The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 60:882-887 (2005)
© 2005 The Gerontological Society of America
Cross-Sectional Associations Between Trunk Muscle Composition, Back Pain, and Physical Function in the Health, Aging and Body Composition Study
Gregory E. Hicks1,,
Eleanor M. Simonsick1,
Tamara B. Harris2,
Anne B. Newman3,
Debra K. Weiner3,
Michael A. Nevitt4 and
Frances A. Tylavsky5
1 Clinical Research Branch
2 Laboratory of Epidemiology, Demography and Biometry, National Institute on Aging, Baltimore, Maryland.
3 Division of Geriatric Medicine, University of Pittsburgh, Pennsylvania.
4 Department of Epidemiology and Biostatistics, University of California at San Francisco.
5 Department of Preventive Medicine, <--?1-->University of Tennessee, Memphis.
Address correspondence to Gregory E. Hicks, PhD, PT, Department of Physical Therapy and Rehabilitation Science, University of Maryland School of Medicine, 100 Penn Street, Baltimore, MD 21201. E-mail: ghicks{at}som.umaryland.edu
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Abstract
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Background. Associations between trunk muscle composition and physical function have not been examined previously in older adults. We hypothesized that lower trunk muscle area and attenuation (higher fat infiltration) are associated with decreased functional capacity.
Methods. The study sample consisted of a biracial cohort of well functioning men (739) and women (788) aged 70–79 from the Pittsburgh site of the Health, Aging and Body Composition (Health ABC) study. Computed tomography was used to measure trunk muscle area (cm2) and muscle attenuation (Hounsfield Unit [HU]) of the following muscle groups: lumbar paraspinals, lateral abdominals, and rectus abdominis. An average score was calculated for both trunk area and attenuation. The Health ABC Physical Performance Battery (PPB) and its individual components (usual and narrow walk, chair stands, and standing balance) were used to measure functional capacity.
Results. Linear regression analyses adjusting for demographic factors, height, body fat, thigh muscle composition, disease status, and low back pain (LBP) found that average trunk muscle area was not associated with any element of functional capacity (p >.10), whereas average trunk muscle attenuation was positively associated with the Health ABC Physical Performance Battery (p <.05) and chair stands (p <.001). Participants reporting higher LBP severity during the past year had lower muscle attenuation (p <.001 for trend), but there was no difference in average trunk muscle area according to LBP status.
Conclusions. Findings suggest a link between trunk muscle composition and history of LBP as well as reduced functional capacity in older adults. Improving trunk muscle quality may lead to reduced LBP severity and improved functional status.
RECENT studies have demonstrated a cross-sectional association between sarcopenia, age-associated loss of muscle mass (1), and poor physical function (2,3). These associations between muscle quantity and physical function have been primarily elucidated through examination of leg muscle mass (3–5). When muscle composition has been examined, higher levels of fat infiltration into the quadriceps muscle, or lower attenuation values, have been linked with reduced strength and poor physical function (6,7). Thus far, only leg muscle composition measures have been examined in relation to physical function, but good performance of lower extremity tasks also requires engagement of the spine-stabilizing muscles. The abdominals and lumbar paraspinals provide stability of the trunk during activities of daily living (8,9). Therefore, it is important to examine the association between trunk muscle composition and physical function, independent of other factors. Additionally, evidence of a relationship between poor trunk muscle function and low back pain (LBP) (10) and between performance-based measures of function and LBP (11–13) suggests a need for considering LBP when examining the relationship between trunk muscle composition and physical function.
This study examines cross-sectional associations of trunk muscle composition and physical function in a biracialcohort of men and women aged 70–79 years participating in the Health, Aging and Body Composition (Health ABC) study. In addition, we examine differences in muscle composition factors and physical function according to LBP severity during the past year. We hypothesize that lower trunk muscle area and higher fat infiltration estimated from attenuation values will be independently associated with decreased functional capacity and that higher severity of back pain will be associated with lower trunk muscle area, lower muscle attenuation, and poorer physical function.
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METHODS
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Participants
The Health ABC study population consists of 3075 well functioning black (42%) and white, men (48%) and women. Potential participants were identified from a random sample of white Medicare beneficiaries and all age-eligible community-dwelling black residents in designated ZIP code areas surrounding Pittsburgh, Pennsylvania, and Memphis, Tennessee. Eligibility criteria included: aged 70–79 during the recruitment period of March 1997 through July 1998; no self-reported difficulty walking one quarter mile, walking up 10 steps or performing basic activities of daily living; no known life-threatening cancers; and no plans to move out of the area in the next 3 years. For the present study, only participants from the Pittsburgh site were included because computed tomography (CT) scans of the paraspinal muscles were done only at this site. The sample for the present analysis consists of 1527 black (44%) and white, men (48%) and women.
Muscle Area and Attenuation
Axial CT scans at the L4–L5 disc space were acquired at the Pittsburgh site (GE9800 Advantage; General Electric, Milwaukee, WI) based on a lateral abdominal scout. A cross-sectional scan of 10-mm thickness was obtained at the L4–L5 disc space (140 kVp, 300–360 mAs). Muscle area and muscle attenuation of the abdominal and paraspinal muscles were calculated from the axial CT images using proprietary developmental software (RSI Systems, Boulder, CO). Visceral fat was separated from subcutaneous fat by manually drawing a line through the abdominal muscles or the fascial plane. Muscle and adipose tissue areas were identified based on a bimodal image histogram resulting from the distribution of CT numbers in muscle and adipose tissue.
After the adipose tissue was separated from the images, the individual muscles were identified. In the instances where the separations are not clear, these borders were outlined manually, with internal controls in place to assure that no bone density pixels were included with the muscle area. Mean muscle area was calculated by multiplying the number of pixels of nonbone, nonadipose tissue within the fascial plane by the pixel area. Mean muscle attenuation was calculated by averaging the CT number (pixel intensity) values of the regions outlined on the images. CT numbers were defined on a Hounsfield Unit (HU) scale, where 0 equals the HU of water and –1000 equals the HU of air. Higher levels of HU are associated with lower levels of fat infiltration (14). This measure of fat infiltration has been previously validated in muscle biopsy studies (14). Reproducibility of muscle area and attenuation values, assessed by reanalysis of a 5% convenience sample, demonstrated a coefficient of variation less than 5%.
Physical Function
Performance-based.--
The Health ABC Physical Performance Battery (PPB) was used to assess physical function. The Health ABC PPB consists of four lower extremity performance tests, including timed performance of five repeated chair stands, timed standing balance (semi-tandem, full-tandem, and single-leg stands), timed 6-m walk at usual gait speed, and timed-narrow 6-m walk test. Ratio scores ranging from 0 to 1 were calculated for each component of the battery, where 1 represents the maximal performance observed for healthy older adults. Participants unable to complete a test were scored 0 for that test. Ratio scores from the four tests were added together for a continuous scale ranging from 0 to 4. Higher scores represent higher levels of physical function and capacity. Specific operational definitions and scoring procedures for the Health ABC PPB have been described in detail previously (15).
Self-report.--
Participants were asked, "Because of a health or physical problem, do you have any difficulty ...?" Those responding "no" were asked, "How easy is it for you to (lift and carry 20 pounds, walk 1 mile, stand up from a chair without using your arms)? Would you say ... is very easy, somewhat easy, or not so easy?"
Covariates
Covariates included age, race, sex, height, total body fat, thigh muscle area or attenuation, prevalent disease status, and LBP status. Standing height was measured to the nearest millimeter using a wall-mounted stadiometer. Total body fat was included because high levels are associated with greater muscle mass (5,16), higher fat infiltration in the muscles (17), and poorer physical performance (5,16). Total body fat was measured using fan beam Dual X-Ray Absorptiometry (Hologic QDR4500A, software version 8.21; Waltham, MA). Because smaller mid-thigh muscle area and higher fat infiltration in the thigh are associated with poorer physical performance, thigh muscle area and attenuation were included as covariates; measurement has been previously described (3). Disease status was based on physician-diagnosed diseases, clinic data, and medication use. The following diseases were included: heart disease, lung disease, stroke, arthritis, hip fracture, and diabetes mellitus. LBP status was categorized based on the following questions: "In the past 12 months, have you had any pain in your back? If yes, would you rate the pain as mild, moderate, severe or extreme?" The severe and extreme categories were collapsed due to small sample size in the extreme group (n = 22). Therefore, LBP status consists of four categories: none, mild, moderate, or severe/extreme.
Data Analysis
All analyses were performed using SAS software (version 8.2; SAS Institute, Inc., Cary, NC). Muscle area and attenuation values for paraspinal muscles, rectus abdominis, and lateral abdominals were averaged to create composite trunk area and attenuation values. In a two-step manner, multiple linear regression analyses were used to examine association of trunk muscle area and attenuation with performance-based physical function. In the first step, we examined associations of both thigh muscle area and thigh muscle attenuation with physical function in separate models, using physical function as the dependent variable. To examine associations, independent of other factors, both analyses were adjusted for age, race, sex, height, total body fat, prevalent disease status, and LBP status. In the second step, trunk muscle area and attenuation were added to the respective thigh muscle area and attenuation models to evaluate their unique contributions in explaining variation in physical function.
Analysis of covariance was used to investigate differences in trunk muscle area, trunk muscle attenuation, and physical function among participants according to back pain status. Means were adjusted for age, race, sex, height, total body fat, and thigh muscle composition. Adjusted means were also calculated to compare trunk muscle factors according to self-reported functional status. Because the "Difficult" and "Not so easy" categories were not found to be different in terms of muscle composition factors, they were collapsed into one category. Thus, three self-reported functional status categories were used: "Difficult/Not so easy," "Somewhat easy," and "Very easy."
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RESULTS
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Table 1 displays baseline characteristics of study participants. Men had lower body fat and higher average trunk muscle area and muscle attenuation (lower fat infiltration) than did women. Also, men had better physical performance scores than did women, as measured by the Health ABC PPB.
Table 2 demonstrates the lack of association between trunk muscle area and lower extremity function. According to model 1, higher thigh muscle area is associated with better physical function, and accounts for about 5% of the variance. In model 2, trunk muscle area offers no additional explanation of variance in the Health ABC battery score or any single component of the battery (p >.10).
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Table 2. Associations Between Trunk Muscle Area and Physical Function as Measured by the Health, Aging and Body Composition (Health ABC) Physical Performance Battery.
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In contrast, Table 3 shows a significant positive association between trunk muscle attenuation and function. Model 1 explains nearly 30% of the variance in physical function score, with thigh muscle attenuation contributing approximately 18% to the explained variance. When trunk attenuation was added in model 2, the variance in performance explained by thigh attenuation was reduced to 5.5%, whereas trunk attenuation explained about 13% of the variance. Trunk muscle attenuation was positively associated with the composite function score and with the individual chair-stands component. The addition of trunk muscle attenuation negated the association between thigh muscle attenuation and chair-stand performance (Table 4).
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Table 3. Associations Between Trunk Muscle Attenuation and Physical Function as Measured by the Health, Aging and Body Composition (Health ABC) Physical Performance Battery.
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Table 4. Associations Between Trunk Muscle Attenuation and Physical Function as Measured by the Chair Stand Ratio Score.
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Tables 5 and 6 delineate differences in trunk muscle composition according to LBP status. In both unadjusted and adjusted models, there was a clear trend toward decreased attenuation in all trunk muscles associated with increased LBP severity, whereas muscle area remained constant across severity levels. Participants with higher LBP severity had significantly lower scores for each component of the Health ABC PPB (p <.01) and for the composite score (p <.0001) in both unadjusted and adjusted models (Tables 7 and 8). The between-groups difference (no LBP vs severe/extreme LBP) for the composite Health ABC PPB score was.27 points. This difference can mean as much as a 24.2-second difference in standing balance time, a difference of.27 chair stands per second, or a.54 meter per second difference in gait speed.
In Table 9, trunk muscle area and attenuation were examined according to self-reported levels of ease in performing specific functional tasks. No differences were found in the amount of muscle area or attenuation across levels of ease for walking 1 mile and rising from a chair, but participants who reported that lifting and carrying 20 pounds was "very easy" had significantly greater trunk muscle area and higher trunk muscle attenuation than those participants who reported that the task was "somewhat easy" or "not so easy/difficult."
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DISCUSSION
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Similar to previous studies examining leg muscle composition, this study demonstrates that muscle attenuation, not quantity, is the most important factor for explaining differences in both observed and reported physical function. The new significant contribution of this work is the finding that the properties of trunk muscle may be even more important than thigh musculature for those tasks that are typically considered lower extremity function. Average thigh muscle attenuation explained approximately 6% of the variance as compared to the 13% explained by trunk muscle attenuation, which suggests that the amount of fat infiltration into the trunk muscles may play a larger role in functional capacity than previously appreciated. Nevertheless, it is important to note that the strongest association between trunk muscle composition and physical performance is with the repeated-chair-stand component of the battery, likely the most taxing of the four tasks to the trunk musculature.
With the self-report assessments, only the 20-pound-lift-and-carry item showed an association with trunk muscle properties. Participants who responded that it was "very easy" to lift and carry 20 pounds had the best muscle quantity and composition, which suggests that this question has good discriminant ability, possibly due to the greater axial specificity of this task. Previous work shows that repetitive isodynamic lifting has good discriminant validity in younger patients with LBP as compared with pain-free controls (18,19). In addition, the lift-and-carry task may be challenging enough and performed often enough that participants were able to give an accurate estimate of their abilities.
These findings related to trunk muscle are not surprising given that the muscles of the trunk comprise the dynamic component of the spine-stabilizing system. According to in vitro studies, the lumbar spine, without assistance from muscles, buckles under compressive loads less than approximately 90 N, whereas the in vivo spine can withstand loads up to 18,000 N (20). The ability of the in vivo spine to tolerate such high loads is mainly attributable to the dynamic stabilizing capacity of the trunk musculature. The trunk muscles support the spine in a fashion similar to guide wires supporting a ship's mast (21). If any of the trunk muscles perform at suboptimal levels, then the stabilizing capacity of the spine will be compromised. Each muscle has a specific role in spinal function with some more suited for segmental stabilization (e.g., multifidus) and others more suited for primary movement of the spine (e.g., erector spinae). These muscles must work together in a coordinated fashion for optimal trunk stability and function. Our data suggest that high levels of fat infiltration into the trunk muscles may act to weaken the force of the muscles on the spine and thus contribute to poorer function.
Our findings also demonstrate that reports of higher LBP severity during the past year are associated with lower muscle attenuation and poorer physical function. This is especially noteworthy given that this is a generally well functioning cohort with no self-reported functional limitations. Associations between LBP, muscle attenuation, and function are likely to be even greater in a population that would include functionally limited or disabled persons. Further, these relationships are independent of any knowledge of LBP chronicity and, thus, may underestimate associations between long-term LBP, muscle quality, and function. Nevertheless, these findings offer support to the results of a small study of men with chronic LBP that found a positive association between fat content within the lumbar paraspinal muscles and self-reported disability (22). As has been suggested previously, poor trunk muscle composition may lead to the onset of LBP and thereby reduced functional capacity (10). It is also possible that poor trunk muscle composition is the result of decreased physical activity level following an episode of LBP. If either of these scenarios is correct, then improving trunk muscle quality through a physical training intervention may lead to reduced LBP severity and improved functional status in healthy older adults.
A major advantage of this study includes use of a large population-based sample with rarely available in vivo CT measurements of muscle area and attenuation as well as validated and reliable measures of physical function. It is important to identify factors associated with loss of functional capacity to help highlight possible intervention points on the pathway to disability. The combination of measures in this study provides a unique perspective on the relationship between muscle factors and functional capacity. The primary limitations of this study concern the lack of major disability in the cohort and the lack of information on current and past back pain history, which limit the generalizability of findings to healthy persons between the ages of 70 and 79 years with or without reports of back pain in the past 12 months. Also, the cross-sectional nature of this data does not allow us to examine whether causal relationships exist.
Conclusion
Trunk muscle attenuation, not quantity, is associated with reduced functional capacity and a history of significant LBP independent of thigh muscle mass and attenuation. Whether poor muscle composition predicts loss of functional capacity, development of LBP, or worsening of LBP requires further examination in a longitudinal fashion.
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Acknowledgments
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This study was funded by National Institute on Aging contract numbers N01-AG-6-2101, N01-AG-6-2103, and N01-AG-6-2106.
Part of this work was presented at the 56th Annual Scientific Meeting of The Gerontological Society of America.
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Footnotes
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Decision Editor: John E. Morley, MB, BCh
Received March 5, 2004
Accepted April 21, 2004
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Abstract
Methods
