Muscular strength and physical function
PATRICIA A. BRILL, CAROLINE A. MACERA, DOROTHY R. DAVIS, STEVEN N. BLAIR, and NEIL GORDON
School of Public Health, University of South Carolina, Columbia, SC 29208, and Cooper Institute for Aerobics Research,
Dallas, TX 75230
BRILL, P. A., C. A. MACERA, D. R. DAVIS, S. N. BLAIR, and N. GORDON. Muscular strength and physical function. Med. Sci.
Sports Exerc., Vol. 32, No. 2, pp. 412–416, 2000. Purpose: The purpose of this study was to evaluate the potential association of
muscular strength and endurance at baseline with the prevalence of functional limitations at follow-up. Methods: Study participants
were 3,069 men and 589 women (30–82 yr) who received a clinical examination including a strength evaluation at the Cooper Clinic
between 1980 and 1989 and responded to a 1990 mail-back survey. Participants also had to achieve at least 85% of their age-predicted
maximal heart rate on a maximal exercise treadmill test and have no history of heart attack, stroke, diabetes, high blood pressure, cancer,
or arthritis at their first visit. A strength index composite score (0–6) was calculated using age- and sex-specific tertiles from bench
press, leg press, and sit-up tests. Those scoring 5 or 6 were categorized in the high strength group. Functional health status was assessed
by responses to questions about the participant’s ability to perform light, moderate, and strenuous recreational, household, daily living,
and personal care tasks. Results: After an average follow-up of 5 yr, 7% of men and 12% of women reported at least one functional
limitation. A logistic regression model including age, aerobic fitness, body mass index, and new health problems at follow-up found
that, relative to those with lower levels of strength, the odds of reporting functional limitations at follow-up in men and women
categorized as having higher levels of strength were 0.56 (95%CI ? 0.34, 0.93) and 0.54 (95%CI ? 0.21, 1.39), respectively.
Conclusions: These findings, if replicated in other populations, suggest that maintenance of strength throughout the lifespan may
reduce the prevalence of functional limitations. Key Words: ACTIVITIES OF DAILY LIVING, FUNCTIONAL LIMITATIONS,
cardiovascular morbidity or mortality (2,3). Previously, our
group found an inverse gradient of self-reported functional
limitations across both physical activity and aerobic fitness
categories in every age group for both men and women,
particularly among those over age 55 (12). Considerable
evidence suggests that the ability to perform a physical task
is determined by a threshold level of muscular strength and
endurance (4–6,8). Individuals lacking the requisite strength
may not be able to perform various activities of daily living
that are important determinants of independence. A decline
in functional status is determined at least in part by muscle
strength, flexibility, range of motion, physical fitness, and
body composition (4,8,10,11,13,15,17). Those who already
report some limitations are more likely to develop additional
limitations over time (19). The changes leading to functional
ost of the research on the benefits of physical
activity and health relates aerobic activity or aer-
obic fitness to some measure of health, often
limitation do not generally occur suddenly and may have
their origins in lifestyle habits developed over many years.
While the incidence of disability rises sharply with age, it
is important to study functional limitations among adults of
all ages because of the potential to prevent or minimize
subsequent disability. However, in contrast to studies of
physical activity and aerobic fitness, there is limited infor-
mation on the association of muscular strength and endur-
ance to functional limitations among relatively healthy
adults under age 65. Thus, the purpose of this report is to
examine these associations among healthy men and women
Study population. The study subjects were selected
from those who received preventive medical examinations
at the Cooper Clinic in Dallas, Texas, between 1980 (when
the strength measurements were first available) and 1989 (1
yr before the follow-up mail survey). Approximately 70% of
the clinic patients reside in Texas; over 80% are college
educated and more than 99% are white. The study protocol
was approved by the institutional review boards of both the
Cooper Institute for Aerobic Research and the University of
MEDICINE & SCIENCE IN SPORTS & EXERCISE®
Copyright © 2000 by the American College of Sports Medicine
Submitted for publication July 1998.
Accepted for publication April 1999.
South Carolina. Participants were included if they signed a
consent form, completed a muscular strength and endurance
evaluation, achieved at least 85% of their age-predicted
maximal heart rate on a maximal exercise treadmill test, and
responded to a 1990 mail-back survey (65% response rate).
Additionally, we excluded those under age 30 or those with
a history of heart attack, stroke, diabetes, high blood pres-
sure, cancer, or arthritis at their first visit. The study subjects
meeting the inclusion criteria consisted of 3,069 men and
589 women between the ages of 30 and 82 yr.
Laboratory assessments. The strength evaluation in-
cluded a 1RM lift using Universal bench press and leg press
machines (Universal Equipment, Cedar Rapids, IA). The
test protocol for the bench press had the participant lie on
his/her back with the bar of the bench press machine posi-
tioned between the nipple and axilla, and the feet were
placed flat on the floor or on the bench. Keeping the lower
back flat on the bench, arms were extended slowly upward,
then slowly brought back to the starting position. The initial
load was 70% of body weight; additional weight (5 or 10 lb.)
was added depending on the participant’s age, gender, and
previous physical activity history, and the exercise was
repeated until maximum effort was achieved. The test pro-
tocol for the leg press had the participant in a seated position
with the ball of the foot placed in the crease of the foot plate
of the leg press machine. The knee angle was set at 70° by
moving the adjustable seat forward or backward. With the
lumbar spine placed firmly against the seat back, partici-
pants were instructed to slowly extend their legs, then
slowly return them to the starting position. Initial load was
100% of body weight; additional weight (5 or 10 lb.) was
added depending on the participant’s age, gender, and pre-
vious physical activity history and the exercise was repeated
until maximum effort was achieved. Scores used in analyses
were bench press or leg press/body weight ratio. Muscular
endurance was measured by a 60-s bent leg sit-up test. The
test protocol for the sit-up test had the participant lie on
his/her back with knees bent and feet flat on the floor. Hands
were placed on the side of the head with fingers placed over
the ears. Participants then were instructed to curl up, touch-
ing their elbows to their knees and then lower their trunk
until the shoulders touched the mat. Participants completed
as many sit-ups as possible during 1 min. This test was used
because muscular endurance of the rectus abdominus mus-
cles relates to balance and walking.
A muscular strength and endurance index was calculated
using a composite score from the bench press, leg press, and
sit-up tests. Tertiles were determined for gender and age
groups (30–39, 40–49, 50–59, and 60?) for each measure.
The first tertile scores were coded as 0 and the second and
third tertiles were coded as 1 and 2, respectively. Scores
were added for the three tests for a total score ranging from
0 to 6. Individuals scoring 5 to 6 points were assigned to the
high strength group and individuals scoring 0 to 4 were
assigned to the low strength group.
Maximal graded exercise treadmill testing was completed
by all participants, but only those who achieved at least 85%
of their age-predicted maximum heart rate were included.
The test protocol is described in detail in earlier reports (2),
and total treadmill time (in minutes) is the variable used in
analysis. Treadmill time from this protocol is highly corre-
lated with measured maximal oxygen uptake in men (r ?
0.92) and in women (r ? 0.94).
Clinical assessments. Other clinical assessments in-
cluded a thorough preventive medical evaluation that in-
cluded a personal and family health history, a physical
examination, a questionnaire on demographic characteris-
tics and health habits, anthropometry, resting and exercise
ECG, blood chemistry tests, and blood pressure measure-
ment. Trained technicians administered these evaluations
following a standard manual of procedure. These methods
are described in earlier papers (2,3).
Functional health status assessments from 1990
survey. Because these individuals were young and mostly
healthy, available functional health measures that were com-
monly used in the late 1980s were not appropriate for this
group. The questions used in the 1990 survey were the result
of deliberation from a panel of experts in the area of phys-
ical activity and aging. Functional health status was assessed
by responses to 10 questions. These questions focused on
the participant’s ability to perform light, moderate, and
strenuous recreational, household, daily living, and personal
care tasks. For example, participants responded to the ques-
tion, “Are you currently physically able to do the following
activities? Please circle YES, YES with ASSISTANCE, or
NO for each activity. These questions refer to whether or not
you can do the activity and not whether you actually do it.”
(See Appendix A for specific questions on functional
A score of zero was assigned for each YES response and
a score of one was assigned for each yes with ASSIS-
TANCE or NO response. Individuals scoring one or more
were considered to have some functional limitation. Indi-
viduals who checked YES with ASSISTANCE or NO to the
light or moderate household activities but could perform the
strenuous household activities were coded as having no
Other health assessments from 1990 survey. The
range of questions on the 1990 mail-back survey also in-
cluded information on health conditions and the date of first
occurrence. We identified as “new health problems” those
who reported a physician diagnosis of heart attack, stroke,
diabetes, high blood pressure, cancer, or arthritis since their
first clinic visit. This item was used as a control variable in
the regression analyses.
Statistical analysis. Descriptive statistics (means or
percentages) were calculated for all variables by strength
group. Logistic regression models using functional limita-
tion (yes or no) as the dependent variable were developed
separately for men and women. Independent variables in the
models included items from the baseline examination such
as strength group (high, low), age, body composition (mea-
sured by BMI; weight (kg) divided by height (m)2), and
treadmill time (in minutes), as well as new health problems
identified at follow-up (yes, no). These variables were cho-
sen because they have the potential to affect the relationship
STRENGTH AND PHYSICAL FUNCTION Medicine & Science in Sports & Exercise?
between strength and functional limitation. The time from
the subject’s first visit to the 1990 survey was coded as years
of follow-up and also included in the model.
Results. The range of follow-up time from the strength
evaluation to the 1990 survey was 1–8 yr with an overall
average of 5 yr. At follow-up, 7% of men and 12% of
women reported functional limitations (unable to perform or
required assistance to perform at least one of the activities).
As shown in Table 1, weaker men and women had a higher
BMI, lower treadmill times, and a higher percentage re-
ported functional limitations than their stronger peers. There
were no statistically significant differences between strength
groups by age or by the percentage who developed new
health problems during follow-up.
The association of strength (measured at baseline) and
functional limitations (measured at follow-up) was evalu-
ated using logistic regression. Even though age and the
percentage of subjects with new health problems did not
vary by strength group, both of these factors are associated
with functional limitation and were included in the logistic
regression model. After adjusting for age, BMI, treadmill
time, new health problems, and follow-up year, men in the
high strength group were about half as likely to report
functional problems as men in the low strength group
(OR ? 0.56, 95% CI ? 0.34–0.93). The same general
relationship between strength and functional limitations was
found for women (OR ? 0.54, 95% CI ? 0.21–1.39), but
the results were not statistically significant, probably be-
cause of the small sample size (Table 2). Other factors that
were statistically significantly associated with functional
limitations included older age, lower levels of aerobic fit-
ness (measured by treadmill time), and developing new
health problems during follow-up.
Discussion. Functional limitations are an important in-
dicator of quality of life and the preservation of indepen-
dence in older persons (6,14). Although functional limita-
tions represent a major source of disability for older adults,
the problem may stem from patterns of lifestyle behaviors
developed during middle age. This analysis was conducted
using over 3,000 heathy men and 589 healthy women aged
30–82 at baseline. Only a small number of these individuals
(7% of men and 12% of women) reported any type of
limitation after an average follow-up of 5 yr. Compared with
those in the low strength group, men and women in the high
strength group had a lower prevalence of functional limita-
tions (P ? 0.05; Table 1). This relationship persisted after
adjusting for several possible confounders (age, BMI, tread-
mill time, and health problems at follow-up). These results
also suggest that, in addition to strength, low levels of
aerobic fitness at baseline and the development of new
health problems during follow-up were also associated with
the prevalence of functional limitations at follow-up (Table
2). It is worth noting that treadmill time and muscular
strength make independent contributions to the prevalence
of functional limitations in this analysis. This is consistent
with our previous work (12) but also supports the theory that
a threshold level of strength may be required to perform
basic activities of daily living and to participate in activities
designed to maintain cardiorespiratory fitness (6,9).
Other studies have found that body composition is asso-
ciated with disability (10,11,15). The effect seemed to be
present for both underweight and overweight individuals. In
our study the relationship between strength and functional
limitations was not affected by body composition, probably
because we did not have many individuals who were ex-
tremely heavy or extremely light. In other more diverse
populations, body composition could play an important role
that could not be detected in our study.
While other investigations on strength and function are
short-term clinical studies or cross-sectional analyses with
few subjects, our study includes objective laboratory assess-
ments of physical fitness and strength, a follow-up study
design, and over 3,000 men. One major limitation of our
study is the lack of baseline information on functional
limitations. To minimize this problem, we selected only
those who were free of health problems at baseline (heart
attack, stroke, diabetes, high blood pressure, cancer, and
arthritis) because these conditions may affect the strength
measurements and also may be associated with functional
limitation. Additionally, we eliminated all those who (at
baseline) could not reach 85% of their age-predicted max-
imal heart rate on a maximal exercise treadmill test, seeking
to ensure that our cohort was relatively healthy and not
likely to have pre-existing functional limitations. However,
there is still the possibility that some members of our cohort
had pre-existing functional limitation.
TABLE 1. Characteristics of men and women by musculoskeletal strength and
endurance classification (Means and SD or percentages).
Men (N ? 3,069)
N ? 600
Age (yr)45.8 (9.2)
Treadmill time (min)*23.4 (4.3)
% with new health problems19.2
% with functional problems*3.2
Follow-up years*5.1 (2.0)
* Significant difference between strength groups (P ? 0.05) for both men and women.
BMI (Body Mass Index), weight (kg)/height (m)2; new health problems (yes or
no); heart attack, stroke, diabetes, high blood pressure, cancer, and arthritis;
functional limitations (yes for one or more).
Women (N ? 589)
N ? 129
N ? 2,469
N ? 460
44.8 (8.1) 44.1 (8.3)
20.9 (1.7) 22.1 (3.0)
18.6 (4.2) 14.1 (3.9)
TABLE 2. Logistic regression models for functional limitations in men and women:
odds ratios and 95% confidence intervals.
High strength group0.56
Treadmill time (min)0.90
New health problems2.14
§ Odds ratio (OR) and 95% confidence intervals (CI) adjusted for all variables listed.
New health problems: heart attack, stroke, diabetes, high blood pressure, cancer,
Official Journal of the American College of Sports Medicinehttp://www.msse.org
The strength measures used in this study were chosen
because of their potential relationship to physical function.
However, previous validity and reliability tests of the 1RM
bench/leg press and the 60-s maximum sit-up test suggest
the possibility of misclassification of some persons (18).
Although we tried to minimize this potential bias by com-
bining the scores and taking only the highest third (those
scoring 5 to 6) as the high strength group, this remains a
limitation of this study.
Other drawbacks of our study included the small numbers
of women and the self-report nature of the outcome variable
(functional limitations). The high educational level of this
group and documented reliability of other self-reported
measures, such as hypertension (2), serve to minimize the
effect of self-report bias in this population. Additionally,
self-reported functional status has been shown to be rela-
tively accurate when compared with performance measures
(7). Furthermore, our findings on the overall prevalence of
functional limitations and the higher prevalence among
women are consistent with other work (16), although the
select nature of this population precludes generalizing these
findings to other populations.
Although older cohorts have the highest risk of develop-
ing disability, the association between muscular strength
and endurance and the subsequent prevalence of functional
limitations found in this study indicates that this relationship
persists even among middle-aged adults. While most phys-
ical activity recommendations conclude that aerobic condi-
tioning activities are required for adults of all ages, until
recently strength training was primarily recommended for
older adults. Our results support the current ACSM guide-
lines that encourage all adults, in addition to increasing
aerobic physical activity, to include activities that increase
muscular strength (1).
We thank Dr. Kenneth Cooper and the physicians and techni-
cians of the Cooper Clinic for data collection, Kia Vaandrager for
supervising the muscular strength and endurance assessments,
Jennie Hootman for technical review, and Beth Barlow for survey
and data management.
This study was supported in part by grants from the National
Institute on Aging AG-06945 and the National Institute of Arthritis
and Musculoskeletal and Skin Diseases AR39715.
Current address for Dr. Neil Gordon: Center for Heart Disease
Prevention, St. Joseph’s/Candler Health System, 5356 Reynolds
Street, Suite 120, Savannah, GA 31405.
Current address for Dr. Brill: VA Medical Center, Center of Ex-
cellence on Healthy Aging with Disabilities, 2002 Holcombe Blvd.,
Houston, TX 77030.
Address for correspondence: Dr. C. A. Macera, Centers For Dis-
ease Control and Prevention, Division of Nutrition and Physical
Activity, 4770 Buford Hwy, N.E., Mail Stop K46, Atlanta, GA 30341.
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APPENDIX A: PHYSICAL FUNCTIONING
AND ACTIVITIES OF DAILY LIVING
In this section we would like to find out what activities you
are physically able to do. Are you currently able to do the
following activities? (Please circle YES, YES with ASSIS-
TANCE, or NO for each activity). Remember that these
questions refer to whether or not you can do the activity, and
not whether you actually do it regularly.
STRENGTH AND PHYSICAL FUNCTION Medicine & Science in Sports & Exercise?
RECREATIONAL ACTIVITIES Download full-text
Moderate recreational activities such as leisure bicycling,
fishing, ballroom dancing, or volleyball.
Strenuous recreational activities such as jogging, basket-
ball, circuit training, skiing, or tennis.
Light household activities such as cooking, ironing, paint-
ing inside, dusting, or making beds.
Moderate household activities such as general carpentry,
cleaning, food shopping, mopping floor, vacuuming, or rak-
Strenuous household activities such as digging in garden,
mowing, scrubbing floors, shoveling snow, or washing cars.
Light daily activities such as twisting/bending, reaching
overhead/out, grasping with fingers, sitting, or standing.
Moderate daily activities such as lifting/carrying 10 lb.,
stooping, crouching, kneeling, or prolonged sitting/standing.
Strenuous daily activities such as walking 1/4 mile, climb-
ing 10 stairs with no rest, lifting/carrying 25 lb, or moving
large objects such as a heavy chair.
Moderate personal care activities such as bathing/show-
ering, going to the toilet, dressing, or getting in/out bed/
Activities requiring dexterity such as writing, turning
keys, buttoning, or opening jars.
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