Journal of Gerontology: MEDICAL SCIENCES
1999, Vol. 54A, No.6, M288-M292
Copyright 1999 by The Gerontological Society
0/ America
A Cross-Cultural Comparison of Neuromuscular
Performance, Functional Status, and Falls
Between Japanese and White Women
JamesW. Davis,' Michael C. Nevitt," Richard D. Wasnich,' and Philip D. Ross'
'HawaiiOsteoporosis Center, Honolulu, Hawaii.
2Departments of Medicineand Epidemiology and Biostatistics, University of California, San Francisco.
Background. Previous studies have reported that the incidence of falls among Japanese women is about half that of white
women. The difference in incidence might result from differences in neuromuscular performance, such as muscle strength, mobility, and balance. This hypothesis was tested by comparing two community-dwelling populations: Japanese women in the
Hawaii Osteoporosis Study, and Caucasian women in the Study of Osteoporotic Fractures.
Methods. Neuromuscular performance was assessed for women in the two cohorts using standardized procedures. Falls
were monitored longitudinally, using surveys mailed at 4-month intervals.
Results. The Japanese and white women differed substantially in their neuromuscular performance. The Japanese women
had faster walking speeds and chair stands, and performed better on a series of balance tests. The white women had greater
strength, particularly at the quadriceps, and faster hand and foot reaction times. The white women also reported fewer functional disabilities, including fewer difficulties in climbing steps, doing heavy housework, and shopping for groceries. In ageadjusted analyses, the risk of falls was greater for the white women [odds ratio (OR) = 1.8; 95% confidence interval (CI) = 1.6,
2.0]. After adjusting for the neuromuscular test results and the number of functional disabilities, the odds ratio for the risk of
falls remained essentially the same (OR = 1.8; 95% CI = 1.5,2.1).
Conclusions. The Japanese and white women had different advantages and limitations in neuromuscular performance.
These differences, however, did not explain the lower risk of falls among Japanese women.
F
OR predominately white populations above age 65, the
annual incidence of falls reaches 30% or higher (1-4). The
incidence may be lower for Japanese: studies of Japanese community-dwellers report about half the incidence of falls as in
studies of community-dwelling whites (5-7). Methodological
differences across studies, however, make comparisons of incidence rates uncertain, although one comparison of white and
Japanese nursing home residents did standardize the ascertainment of falls (6). The white residents reported four times the incidence of falls compared to Japanese.
In this article we describe comparisons between two community-dwelling populations, Japanese women in the Hawaii
Osteoporosis Study (HOS) and white women in the Study of
Osteoporotic Fractures (SOF). Methods at the Hawaii site were
standardized against the procedures of the SOF, including ascertainment of falls and assessment of functional status and neuromuscular performance. Neuromuscular deficits such as muscle
weakness, limited mobility, and poor balance have been associated with the risk of falls in longitudinal studies (1-3,8-12).
The objective of the comparison was to measure the relative
risk of falls, and to see if differences in functional status and
neuromuscular performance could explain the differing risks of
falls between Japanese and white women.
METHODS
Subjects
The SOF recruited women from 1986 to 1988 who were able
to walk independently and who were at least 65 years of age
M288
from centers in Portland, Oregon; Minneapolis, Minnesota;
Baltimore, Maryland; and the Monongahela Valley, Pennsylvania (13). The women were identified by voter registration and
other lists and contacted through mailings. Women who had
undergone bilateral hip replacement were excluded. The study
group consisted of 9,704 nonblack women. For comparisons to
the HOS, 15 Asian women were also excluded. The mean age
of the women included in the analyses was 72 ± 5 (SD) years.
Their mean height was 159 ± 6 (SD) cm, and their mean weight
was 66.4 ± 12.5 (SD) kg.
The subjects from the HOS were 705 women who participated at the eighth examination conducted from January 1992
through September 1994. The HOS is an extension of the
Honolulu Heart Program (HHP), which originally invited all
noninstitutionalized men of Japanese ancestry to participate, If
born from 1900 through 1919, and living on the island of
Oahu, Hawaii (14). A total of 8,006 men (71.8% of the eligible
population) were examined by the HHP between 1965 and
1968. A random sample of the surviving men was invited to
join the first examination of the HOS in 1981. Wives, if of pure
Japanese ancestry, were also invited; of 1,534 study-eligible
wives, 72.0% participated. The derivation of the HOS from the
HHP has been published in detail (15). At the eighth examination of the HOS, 77% of the surviving women attended. The
analyses included the 690 women who were at least age 65 at
the eighth examination. Their mean age was 75 ± 5 (SD) years.
Their mean height was 150 ± 6 (SD) em, and their mean
weight was 52.9 ± 9.4 (SD) kg.
FAUS IN JAPANESE AND WHITE WOMEN
Examinations
Weight was measured on a standard balance beam scale, and
standing height was measured using a wall-mounted stadiometer. Corrected visual acuity was tested following a published
procedure (16). The strength tests and performance-based neuromuscular measures were assessed using methods from the
SOF (17,18). Maximum grip strength in the dominant hand was
measured using a hydraulic grip strength dynamometer. The isometric strength of the quadriceps (knee extensors) was measured
using a leg extension chair designed for strength testing; the subject, from a sitting position, pushed with the leg on her dominant
side for 4 seconds, with maximum effort, against a padded bar
attached to a strain gauge with an electronic monitor that displayed the average force. Triceps strength was measured with a
hand-held dynamometer; the subject, lying face up on an examination table, elbow flexed at 90°, pushed back against the dynamometer, which was pushed against the subject by the technician. The peak force attained was automatically recorded by the
dynamometer. Neuromuscular performance was assessed using
a series of tests. Chair stands were measured as the time to stand
up from a standard chair five times; the women were asked, if
possible, to not use their arms for assistance.
Walking speed was based on seconds to cover a 6-meter distance, first at a usual walking pace, and subsequently walking as
fast as possible while still feeling safe. The tandem walk was
recorded as the time to complete a 2-meter course, walking heel
to toe along a line marked on the floor. Hand and foot reaction
times were measured as the time, using two switch pads, for the
subject to respond to a stimulus light by moving her hand or foot
from the first pad (initially depressed), and to depress the second
pad (the time was recorded electronically); reaction times were
calculated as the average of 9 trials. For semi- and full-tandem
balance tests, the women were asked to stand with the side of
the heel of one foot touching the side of the great toe of the other
foot (semi-tandem), or with the heel of one foot in front of and
touching the toes of the other foot (full tandem). The procedures
were repeated first with eyes open and second with eyes closed,
testing the women's ability to hold the positions for 10 seconds.
For the SOF: quadriceps strength, walking times, and reaction
times were measured at a second visit held 2 years after the first,
attended by 8,134 women, representing 86% of those surviving.
M289
Functional status was assessed by questionnaire; the women
were asked if they had difficulty with six common activities
such as climbing steps or preparing their own meals. The questionnaire also included questions about how many times per
week they did activities long enough to work up a sweat, how
many times per week they went out of their house or residence,
and how many days per month they left their neighborhoods.
Falls.-A fall was defined as "when you land on the floor, or
other lower level (such as stairs, or a piece of furniture), by accident." During the 2 years after examination, subjects were
asked about falls by mail questionnaire at 4-month intervals.
Data analysis.-Continuous variables were analyzed using
multivariable regression analysis. Associations of functional status by cohort, stratified by age groups, were analyzed using the
Mantel Haentzel extension test. Except for falls, other analyses of
binary variables were performed using logistic regression. Falls
were analyzed using pooled logistic regression (19), using a separate record for each 4-month interval that was monitored for
falls. The outcome was having a fall during the 4-month period.
REsULTS
In age-adjusted analyses, white women in the SOF had nearly
double the risk of falls of Japanese women (OR = 1.8,95% CI =
1.6, 2.0). The neuromuscular performance of the women in the
two cohorts was compared to identify possible explanations for
the difference in risk. The Japanese and white women differed in
grip, triceps, and quadriceps strength, and on six other tests of
neuromuscular performance including chair stands, walking
performance, and hand and foot reaction times (Table 1). The
white women had greater strength and shorter hand and foot reaction times; the Japanese women had better performance on
the other neuromuscular tests. Figure 1 illustrates how the differences persisted across the common age span of the cohorts for
four characteristics. Except for grip strength, the differences remained significant after adjustment for age, height, and weight.
Differences in strength were reduced 41 %-86% in the age-,
height-, and weight-adjusted models compared to the age-adjusted models. The Japanese women also performed better in
balance tests; despite their greater mean age, a higher proportion
Table1. Differences in Strengthand Neuromuscular Performance BetweenJapaneseWomenin the HawaiiOsteoporosis Study(HOS)
and White Womenin the Studyof Osteoporotic Fractures(SOF)
Mean ± StandardDeviation
Strengthor PerformanceMeasure
Grip strength(kg)
Tricepsstrength(kg)
Quadricepsstrength(kg)
Chair stands (sec)
Tandemwalk (sec)
Usual walkingspeed (m/sec)
Rapid walkingspeed (m/sec)
Hand reactiontime (sec)
Foot reactiontime (sec)
SOF
HaS
21.6 ± 4.6
10.5 ± 2.7
67.8 ± 27.6
12.5 ±4.7
12.6 ± 5.9
0.904 ± 0.220
1.28± 0.30
0.357 ± 0.091
0.366 ± 0.092
19.3±4.2
9.38 ± 2.12
35.7 ± 17.2
8.86± 2.93
8.9 ±4.4
1.07±0.26
1.44± 0.33
0.440 ± 0.102
0.428 ± 0.122
Difference ± StandardError*
Age-Adjusted
1.55± 0.18t
0.902 ± 0.107t
28.0 ± i.i4.31 ± O.17t
4.50± 0.23t
-0.213 ± 0.OO8t
-0.210 ± 0.011t
-0.0703 ± 0.OO36t
-0.0497 ± 0.OO380t
Age-, Height-,Weight-Adjusted
0.22 ±0.19
0.275 ±0.110t
16.6± 1.13t
3.07 ± 0.18t
4.84 ± 0.25t
-0.219 ± O.OO9t
-0.211 ± 0.012t
-0.0616 ± 0.OO392t
-0.0475 ± 0.0041t
*Differences between the study cohorts in multivariableregressionmodels,which included strengthor performancemeasuresas the dependentvariables,and cohort and age, or cohort, age, height, and weightas independentvariables. Positivesigns indicategreatervaluesfor the SOp.
tStatistically significantdifference(p < .05) betweenthe SOF and HaS.
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Figure 1. Age-specific comparisons of quadriceps strength, chair stands, usual walking speed, and hand response times between Japanese women in the Hawaii
Osteoporosis Study (open circles) and white women in the Study of Osteoporotic Fractures (closed circles). Bars represent standard deviations.
completed both the semi- and full-tandem tests (Figure 2). In logistic regression analyses, the cohort differences on the four balance tests remained statistically significant after adjusting for
age, height, and weight (P < .001). Comparing white to Japanese
women, the odds ratios for failure to complete the semi- and
full-tandem balance tests with eyes closed were 2.4 (95% Cl =
1.9, 2.9) and 3.2 (95% CI = 2.6, 4.0).
Comparisons were extended to measures of functional status; of six items examined by age stratification, the Japanese
women more often reported difficulty with four items: climbing
up stairs, climbing down stairs, doing heavy housework, and
shopping for groceries (Table 2). In a logistic regression model
adjusting for age, height, and weight, the difference in shopping
for groceries became nonsignificant (OR = 0.8, 95% CI = 0.6,
1.1). In a model further adjusted for quadriceps, grip, and triceps strength, the difference in doing heavy housework also became nonsignificant (OR = 1.0, 95% CI =0.8, 1.3).
The extent that differences in body size, neuromuscular performance, and functional status might explain the differing risks
of falls was examined in subsequent analyses (Table 3). The
Table 2.Age Group-Specific Comparisons of Difficulties
WithActivities Requiredfor Independent LivingBetweenJapanese
Womenin the HawaiiOsteoporosis Study(HOS)and White
Women in the Study of Osteoporotic Fractures(SOP)
Percent With Difficulty
by Age Group
Activity
Cohort
65-69 70-74 75-79
80+I
Difficulty walking 2 or 3 blocks
HOS
SOF
11.4
9.3
11.3
13.1
16.0
18.4
28.6
27.0
Difficulty climbing 10 steps
HOS*
SOF
20.2
12.4
20.4
17.4
24.2
20.7
45.5
26.9 ;
Difficulty walking down 10 steps
HOS*
SOF
14.9
9.5
15.3
12.5
16.9
15.9
36.4
21.5
Difficulty with heavy housework
HOS*
SOF
31.6
26.0
42.6
32.4
40.6
40.5
57.1
54.1
Difficulty shopping for groceries
HOS*
SOF
9.7
4.8
6.6
6.1
11.0
9.4
23.4
17.2
Difficulty preparing your own meals
HOS
SOF
2.6
2.1
3.3
3.2
4.1
4.0
18.2
9.1
*Significant (p < .05) differences between the HOS and SOF cohort after
adjustment for age group.
FALLS IN JAPANESE AND WHITEWOMEN
100 - , - - - - - - - - - - - - - - - - - - - - - - - ,
M291
Table 3. Odds Ratiosand95%Confidence Intervals (as) forFalls
Comparing White Women in the Study of Osteoporotic Fractures
(SOP) to Japanese Women in the Hawaii Osteoporosis Study(HaS)
Covariates in Model
OddsRatio(95%CI)*
Age
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1.8(1.6,2.1)
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2.1 (1.8,2.4)
Abovevariables plushandreaction time
2.2 (1.9,2.5)
Abovevariables pluschairstands, tandemwalk,usualwalking speed
1.7(1.5,2.0)
Abovevariables plussemi-tandem eyesclosed,full tandemeyesclosed 1.6(1.4,1.9)
Abovevariables plusnumberof difficulties withactivities of dailyliving 1.8 (1.5,2.1)
*Oddsratiosgreater than 1.0indicate a greaterriskof falls for womenin the SOF
comparedto women in the HOS.The specificactivities of dailylivingare listedin
Table2.
Figure 2. Comparisons of performance on tandem balance tests between
Japanese women in the Hawaii Osteoporosis Study (open bars) and white
women in the Study of OsteoporoticFractures(shaded bars). *p < .05.
odds ratio with adjustment for age, height, and weight (1.8) was
identical to that with age adjustment alone; but the odds ratio
increased to 2.1 with the addition of strength measures to the
models, and to 2.2 with the addition of hand reaction times.
Further adjustment for the Japanese advantages in chair stands,
tandem walk, and usual walking speed reduced the odds ratio
(OR = 1.7); adjusting for differences in tandem balance lowered the ratio slightly more (OR = 1.6). A further adjustment
for the number of difficulties with activities required for independent living, in which the white women reported fewer difficulties (Table 2), increased the OR to 1.8. Additional variables
were evaluated when added singly to this model. The odds ratios for the risk of falls with the added variable included were,
for visual acuity, 1.7 (95% CI = 1.4,2.0); for hours per week of
activity causing sweating, 1.8 (95% CI = 1.6, 2.1); for how
often they left their neighborhood, 1.8 (95% CI = 1.5,2.1); and
for how often they left their house.L? (95% CI = 1.5,2.0).
Of the lifestyle questions, the Japanese women had been less
likely to engage in activities long enough to work up a sweat
even once a week (55% vs 64%), less likely to leave their residence one or more times per day (45% vs 87%), and less likely
to leave their neighborhoods once a day or more (27% vs 44%).
The lifestyle differences were statistically significant, and remained significant after adjusting for age (p < .001).
The Caucasian women in the SOF were more fearful of falling,
despitetheir younger mean age. For the SOF,45.8% of the women
expresseda fear of falling;for the HOS (Japanese women), 24.6%.
In a logisticregression model comparing the white to the Japanese
women, the difference in the fear of falling remained significant
after adjustingfor age (OR =2.9, CI =2.4,3.5).
DISCUSSION
The white women in the SOF had nearly twice the risk of
falls of the Japanese women of comparable age in the HOS (OR
=1.8, 95% CI =1.6, 2.0). The white women were also nearly
twice as likely to report a fear of falling as the Japanese women
(45.8% vs 24.6%). Neuromuscular performance and functional
status were examined as possible contributing factors to the dif-
fering risks of falls. Although the white and Japanese women
differed in several respects, the differences did not consistently
favor one group over the other. The Japanese women had faster
usual and rapid walking speeds, and required less time to complete the tandem walk. The Japanese women also exhibited better balance on a series of tests despite their older average age.
The Japanese women, however, took longer to perform the
hand and foot reaction time tests. The white women also had an
advantage in strength, only partially explained by their larger
body size. At the quadriceps the white women maintained 50%
greater strength than Japanese women of comparable age,
height, and weight. Nonetheless, the Japanese women took significantly less time to complete five chair stands, an indicator of
lower extremity strength.
Differences in functional status generally favored the white
women. The Japanese women more often reported difficulties
going up or down stairs, doing heavy housework, and shopping
for groceries. Except for managing stairs, however, the differences were not statistically significant after adjusting for age,
height, weight, and strength.
Neuromuscular performance and functional status could not
fully explain the differing risks of falls. The age-adjusted OR of
1.8 (95% CI = 1.6, 2.0) was actually increased to 2.1 (95% CI =
1.8, 2.4) after adjusting for the greater height, weight, and
strength of the white women. Adjustment for the Japanese advantages in chair stands, tandem walk, usual walking speed, and
balance reduced the odds ratio to 1.6 (95% CI =1.4,1.9). The
better neuromuscular performance of the Japanese women reflected in these tests appears to have reduced their risk of falling.
A final adjustment for the number of difficulties with activities
required for independent living-an advantage of the white
women-increased the odds ratio to 1.8 (95% CI = 1.5,2.1).
Thus, the differences in the risk of falls remained after adjusting
for both functional status and neuromuscular performance.
Factors other than neuromuscular performance and functional
status may also contributeto differences in the risk of falls, including stroke, Parkinson's disease, arthritis, chronic lung conditions,
certain medications,cognitiveimpairment, affective disorders, and
home or environmental hazards (1,2,9,10,12,20-22). Asian
women have a longer life span than white women (23,24), and
M292
DAVIS ITAL.
differences in diseases or other factors that are difficultto evaluate
might explain the lower risk of falls compared to white women.
Beyond risk factors, risk taking also deserves consideration
as a possible explanation of the observed risk differences. The
Japanese women may not place themselves at risk of falling as
often as the white women. In the present study, the Japanese
women left their homes and neighborhoods less often than the
white women, suggesting behavioral dilferences. Social supports and care from family or friends may also differ between
Japanese and white women, and influence the frequency of risk
taking. Anecdotal information suggests that elderly Japanese in
Japan and Hawaii are less likely to leave their homes unaccompanied, but we do not have such measures to compare.
Among other explanations, differences in measuring falls
might explain the observed difference in the risk of falls.
Although a strength of the study is the standardization of techniques, better recall by white women or a reluctance to report
falls by Japanese women could yield apparent differences in risk.
One study examining recall of falls found that a third of falls
identified by regular monitoring were forgotten on contact after 3
to 6 months (25). Both the HOS and SOF studies contacted participants at 4-month intervals. Differences in recall would have to
be substantial, however,to explain the twofold differencesin risk.
The apparent differences in neuromuscular petformance and
functional status may also reflect methodological differences
between the cohorts. Although techniques were standardized,
tests for strength, walking speed, and other neuromuscular abilities depend upon the efforts of the participants. Greater exertion by white women in the strength tests, or greater effort by
the Japanese women in the walking and other tests, could yield
measurable differences in petformance. Functional status was
assessed by a questionnaire that asked about difficulties with a
series of activities required for independent living. Whether a
woman perceived she had difficulty may depend upon comparisons to her peers, which would differ between the cohorts.
Cultural differences may affect theperception of difficulty,
In summary, Japanese women in the HOS had about half the
risk compared to falls of white women of similar age in the
SOP. The differences in risk were comparable to previously reported differences for community-dwelling elderly Japanese
and white populations (5,6), although previous studies lacked
standardized methodology. The differing risks between women
in the HOS and SOF, however, could not be explained by differences in body size. functional status, or neuromuscular performance. Other factors, such as differences in health, disease,
medication use, risk taking, or social supports, may contribute
to the observed difference in the risk of falls.
The contrasts in neuromuscular performance between
women in the HOS and SOF are of interest in their own right:
the greater strength and shorter reaction times for the white
women and the faster usual and tandem walking speeds, chair
stands, and better balance for the Japanese women. The results
suggest that Japanese and white women face aging, and the risk
of falls, with differing advantages and limitations.
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ACKNOWLEDGMENTS
This study was supported in part by grant AG10412 from the National
Institutes of Health, National Institute on Aging, under the auspices of the
Pacific Health ResearchInstituteand the Hawaii OsteoporosisFoundation.
Address correspondenceto Dr.James W. Davis, Hawaii OsteoporosisCenter,
401 Kamakee Street, Honolulu,HI 96814. E-mail:[email protected]
ReceivedFebruary 23, 1998
AcceptedAugust19,1998