attempted
to understand the intrinsic risk factors that contribute to
falls in the elderly population (19). Some of these factors
AGE-RELATED PHYSIOLOGICAL CHANGES AND POSTURE
This study used an approach that was different than the
traditional measures. Instead of examining either the excursions of the center of pressure under the foot or the muscular response latencies, the postural movement of each of the
individual body segments (i.e., head, trunk, thigh, shank,
and foot) was examined. It is believed that this approach
can provide in-depth information on how whole body balance is maintained through the coordination of individual
body segments and therefore is more sensitive to the subtle
changes in postural stability arising from the changes in the
postural control system (20).
The subject pool in this study was selected based on a set
of rigorous exclusion criteria to ensure a healthy population
over a wide range of ages. Nevertheless, this pool of subjects has shown a significant age-related increase in vibratory perception threshold of the plantar surface of the foot.
This is consistent with the studies by Kokman et al. (21)
and by Bloom et al. (22). Another significant age-related
change has been found in plantarflexor strength but not in
dorsiflexor strength. This is also consistent with a study by
Sepic et al. (23), who reported a reduction in plantarflexor
strength in older women compared to young women but
similar dorsiflexor strength for both groups.
The main question that this study is intended to address is
whether age-related changes in sensory and musculoskeletal
systems relate to the balance of upright posture. The results
presented here have shown that even though both cutaneous
vibratory perception threshold and ankle strength are changed
significantly with age, it is plantarflexor strength that most
significantly correlates with the postural movement of the
body in response to a sudden platform movement. Moreover, this relation is sensitive to the direction of the platform
movement. It is only when the platform moves backward,
which requires mainly the posterior muscles to stabilize the
upright posture, that a declined plantarflexor strength becomes significant in altering the range of body movement.
This finding indicates that maintaining upright balance in
response to a forward platform movement may not require
as much posterior torque as a forward platform movement
does; therefore, a sudden backward foot disturbance, such as
tripping, is most dangerous for the healthy elderly person
whose plantarflexor strength has declined more than his or
her dorsiflexor strength. In fact, this finding is supported by
the results of other studies, which showed that posterior
muscles are mainly inactive in response to a sudden forward
disturbance of the foot, whereas they are active when the
foot is disturbed backward (24,25).
It is surprising to find in this study that the age-related
change in ankle strength does not relate to the movement
magnitude of the lower body segments but instead is
related to the head. This result suggests that (i) stabilizing
the head during a dynamic posture is not only controlled by
the neck muscles (20) but also is associated with the muscles from the distal parts of the body, such as the ankle
plantarflexors; and (ii) the stability of the head may represent the overall stability of the upright balance. This second
notion is supported by the work of Kilburn et al. (26), who
found that the head tracking measurement produced results
equivalent to the force platform measurement for a group
of individuals who are posturally unstable.
M325
In addition to the plantarflexor strength, age is another
factor that has a significant relation with movement of the
head. Although different from strength, the age effect exists
regardless of the supporting surfaces and the platform
movement directions. This difference may suggest that
there are possibly other age-related factors (which are not
investigated in this study) that may relate to postural balance, especially for a forward movement of the platform.
These factors may include reaction time, proprioception
function, and central control of balance. Further studies are
needed to address these issues.
Cutaneous vibratory perception threshold also has a positive, although weaker, relation with the head movement.
This is consistent with the findings by Era et al. (13), who
found that lower vibration sense is one of the factors for
poor balance ability in the elderly population. However, the
results of this study further indicate that when standing on
either a hard or soft surface and when the platform moves
in the backward direction, it is the vibratory perception at
the 5th metatarsal head that affects head movement. Moreover, when standing on a reduced surface that was supporting the posterior portion of the foot (i.e., the heel), the vibratory perception at the heel becomes effective (although
the effect is weak). This finding suggests that maintaining
upright balance during a dynamic posture is associated
more with dynamic loading information at the lateral-anterior portion of the foot than at both the medial-anterior and
the posterior portion of the foot. The dynamic loading at
the heel is not so important unless it is the only part of the
foot that contacts the ground. Moreover, as the results
show, the relation between cutaneous vibratory perception
threshold increases with age, and postural movement is
much weaker than either age or plantarflexor strength, indicating that it does not play a primary role in maintaining
postural balance. This finding is consistent with an earlier
simulation study by Wu and Zhao (27), who concluded that
the use of cutaneous mechanoreceptive information from
the foot is not necessary, although helpful, to maintain a
standing balance.
In summary, the aim of this study was to identify some
critical factors whose functional changes with age would
potentially relate to the maintenance of a standing balance.
The results showed significant age-related changes in cutaneous vibratory perception threshold at all four locations of
the foot (cc = .46 to .60), plantarflexor strength (cc = -.64),
and touch sensation at the heel region. Although postural
movements of the body had significant correlations with
age regardless of the supporting surfaces and the movement
directions of the platform (cc = .31 to .48), they were significantly affected by plantarflexor strength only when the
platform moved in the backward direction (cc = -.44 to
-.50). Moreover, the postural responses of the body were
weakly correlated with the change in cutaneous vibratory
perception threshold with age.
ACKNOWLEDGMENT
This work was supported in part by National Institutes of Health grant
1R29AG11602-01A2 and by a grant from the Whitaker Foundation.
I thank Mary Becker for testing subjects and Weifeng Zhao and Tracy
Spaulding for assistance in data collection and analysis.
M326
WU
Address correspondence to Ge Wu, PhD, Department of Physical Therapy, University of Vermont, 305 Rowell Building, Burlington, VT 05405.
E-mail: [email protected]
15.
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Received December 13, 1996
Accepted December 20, 1997