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Age-Related Declines in Knee Extensor Strength and Physical Performance in Healthy Hispanic and Caucasian Women

^a Department of Kinesiology and Applied Physiology, University of Colorado, Boulder
^b Department of Medicine, Divisions of Cardiology and Geriatric Medicine, University of Colorado Health Sciences Center, Denver

Douglas R. Seals, Department of Kinesiology and Applied Physiology, Campus Box 354, University of Colorado, Boulder, CO 80309 E-mail: seals{at}spot.colorado.edu.

Decision Editor: John A. Faulkner, PhD

	Abstract

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Hispanic women may undergo greater age-related reductions in physical functional capacity compared with Caucasian women. If so, a greater rate of decline in muscle strength with age could contribute. We tested this hypothesis in 82 healthy sedentary Caucasian (n = 37) and Hispanic (n = 45) adult women aged 21–78 years of similar socioeconomic status. Absolute one-repetition maximum (1-RM) strength of the knee extensors (KE) declined with advancing age in the Caucasian (r = -.55, p < .01) and Hispanic (r = -.45, p < .01) women; the rates of decline were similar in the two groups (-7% to 8%·decade^-1, p = .60). KE strength normalized for thigh fat-free mass (FFM) also declined with age in the Caucasian (r = -.52, p < .01) and Hispanic (r = -.41, p < .01) women, the rates of decline being similar (-6% to 7%·decade^-1, p = .66). For all functional performance tasks (10-m walk, stair ascent, stair descent, and chair stand), performance time increased with advancing age (mean of four tasks vs age: Caucasian, r = .64, p < .01; Hispanic, r = .56, p < .01). Absolute and normalized KE 1-RM were inversely related to the mean time for the four performance tasks (r = -.34 to -.58, all p < .01). Normalized KE 1-RM was the best independent predictor of the age-related decline in task performance in both groups. These cross-sectional findings do not support the hypothesis that Hispanic ethnicity per se is associated with a greater decline in KE strength and performance tasks associated with KE strength in healthy women.

MUSCLE strength declines with age in adult humans ^{(1) (2) (3)}. This process is associated with an increase in functional disability, deterioration in health status, loss of independence and quality of life, and increases in health care services use and related costs ^{(4) (5) (6) (7)}.

Muscle strength appears to be an important determinant of physical functional capacity with age. This concept is based in part on the consistent relation observed between strength and the ability to perform physical tasks associated with standardized test instruments such as frequently performed activities of daily living (ADL) and the more complex instrumental activities of daily living (IADL) ⁽⁸⁾. In these investigations, age-related reductions in lower extremity strength have been shown to be associated with impairments in gait speed, balance, stair climbing, and chair rising ^{(9) (10) (11) (12) (13) (14)}.

Age-related reductions in physical functional capacity and increases in functional disability have been reported in some cases to be greater in Hispanic than in Caucasian adults, especially in women ^{(15) (16) (17) (18) (19) (20) (21)}. These observations cannot be completely explained by differences in other potentially influencing factors such as socioeconomic status (SES), habitual physical activity levels, or disease, suggesting a possible link with ethnicity per se. The physiological basis for a greater functional decline with age in Hispanic women, when observed, is unclear. In this regard, greater age-associated decreases in muscle strength could be one important contributing mechanism. Indeed, Rantanen and colleagues ⁽²²⁾ recently reported differences in muscle strength between African American and Caucasian older women after stratifying for several influencing factors. However, to our knowledge there are no published data on age-related strength declines in Hispanic populations.

Accordingly, the primary experimental aim of the present investigation was to determine if the age-related decline in muscle strength is greater in healthy Hispanic compared with Caucasian women. Additionally, we sought to determine whether a greater age-related decline in muscle strength in Hispanic women, if observed, is associated with impaired performance in strength-dependent tasks. To determine if any group differences observed in the decline in muscle strength with age could be attributed to Hispanic ethnicity per se, subjects of similar overall health, SES, and habitual physical activity status were studied. Finally, we chose to measure knee extensor (KE) muscle strength because of the importance of the muscle group in performing common and functionally important physical tasks associated with daily living ^{(4) (8) (13) (14) (23)}.

	Methods

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Subjects
We studied 82 healthy sedentary community-dwelling women aged 20–78 years; 37 were Caucasian and 45 were Hispanic. Ethnicity was determined by asking subjects to which ethnic group they belonged. Those who stated "Caucasian," "white," or "non-Hispanic white" were considered Caucasian, and those who chose "Mexican/Mexican American," "Central American," or "South American" were considered Hispanic as described by Winkleby ⁽²⁴⁾. Hispanic women also volunteered that they, their parents, or grandparents were born in Mexico, Central America, or South America. Approximately 90% of the Hispanic women were of Mexican descent.

Subjects were healthy as assessed by medical history. They reported no orthopedic or neuromuscular disorders, or difficulties in performing any ADL or IADL upon subjective examination. Subjects 50 years of age and older were further evaluated for clinical evidence of cardiopulmonary disease with a physical examination and electrocardiograms during rest and maximal exercise. Subjects were nonsmokers and were not taking any regular medications that are known to affect the key outcome variables. They were normally physically active but not engaged in any type of regular exercise. The nature, purpose, and risks of the study were explained to each subject in English or Spanish, as needed, before written informed consent was obtained. The experimental protocol was approved by the Human Research Committee at the University of Colorado at Boulder.

Body Mass and Composition
Body mass was measured to the nearest 0.1 kg on a physician's balance scale (Detecto, Webb City, MO). Whole body composition was measured using dual-energy x-ray absorptiometry (DXA; DXA-IQ, Lunar Radiation Corp., Madison, WI, software version 4.1). DXA scans were used to determine fat mass, fat-free mass (FFM), and bone mineral content. Regional measures of nonosseous fat and lean tissue were determined using the extended research analysis of LUNAR software ⁽²⁵⁾. The region below the joint line of the knee was subtracted from the total leg region to determine thigh FFM. Thus, thigh FFM included the hamstrings and hip adductor muscle groups in addition to the quadriceps.

SES
We used education as our measure of SES instead of using income, occupation, or a combination of the three, because education is more closely associated with risk factors for chronic degenerative diseases than is income or occupation ^{(26) (27) (28)}. Education was defined as the highest number of completed school years.

Habitual Physical Activity
Daily energy expenditure was estimated using the Stanford Physical Activity Questionnaire ⁽²⁹⁾ as employed previously by our laboratory ^{(30) (31)} as a measure of habitual physical activity. This questionnaire provides a gross estimate of total daily leisure time and work-related physical activity over a 7-day time period. Subjects are asked about their time spent in moderate, hard, and very hard physical activity. This method has been validated with a wide age range of subjects ⁽³²⁾.

KE Strength
Maximal voluntary dynamic KE strength was determined bilaterally using the one-repetition maximum (1-RM) strength test. Testing was done on a commercially available knee extension weight stack machine (Icarian, Sun Valley, CA). Subjects were positioned against a properly adjusted backrest with regard to knee alignment, had straps placed across the thigh and pelvis, and performed several warm-up repetitions with a light load. Subjects were required to lift the load through a standard, objectively determined range of motion from 90° through 165°. The hips were kept at 90°. After familiarization and warm-up, a load thought to be below the 1-RM value was chosen and subjects performed one repetition of the knee extension movement. After approximately 60 seconds of rest, another attempt was made with a higher load. Subsequently, increases of 1.13 kg of resistance were adjusted to minimize the total number of trials required to obtain a valid 1-RM value (i.e., the highest load at which one repetition could be completed) ⁽³³⁾. The same verbal encouragement was given to all subjects, and the heaviest load lifted was recorded as the 1-RM. All tests were performed by the same investigator.

Functional Performance Measures
Performance measures were adapted from previous investigations ^{(8) (34) (35)}. Functional performance was determined with three timed measures: a 10-m walk, repetitive chair stands, and stair ascent and descent. All tests were demonstrated by the investigator prior to being performed by the subject. One investigator (YGC) conducted the measurements on all subjects.

10-m walk..-- Subjects were instructed to walk 10 m as quickly as possible on a standard course. Each subject was timed for two trials. The fastest time was used for analysis.

Repetitive chair stand..-- To test chair rise ability, a straight-backed chair was placed against a wall, and subjects were asked to fold their arms across their chest and to stand up from the chair. If successful, subjects were instructed to stand up and sit down five times as quickly as possible and were timed from the initial sitting position to the final standing position at the end of the fifth stand.

Stair ascent and descent..-- To test stair-climbing ability, subjects were asked to climb and descend a flight of 16 stairs rising approximately 3.6 m as rapidly as possible without running. Subjects were instructed to perform the task safely and to use a handrail if they felt unstable. Three trials were performed, with the fastest time used for analysis.

All subjects were able to complete all three performance tasks without the use of assistive devices. No subject reported difficulty with any task. To determine an overall performance test score, the best times for each of the tasks were combined and an average time was calculated.

Statistics
Group differences among the dependent variables were determined by one-way analysis of variance (ANOVA). Linear regression analyses were performed to determine relations between dependent variables of interest and subject age. Pearson product moment correlation coefficients were used to express the direction and magnitude of the relations. The slopes of regression lines were compared using analysis of covariance (ANCOVA). Forward stepwise multiple regression analysis was used to identify independent physiological correlates of muscle strength and functional performance. The level of statistical significance was set at p < .05 for all analyses.

	Results

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Subject Characteristics
Selected subject characteristics are shown in Table 1 . The two groups were not significantly different in age, body mass, whole-body FFM, SES, or habitual physical activity. Percentage of body fat was slightly higher in the Hispanic women ( p < .05).

View larger version (18K): [in this window] [in a new window]   Figure 1. A, age-related and B, group mean knee extensor one-repetition maximum (KE 1-RM) strength in Caucasian and Hispanic women.

View larger version (16K): [in this window] [in a new window]   Figure 2. Age-related and group mean thigh fat-free mass (FFM) in Caucasian and Hispanic women.

Normalized KE 1-RM Strength
Fig. 3 illustrates the relation of KE strength normalized for thigh FFM (kg·kg FFM^-1) with age. Both groups demonstrated inverse correlations with age (r = -.41 to -.52, both p < .01), with no difference in rates of decline between the Caucasian (-7%·decade^-1) and Hispanic (-6%·decade^-1) women (p = .70). There was also no difference in mean levels of normalized KE strength ( Fig. 3) between the Caucasian (15.3 ± 0.5 kg·kg FFM^-1) and Hispanic (15.9 ± 0.5 kg·kg FFM^-1) women.

View larger version (17K): [in this window] [in a new window]   Figure 3. Age-related and group mean normalized knee extensor one-repetition maximum (KE 1-RM) strength in Caucasian and Hispanic women.

View larger version (17K): [in this window] [in a new window]   Figure 4. Age-related and group mean combined functional performance time in Caucasian and Hispanic women.

	Discussion

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Regardless of the muscle groups examined or the testing modes, reductions in muscle strength are commonly observed to occur with age in adult humans ^{(1) (3) (36) (37)}. Among individuals, and possibly populations, there is marked variability in the age-related rates of decline in strength ^{(3) (38)}. In the present study, we hypothesized that Hispanic adult females would demonstrate a greater age-associated decline in muscle strength compared with their Caucasian peers. In contrast to this working hypothesis, we found evidence for similar rates of decline based on our cross-sectional data. The rates of decline with age in absolute KE strength observed in our two populations, 7% to 8% per decade, are similar to cross-sectional data recently reported in healthy, primarily Caucasian, adults from the Baltimore Longitudinal Study of Aging ⁽³⁾.

The absence of ethnicity-related differences in the mean levels and age-associated declines in KE strength in the present study is likely the result of a combination of factors. First, in some previous studies ^{(1) (37)}, muscle mass has been reported to be the primary determinant of age-associated reductions in strength. Consistent with this, in the present investigation, thigh FFM was the strongest physiological correlate of absolute KE muscle strength. The lack of differences in thigh FFM between the Hispanic and Caucasian women would, therefore, contribute to similar levels of KE strength. Second, habitual physical activity levels are thought to significantly modulate declines in muscle strength with age ^{(1) (22) (39) (40) (41)}. There were no differences in estimated energy expenditure in the Hispanic and Caucasian women in the present study, suggesting that habitual physical activity levels may have been similar in the two groups. Third, by design, SES was similar in our Hispanic and Caucasian women. The exact relation between SES and muscle strength is unknown. However, based on the relations between SES and measures of physical performance and functional disability with age ^{(20) (28)}, it is likely that SES is positively associated with muscle strength across age in the overall population. Thus, this potential influence could have contributed to the similar levels of strength in the two groups. Fourth, most of the present subjects were under 70 years of age. As such, it is possible that differences in muscle strength and physical performance between Hispanic and Caucasian women occur at older ages and contribute to ethnicity-related differences in functional disability. Fifth, we studied only healthy, nondisabled women. It is possible that ethnic differences may have been observed in a less healthy population. In this context, Rantanen and colleagues ⁽²²⁾ recently reported a significant relation between disability and muscle strength among older African American and Caucasian women. Finally, other factors known or suspected to influence muscle strength and how it changes with age include the absolute number/loss with age in type IIb muscle fibers and changes in motor unit properties and behavior. We have no information on these modulatory influences, so they may also have been similar in the Hispanic and Caucasian women in the present study.

In older adults (65–90 years of age), both community dwelling and those living in chronic care facilities, KE strength correlates with the ability to perform basic physical tasks associated with daily living ^{(4) (9) (12) (13)}. In addition, increases in muscle strength (e.g., with resistance exercise training) are associated with improvements in physical performance in older adults ⁽⁴²⁾. The results of the present study demonstrate that the relation between muscle strength and performance of basic physical tasks shown previously in older populations including frail elderly also are observed in a population that includes young and middle-aged healthy Hispanic and Caucasian women. Although performance declined (the time to complete tasks increased) with age, the rate of decline appeared to be similar in the two ethnic groups. This likely was due in large part to their similar declines in absolute and normalized KE strength across age. The fact that normalized KE strength was the primary predictor of task performance (explaining 40% of the interindividual variance) supports this idea. Moreover, the two groups were similarly healthy and of similar SES. Performance of these tasks has previously been shown to be related to the total number of medical conditions as well as SES in Mexican American populations ⁽⁴³⁾.

There are several limitations of the present study that should be emphasized. First and foremost, as with data from previous cross-sectional studies ^{(1) (3) (37) (41) (44)}, our results cannot be used to definitively determine the rate of decline in muscle strength with age. Although longitudinal investigations have their own important limitations ⁽⁴⁵⁾, such designs would be preferable in this regard. However, we believe that the present study is an appropriate first step in determining if there is an adequate experimental basis for the substantial cost, time, and effort associated with longitudinal investigations. Our results provide no such support. Second, we only measured KE strength. As such, it is possible that the two groups studied differed in their mean levels or rates of decline with age in the strength associated with other muscle groups/tasks. Third, as mentioned previously, the Hispanic and Caucasian women in the present study did not differ in health status, SES, or habitual physical activity status. Had the Hispanic women been in poorer health, less physically active, and/or of a lower SES (a common situation in the literature), we may well have observed lower levels of KE strength in that population compared with our Caucasian controls. Fourth, our overall number of subjects for the comparisons across age was limited by the availability of Hispanic women of sufficiently high SES to compare with our Caucasian women. Finally, the Hispanic women in the present study were primarily of Mexican descent; thus, our results cannot be generalized to other populations of Hispanic women. Collectively, these limitations restrict the generalizability of the present findings to the overall population of women across age in the United States.

The present findings may have important clinical implications for gerontology and geriatric medicine. There will be a larger number of Hispanic adults in the future than any other minority. The age-associated increases in functional disability in these and other older adults result in increases in use of health care services and their related costs, as well as in a loss of independence and reduction in quality of life. Our findings provide experimental support for the concept that nothing obviously associated with Hispanic ethnicity per se (i.e., cultural and/or genetic) appears to modulate age-related declines in muscle strength. If so, this suggests that other factors that co-vary with Hispanic ethnicity (e.g., lower SES) contribute to the reduced functional capacity and increased disability previously reported in some older Hispanic compared with Caucasian women ⁽¹⁷⁾.

In conclusion, the results of the present study do not support the hypothesis that the declines in leg muscle strength with age are greater in healthy Hispanic compared with Caucasian women of similar SES. Moreover, the decrease in leg muscle strength with age is the primary predictor of age-associated declines in the performance of basic physical tasks of daily living in this overall study population of healthy women.

	Acknowledgments

 
This study was supported by National Institute on Aging (NIA) R03 Award AG16387, NIA RO1 Awards AG06537 and AG13038, and by Public Health Service research Grant 5 01 RR00051 from the Division of Research Resources of the National Institutes of Health. Ms. Casas was supported by an NIA Research Supplement for Underrepresented Minority Graduate Research Assistants to AG13038. Dr. DeSouza was supported initially by Research Supplements to Minority Individuals in Postdoctoral Training to HL39966 and AG13038 and later by Mentored Research Scientist Award HL03840. We thank the Neural Control of Movement Laboratory for facility and equipment use.

Received January 4, 2000

Accepted May 5, 2000

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Schiller, B. C. Articles by Seals, D. R. Search for Related Content PubMed PubMed Citation Articles by Schiller, B. C. Articles by Seals, D. R.