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Journal of Geriatric Physical Therapy:
doi: 10.1519/JPT.0b013e31823c4bef
Research Reports

Sarcopenic Indices in Community-Dwelling Older Adults

Merriwether, Ericka N. PT, DPT, ATC, CSCS; Host, Helen H. PT, PhD; Sinacore, David R. PT, PhD, FAPTA

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Author Information

Applied Kinesiology Laboratory, Program in Physical Therapy, Washington University School of Medicine, St Louis, Missouri

Address correspondence to: Ericka N. Meriwether, PT, DPT, ATC, CSCS, Applied Kinesiology Laboratory, Washington University, 4444 Forest Park Blvd, Campus Box 8502, St Louis, MO 63108 (emerriwether@wustl.edu).

This Work previously presented at the American Physical Therapy Association Combined Sections Meeting 2009 and Exercise and Physical Activity in Aging Conference 2010

The authors declare no conflict of interest.

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Abstract

Background: Sarcopenic (SP) indices are used to estimate loss of skeletal lean mass and function and to determine the prevalence of SP in older adults. It is believed that older women and men with lower skeletal lean mass will be weaker and have more functional limitations.

Purpose: (1) To classify community-dwelling older adults using 2 common SP indices: appendicular lean mass/height2 (ALM/ht2) and skeletal muscle index (SMI), and (2) to determine each indices value as indicators of lower extremity strength and physical function.

Methods: The sample consisted of 154 community-dwelling older adults (111 women and 43 men; mean age = 82.4, SD = 3.6 years; mean body mass index = 25.8, SD = 4.4 kg/m2). Each underwent whole-body dual-energy x-ray absorptiometry to assess lean mass. The 9-item modified Physical Performance Test and self-selected walking speed were used to evaluate function. Lower extremity strength was measured bilaterally using isokinetic dynamometry.

Results: The ALM/ht2 index classified 75 participants (49%) as SP and 79 (51%) as nonsarcopenic (NSP). The SMI classified 129 participants (84%) as SP and 25 (16%) as NSP. There were no differences in functional measures between groups by gender using either index after classification. The ALM/ht2 index was more strongly correlated with peak torque of all lower extremity muscle groups (r = 0.276–0.487) compared with the SMI (r = 0.103–0.344). There was no relationship between SP index and physical function.

Discussion: There were marked differences in how 2 SP indices classified community-dwelling older adults. Lower extremity strength was lower in older women classified as SP than NSP using the ALM/ht2 index, but LE strength was not different in older men. However, no lower extremity strength differences were observed between SP and NSP men or women using the SMI classification. None of the SP index uniformly identified community-dwelling older adults with functional or strength deficits.

Conclusions: Detection of strength deficits using SP indices alone may be gender-specific and may not reflect strength or functional decline in community-dwelling men aged 80 years or older. Given associations between lower extremity strength and physical function, strength measures remain a better predictor of physical performance than SP indices for community-dwelling older men and women.

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INTRODUCTION

Sarcopenia (SP), an age-related loss of skeletal muscle mass and strength, has been well-established as a prominent feature of currently existing paradigms that characterize physical frailty in older adults.16 Sarcopenic indices were developed to (1) establish a critical threshold of age-related muscle loss associated with functional decline and disability and (2) estimate the prevalence of SP in older adults.5,79 The appendicular lean mass/height2 (ALM/ht2) and skeletal muscle index (SMI) are 2 commonly used SP indices. Appendicular lean mass/height2 is an estimate of appendicular lean mass (ALM) adjusted for stature, whereas SMI is an estimate of total body lean mass adjusted for stature and nonskeletal tissue mass (fat, organ, and bone).5,8 Both indices have previously been shown to predict disability and functional limitations in large, epidemiologic studies of older adults.5,7,8 However, it has been suggested that the cutoff values for ALM/ht2 used to classify individuals as SP or nonsarcopenic (NSP) in the New Mexico Elder Health Survey overestimate the prevalence of SP in a variety of older populations.10 Because previous evidence suggests a highly variable relationship between skeletal muscle mass, strength, and physical performance,1113 none of the SP measure has been firmly established as the index that best reflects decrements in muscle performance and functional decline in community-dwelling older adults. This lack of consensus may be due in part to the diversity of measures used to describe physical function and to the variety of muscle groups assessed.14

Muscle strength and power are frequently used by physical therapists and rehabilitation specialists as prognostic indicators of functional decline and to discriminate functionally independent, community-dwelling older adults from functionally dependent older adults.15 Lower extremity muscle strength and power decrease with age, with the decline in lower extremity power having an earlier onset and more precipitous decline than upper extremity muscle power.16,17 Furthermore, several studies support the relationship between loss of lower extremity muscle strength and power and decline in physical functional performance.1618 However, the value of classifying older adults using any SP index for predicting lower extremity strength decline, power loss, and functional limitations in community-dwelling older adults has yet to be fully examined.

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PURPOSE

The purposes of this study were to (1) classify community-dwelling older adults using 2 indices of SP, ALM/ht2, and SMI and (2) determine the relationship of these indices to measurements of lower extremity strength and physical function. On the basis of the previous studies, we hypothesized that (1) the 2 SP indices would classify community-dwelling older adults as SP or NSP similarly, (2) there would be differences in muscle strength, walking speed, and physical performance between participants classified as SP versus NSP for both ALM/ht2 and SMI, and (3) there would be strong associations between SP indices and select lower extremity strength and functional performance measures in community-dwelling older women and men.

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METHODS

Subjects

We studied 154 community-dwelling older adults (111 women and 43 men, mean body mass index [BMI] = 25.8, SD = 4.4) who were recruited to participate in a randomized, controlled study of supervised exercise training. Inclusion and exclusion criteria have been described previously.14,19 Briefly, participants were included in the study if they (a) were 78 years or older, (b) achieved a composite score between 18 and 32 on the modified Physical Performance Test (modified PPT), (c) self-reported a need for assistance or difficulty with at least 1 basic activity of daily living or 2 or more instrumental activities of daily living, and (d) had a measured

Equation (Uncited)
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O2 peak, a measure of peak aerobic power, between 10 and 18 mL of O2/kg/min. These inclusion criteria reflect mild to moderate physical frailty.14,19 For the purposes of this investigation, SP indices and classifications were derived from body composition assessments, and all strength and functional measures reported were obtained at baseline before being assigned to an exercise training group or participating in any exercise sessions. To our knowledge, none of the participants reported being actively involved in recreational activities prior to the study. Participants volunteered for participation, were not provided remuneration, and signed an informed consent approved by the university institutional review board.

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Assessments
Sarcopenic Indices

Each participant had a whole-body, dual-energy x-ray absorptiometry (DXA) (Hologic QDR 1000/W, software version 6.2 OD; Waltham, Massachusetts) scan to assess composite and regional lean and fat mass. Bilateral upper and lower extremity fat-free masses were summed to calculate ALM and expressed in kilograms (kg).9 Appendicular lean mass was divided by height in meters (m) and squared to calculate each participant's ALM/ht2 index score.5 Using the cutoff values for ALM/ht2 developed by Newman et al,20 women with scores more than 7.23 kg/m2 and men with scores more than 5.67 kg/m2 were classified as NSP.5,20,21

We also calculated the SMI, where SMI = [total body skeletal muscle mass (kg)/total body mass (kg)] × 100.8,22 Total body skeletal muscle mass was determined using the following formula: total body skeletal muscle mass = (1.13 × ALM) − (0.02 × age {years}) + (0.61 × sex {0 = female; 1 = male}) + 0.97 for DXA-derived measurements.20 For the SMI SP index, we used the cutoff values proposed by Janssen et al,8 where scores more than 37% for men and more than 28% for women were considered NSP. Body weight (kg) and height (m) were assessed while standing without footwear using a balance scale with stadiometer (Detecto, Webb City, Missouri).

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Physical performance

Physical function was evaluated using the 9-item modified PPT.14 In addition, self-selected walking speed was determined by having participants perform 2 timed walking trials more than a 50-foot distance (15.24 m). We report the average walking speed determined from 2 trials (m/min).14

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Strength measures

Lower extremity peak torque values were averaged more than 3 trials for knee extension, knee flexion, ankle dorsiflexion, and ankle plantar flexion at 0°, 60°, 120°, and 180° per second, using the Cybex II isokinetic dynamometer (Cybex, Division of Lumex, Inc, Ronkonkoma, New York). Participants were positioned as previously described by Brown et al.14 Values are reported in Newton-meters.

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Analysis

Of 182 potential participants, 85% of participants who had a complete complement of baseline measures of lean mass, modified PPT scores, walking speed, and lower extremity peak torque were included in the statistical analysis (N = 154). Classifications of SP and NSP were derived using cutoff values for the ALM/ht2 and SMI described previously. We first determined the percentages of those classified as SP and NSP for each SP index (ALM/ht2 and SMI) for all participants together, as well as for men and women separately. These percentages were compared using χ2 with Yates correction. Data were tested for normality using the Shapiro-Wilk test. Once the percentages of participants classified as SP and NSP for each SP index were determined, we tested for group differences in anthropometric, body composition, functional performance, and lower extremity peak torque measures, using an analysis of variance (group × measure). This was done for the entire group and then for groups separated by sex. The relationships between SP indices, strength, and functional performance measures were determined using Pearson product moment correlation coefficient. All data were analyzed using Systat 11.0 statistical package (Systat Software, Chicago, Illinois).

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RESULTS

Participant Characteristics and SP Indices

Table 1 summarizes key participant characteristics. The mean age for all participants was 82.4 (SD = 3.6) years (range: 78–95 years), with an average modified PPT score of 28.2 (SD = 4.4) points out of a maximum of 36. The mean walking speed for all participants was 65.5 (SD = 14.1) m/min. The NSP group had greater ALM than the SP group when using the ALM/ht2 index (Table 2), but there were no group differences observed in ALM using the SMI index (Table 3).

Table 1
Table 1
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Table 2
Table 2
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Table 3
Table 3
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Sarcopenic Classification

Cutoff values using the ALM/ht2 index for the NSP classification were more than 7.23 and more than 5.67 kg/m2 for men and women, respectively. Using the ALM/ht2 cut-off scores, 75 participants (49%) were classified as SP and 79 participants (51%) as NSP (Figure 1; Table 2). When analyzed by sex, 47% of women were classified as SP whereas 53% of men were classified as SP (Figure 1; Table 2).

Figure 1
Figure 1
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When using the SMI cut off values for NSP, which was a score more than 37% for men and more than 28% for women, only 16% of participants were classified as NSP (Figure 1; Table 3). The SMI classified 78% of women and 98% of men as SP versus 22% of women and 2% of men as NSP when analyzed by sex (Figure 1; Table 3). There were significant differences in body weight and BMI between SP and NSP groups for men and women for both SP indices (P < .05) (Tables 2 and 3).

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Sarcopenic Indices and Physical Function

As summarized in Tables 2 and 3, there were no significant differences between SP and NSP groups in mean values for the modified PPT or for walking speed in women or men using either SP index. As summarized in Table 4, neither SMI nor ALM/ht2 indices correlated with the modified PPT or with walking speed. A similar trend was observed for both sexes.

Table 4
Table 4
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Sarcopenic Indices and Lower Extremity Peak Torque

There were group differences in peak torque values for select lower extremity muscle groups. For ALM/ht2 index, the NSP group, considered as a whole, had significantly greater peak torque for the knee extensors. In women, the NSP group demonstrated greater peak torque in 2 of 4 selected muscle groups (knee extensors and ankle dorsiflexors) at all speeds. However, in men there were no group differences in peak torque for any of the selected muscle groups at any speed using the ALM/ht2 index (P > .426). No group differences in peak torque for most of the selected muscle groups were found at any speed using SMI (P > .198) when analyzed by sex. Because a similar trend was observed for all muscle groups at each speed, values at select speeds for each group are given in Table 2.

The ALM/ht2 index and lower extremity peak torque demonstrated a small but statistically significant relationship with knee extensor peak torque in women (r = 0.289–0.423) but not in men (r = 0.059–0.230). Data from select lower extremity peak torque measures are shown in Table 4.

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DISCUSSION

The primary purposes of this study were to classify community-dwelling older adults using 2 commonly used SP indices and to determine the relationship of these indices to objective measures of lower extremity strength and physical function. To our knowledge, this is the first study to examine how well validated SP indices reflect lower extremity strength and physical function in community-dwelling older adults. In our study, the SMI and ALM/ht2 SP indices were markedly different in how they classified community-dwelling older adults as SP or NSP. The ALM/ht2 index yielded similar percentages of those classified as SP versus NSP in older adults aged 80 years or older to those of Baumgartner et al5, where 53% to 57% of men and 43% to 60% of women aged 80 years or older were classified as SP when stratified by age and ethnicity. Interestingly, the SMI classified significantly more community-dwelling older adults as SP than the ALM/ht2 index, a trend more profound in men than in women. The percentages of women older than 80 years classified as SP in this study (78%) are similar to those of Janssen et al,8 in which 72% were classified as SP. However, the percentages of men classified as SP contrast with findings from Janssen et al,8 in which 50% of men older than 80 years were classified as SP compared with 98% in our study. One possible explanation for this is the difference in measurement technique for body composition measures between studies. Janssen et al8 used bioelectrical impedance to obtain body composition measurement, whereas we used DXA to determine lean mass. Cut-points for SP classification of SP or NSP often differ on the basis of the measurement technique used to obtain measures of fat and lean mass.10 These findings affirm the significance of how the classification of older adults as either SP or NSP is highly influenced by the method used to assess body composition, particularly lean mass.11

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Sarcopenic Indices and Physical Function

Our findings suggest that none of the SP index is strongly associated with common functional measures that physical therapists routinely use and, therefore, may be insufficient determinants of baseline physical function in community-dwelling older adults. This occurs despite small group differences in ALM. These observations differ from those of Baumgartner et al5 and Janssen et al,8 in which both groups concluded that ALM/ht2 index and the SMI are associated with risk for functional limitation and disability, with adjustment for confounding variables such as age, race, and comorbidities. However, the established relationship between SP classification, physical function, and disability in these previous studies was based on self-reported measures of functional limitation and disability. The modified PPT used in the current study represents an objective, performance-based functional measure, which we used to evaluate the relationship between SP classification and physical function in community-dwelling older adults. Furthermore, since SP indices serve primarily to describe lean mass characteristics, the fact that ALM/ht2 and SMI are poorly correlated with function is consistent with previous findings that lean mass measures are inadequate predictors of functional decline.1113

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Sarcopenic Indices and Lower Extremity Peak Torque

In this study, the ALM/ht2 index more effectively showed differences in LE strength in women than in men 80 years of age or older. However, no lower extremity strength differences were observed between SP and NSP men or women using the SMI classification. These results suggest that the detection of strength differences using SP indices alone could be sex-specific and may not reflect strength or functional decline in community-dwelling men aged 80 years or older.

Recent literature advocates for a more expansive definition of SP that includes the loss of measurable variables such as strength and physical performance, the influence of increasing adiposity with decreasing ALM, termed “sarcopenic obesity,” on declines in lower extremity strength and physical performance23, the loss of composite and regional lean mass, and more definitive cutoff values.10,11,24 In addition, some propose the incorporation of the term “dynapenia,” the age-related decline in muscle performance (strength and power) independent of decreases in lean mass, into gerontological discourse because it is thought to create a clear distinction between decrements in muscle mass and muscle performance in older adults.25,26 Our results indicate that dichotomized SP classifications using commonly used SP indices (ALM/ht2 and SMI) have a variable relationship to lower extremity strength, confirming the potential dissociation of decreases in ALM and strength of major lower extremity muscle groups. Therefore, use of an SP classification alone as a proxy indicator of mass, strength, and functional deficits may not accurately reflect functional decline in community-dwelling older adults. Given the established association of lower extremity strength and power, functional performance, and disability, lower extremity strength may continue to have greater utility as an indicator of physical functioning in community-dwelling older adults than appendicular or total body lean mass.24,17,14,25

There are limitations to this study. First, we studied community-dwelling older adults with relatively few functional limitations, the majority (∼82%) of whom had a BMI less than 30 kg/m2, which would not classify them as obese. Older adults who are more functionally compromised may exhibit different relationships between SP indices and physical function. Furthermore, the inclusion of a larger number of older adults with a BMI more than 30 would allow for an additional classification of SP obesity and for an analysis of the influence of this classification on physical function and lower extremity strength.29 Second, we examined the utility of 2 SP indices. It is possible that other SP indices may better relate to function and strength in older adults than those included in this study. Third, our research design and analysis were cross-sectional, so it is difficult to firmly establish a causal relationship between SP classification and its impact on lower extremity strength and physical function. Future studies should continue to explore the relationships between SP indices and classifications (with consideration of the impact of fat mass), lower extremity strength, and physical performance in more functionally compromised older adult populations and after therapeutic exercise interventions to determine how SP indices are impacted.

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CONCLUSIONS

The SMI and ALM/ht2 are markedly different in how they classify community-dwelling older adults as SP or NSP. None of the index was strongly associated with modified PPT score or walking speed. Given previously described associations between lower extremity strength and physical function, strength measures remain a better predictor of physical performance than SP indices for community-dwelling older men and women.

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ACKNOWLEDGMENT

This study was funded by NICHD T32 HD 07434-17, NIA P60 A613629.

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lean mass; older adults; physical function; strength

Copyright © 2012 the Section on Geriatrics of the American Physical Therapy Association

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