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«Muscle Strength and Postural Stability in Healthy, Older Women: Implications for Fall Prevention Elaine J. Trudelle-Jackson, Allen W. Jackson, and ...»

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Journal of Physical Activity and Health, 2006, 3, 292-303

© 2006 Human Kinetics, Inc.

Muscle Strength and Postural Stability

in Healthy, Older Women: Implications

for Fall Prevention

Elaine J. Trudelle-Jackson, Allen W. Jackson,

and James R. Morrow, Jr.

Background: Effect of muscle strength and balance on falls has not been well

researched in healthy older women. The purpose of this study was to compare

lower extremity strength and balance in older healthy women during each decade of life and to investigate which factors are different in women with a history of falling. Methods: We retrospectively studied 240 women age 50-89 y. Measures of muscle strength, postural stability, and incidence of falls over the past year were obtained from client charts at Texas Womanʼs Universityʼs Health Promotion & Research Center from 1996 to 2002. Results: Strength declined significantly with age in all muscle groups except knee extensors. Age, hip flexor and abductor strength, and postural stability were significantly different in women who had fallen. Conclusions: Strength decline was not consistent across muscle groups.

Women who were older, had less hip flexor or abductor strength, or less balance were more likely to have fallen.

Key Words: aging, balance, physical activity Impairments in muscle strength have been associated with falls, reduced function, and disability.1, 2 About one-third of community dwelling people who are age 65 and older fall resulting in approximately 400,000 fall-related fractures annually.3, 4 Furthermore, a fall often has a deleterious effect on the individualʼs independence and quality of life leading to inactivity and further decline.5 Several studies have identified lower extremity muscle strength as a risk factor for falling,6, 7, 8, 9, 10 but most studies included both frail and healthy older adults. Because the frail fall more frequently, they tend to influence the identification of risk factors more heavily,8 making the effect of reduced muscle strength on incidence of falls in healthier older people less well understood. In a recent epidemiological study of high functioning 70-79 y old men and women, investigators found that fallers were more likely to be white females who reported more chronic diseases, used more medications, and Trudelle-Jackson is with the School of Physical Therapy, Texas Womanʼs University, Dallas, TX 75077.

Jackson and Morrow are with the Dept of Kinesiology, Health Promotion and Recreation, University of North Texas, Denton TX 76203-0769.

Strength and Balance in Older Women 293 had poorer balance and lower extremity strength.11 Muscle strength and balance are known to decrease with aging12, 13 but more importantly, both have beenfound to be modifiable with physical activity in both frail and healthy older adults.14-19 Consequently, as people live longer, maintaining muscle strength and balance should be considered a major goal in preserving quality of life.

Beginning approximately at age 30, muscle strength declines in men and women at a rate of 10% to 15% per decade.20 Studies also show that the rate of loss may be accelerated beyond 50 y of age.20, 21 This accelerated loss, especially after age 55, appears to be more pronounced in women.22 What is not well known is whether strength decline is consistent in various muscles. In a longitudinal study of muscle strength changes in older adults, Hughes, et al.23 found that elbow and knee flexors and extensors all declined in strength with time but that women had a smaller decline (P 0.05) in elbow flexor and extensor strength than men. In addition, womenʼs knee extensor strength declined less than their knee flexors (P 0.01) and more than their elbow flexors (P 0.0001). That is, over a 10 y period, womenʼs knee extensor strength declined 11.8% ± 15.5% compared to knee flexor strength which declined 17.4% ± 16.1%. Elbow flexor strength increased 2.0% ± 33.8% while elbow extensor strength declined 2.4% ± 32.9%. These findings suggest that strength may decline differently for men and women and for different muscle groups in normal aging adults. If certain muscle groups decline more rapidly with aging, then this should be considered in designing physical activity programs for maintaining muscle strength in older, well adults. That is, the muscle groups that decline most in aging women could be targeted in physical activity programs designed for older, well women.

Although several studies have shown muscle strength in the quadriceps and ankle plantar flexors to be an important factor in falls, the strength of hip musculature and its potential effect on falls has not been investigated. In a systematic review of muscle strength and falls in older adults from 1986 to 2002, Moreland et al.24 did not identify any studies that investigated muscle strength at the hip as a risk factor for falls.

The purpose of this study was to compare muscle strength of different lower extremity muscle groups and postural stability in apparently well women during each decade of life beginning at age 50. A second purpose was to determine whether the declines in muscle strength were consistent across muscle groups. A third purpose was to determine whether age, muscle strength, or postural stability were different in women who had fallen over the past year compared to women who had not fallen.

Methods A retrospective, cross-sectional study of 240 apparently healthy women ranging in age from 50 to 89 y was conducted. Measures of lower extremity muscle strength, postural stability, and number of falls were obtained from charts of clients seen at the Texas Womanʼs University Health Promotion & Research Center (HPRC) from 1996 to 2002. Women seen at the HPRC were primarily seen to institute a wellness exercise program that addresses their specific needs such as muscle strength, balance, postural faults or flexibility. However, all information collected at the HPRC 294 Strength and Balance in Older Women was placed in a research database. Therefore, all measures were carefully taken using standardized measurement procedures established for all data collected at the HPRC. Informed consent was not required as the study qualified for exempt status as determined by the Texas Womanʼs University Institutional Review Board.

All subjects did, however, sign a form allowing information from their charts to be used for research purposes.

Participants Exclusion criteria included neurological, musculoskeletal, or cardiopulmonary complications that would prevent clients from participating in a simple exercise program at the HPRC. Women diagnosed with osteoporosis, defined as having bone density equal to or greater than 2.5 standard deviations below the young adult mean at any of the lumbar or femoral sites, were also excluded. The women who qualified for this descriptive study were subdivided into four age groups: 50 to 59 (n = 91), 60 to 69 (n = 76), 70 to 79 (n = 57), and 80 to 89 (n = 16).

Instrumentation Lower extremity muscle strength and postural stability were measured using the Human Performance Measurement (HPM) system (Human Performance Measurements, Arlington, TX). The HPM is a computer-automated system that integrates a battery of tests used to evaluate a broad range of sensorimotor functions called basic elements of performance (BEP).25 The BEP IIIa and BEP IV are components of the HPM system that were used in this study to measure muscle strength and postural stability, respectively. The BEP for Windows software was used to operate the BEP modules, record, and store data.

The BEP-IIIa is a hand-held dynamometer used to measure maximal isometric muscle force. The device is factory calibrated with respect to gain and has a gain accuracy of 1% of full scale. An offset calibration check is automatically performed each time the system is powered. Muscle torque in Newton-meters is calculated by the BEP for Windows software using estimated segment lengths based on each subjectʼs height. The standard error associated with estimated segment lengths that are based on stature has previously been shown to be approximately 1.0 cm when compared to measured segment lengths.26 The criterion validity of hand-held dynamometers as a measure of muscle strength has been previously determined.27, 28 The intrarater reliability of the BEP-IIIa for measurement of lower extremity muscle strength in older adults (mean age 59.5 y) has previously been determined to be excellent (ICC3, 2 = 0.94).29 The BEP-IV postural stability measurement system is a lightweight, portable force platform that measures medial-lateral stability, anterior-posterior stability, and total stability by tracking changes in the center of pressure (COP) over time as the subject stands erect over one or both feet. For this study, only total stability was used for data analysis. Total stability is a measure of how well the subject is able to keep the COP centered over the base of support. Intrarater reliability for measures of postural stability obtained using the BEP-IV and the same measurement protocol used in this study was found to be excellent (ICC3, 2 = 0.95) in a previous study.29 To measure postural stability in single stance, the BEP-IV samples COP Strength and Balance in Older Women 295 movement at a rate of 60 samples per second over a 10 s time period. The resulting samples are then averaged over time and normalized for base of support by calculating the ratio of average movement of the COP to the size and placement of the stance foot. The resulting normalized score represents a “percent instability” score. This score is then subtracted from 100% to provide the score of percent stability used for data analysis.

Measurement Procedures This was a retrospective study performed on existing chart data. Information available through the charts consisted of a detailed client history including documentation of fall history. For this study, number of falls experienced over the past year was the only piece of information extracted from the medical history. Next, measures of muscle strength and postural stability were extracted from the charts. Although four different physical therapists contributed to the clinical database, all therapists were trained to use the same standardized measurement procedures.

A written manual of standardized procedures developed by the primary investigator of this study was used in all participant testing at the HPRC. The following is a description of the standardized measurement procedures used to obtain all muscle strength and postural stability measurements.

First, standardized test procedures were used to obtain height measurements using a stadiometer. Height was recorded in centimeters. Next, body weight in kilograms was obtained using a platform scale with beam and moveable weights.

All muscle force measurements were taken in a gravity-lessened position, using a “make” test. To perform a make test, the examiner held the dynamometer steady with one hand while manually stabilizing the client with the other hand.

Instructions to the clients were to gradually start pushing against the dynamometer, and then to increase their force until they were pushing as hard as they could.

Muscle tests were performed on the following muscle groups: hip flexors, hip extensors, hip abductors, and knee extensors. The specific muscle testing position and force transducer placement used for testing each muscle group are published elsewhere.30 Clients practiced once prior to performance of two maximum effort test trials. The mean of the two trials was then adjusted for body weight by dividing the mean strength value for each muscle group by the clientʼs body weight and multiplying by 100.

Plantar flexor strength was measured using the unilateral heel raise test. To perform this test, the client stood facing a treatment table, with each index finger resting on the table for balance. The examiner instructed the client to stand straight with the opposite knee bent to 90° and to rise up onto her toes using her calf muscle.

She was instructed to perform this motion repeatedly without leaning forward and without pushing down on the table with her index fingers. As the client performed this motion repetitively, the therapist watched carefully that the motion was performed with no breaks in form such as leaning forward, pushing up with the index finger(s), bending the knee, or not completing the available ankle range of motion when rising up onto the toes. The number of repetitions (up to 20) the client was able to perform with good form was adjusted for body weight as described above and used for data analysis.

296 Strength and Balance in Older Women Postural stability in single leg stance with eyes open was measured as clients attempted to stand steadily on one barefoot extremity. The examiner first instructed each client to step onto the force platform, stand on both feet, and look straight ahead. Upon hearing an audible beep from the HPM system, the client flexed one knee to 90° and removed her hands from the table in front of her. The client then maintained this single stance posture as steadily as possible until a second beep sounded, signifying the end of the 10-s trial. The trial was terminated by the examiner before the end of the 10-s interval if clients lost their balance, and had to touch down with a hand or foot. Three trials of postural stability were measured and the BEP for Windows software calculated mean percent stability using the two trials with the most similar values. When the timed trial had to be terminated early (less than 10 s), percent stability for that trial was based on a shorter period of time. Each client was allowed one practice trial prior to the three test trials, and a 5-s rest was taken between trials.

Data Analysis Means, standard deviations, and percent decline across each decade of life for each muscle strength variable (hip flexors, extensors, abductors, knee extensors, and plantar flexors) and for postural stability were calculated for each of the four age groups. Strength and postural stability across four decades of life were analyzed using multivariate analysis of variance (MANOVA). Univariate trend analyses were conducted across age groups for the strength and postural variables. Age, muscle strength, and postural stability were compared between women who had fallen over the past year and women had had not fallen using a second MANOVA. A significant MANOVA was followed with univariate ANOVAs and a post hoc discriminant function analysis. An alpha level of 0.05 was used for all statistical analyses.

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