Author Affiliations: Department of Medicine, School of Medicine (Drs Xue and Walston), Center on Aging and Health (Drs Xue and Walston), Johns Hopkins University, Baltimore, Maryland; Mailman School of Public Health, Columbia University, New York, New York (Dr Fried); and Division of Gerontology and Geriatric Medicine, Baltimore VA Medical Center, Geriatric Research, Education, and Clinical Center (GRECC), University of Maryland School of Medicine, Baltimore (Dr Beamer).
Aging people progressively lose skeletal muscle mass and strength.1 Epidemiologically, a single measurement of grip strength has repeatedly proven to correlate with subsequent adverse health outcomes2 even when measured in midlife to predict physical disability decades later.3 This has led some to propose that grip strength be used clinically as an indicator of risk for decline in health, or even as a new “vital sign.”2,4 However, it remains unclear whether the risk of developing adverse outcomes is greater or less for a person who is relatively weak compared with other age-matched individuals than it is for a relatively strong person whose strength is declining rapidly. We hypothesized that those with a faster rate of decline would be at higher risk compared with those who had a weaker single grip measurement. We tested this hypothesis using longitudinal data from older women studied for the evolution of disability and functional decline with aging.
The Women's Health and Aging Study (WHAS) II is an institutional review board–approved, prospective cohort study of 436 women aged 70 to 79 years with high functional ability at baseline, as previously described.5 Data were collected at baseline and 6 follow-up examinations approximately 18 months apart, except for the interval between the third and fourth examination, which was on average 3 years, resulting in a median follow-up time of 9 years (ranging from 1.5 to 13.3 years) between 1994 and 2008. The analytic sample for this report consisted of 352 women who had baseline data available on all covariates and had at least 2 measurements on grip strength during the follow-up period.
Grip strength was measured using a JAMAR hand dynamometer (Model #BK-7498; Fred Sammons Inc, Burr Ridge, Illinois).5 The maximum measurement of 3 trials in the nondominant hand was used in the analyses.6 Outcomes of the study included incident health events: falls, walking speed slower than 0.4 m/s, the WHAS frailty phenotype,7 and difficulty in 1 or more task (“disability”) of the Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL) scales.8
We used a linear random effects model (REM) to assess population average rate of change in grip strength over time (ie, fixed effects) by including age in the model as a time-dependent covariate. To account for the observed nonlinear time trend in change of grip strength, a 2-piece linear spline was used in the REM with 1 knot fixed at age 75 years.9 To account for between-person heterogeneity in terms of individual deviation from the population mean trajectory, intercept (ie, grip strength at age 70 years, termed baseline strength henceforth) and age slope before age 75 years were modeled as random effects with an unstructured variance-covariance matrix.
Next, we performed joint analysis of the repeated measurements of grip strength and the time-to-event data using a method that has been previously published.9 The model assessed the effects of the rate of change in grip strength on the outcomes independent of baseline strength. For each outcome, analysis was restricted to women who were event-free at baseline and included only strength measurements up to the time of event onset or censoring. All analyses were adjusted for age, race, education, body mass index (BMI), number of chronic diseases, smoking status, physical activity, depressive symptomatology, and serum interleukin 6 and albumin, as in prior research.9 Statistical analyses were conducted in SAS (version 9.2; SAS Institute Inc, Cary, North Carolina).
Of these 352 women included, 17% were African American and 70% were either overweight or obese. Their mean age was 74 years at baseline, with a mean 12.7 years of education. At baseline, 20% reported a history of falls in the past 12 months; 3% of women were frail; 11% and 5.4% reported any ADL and IADL disability, respectively; and none had a walking speed slower than 0.4 m/s.
The REM model estimated that the mean grip strength was 26.5 kg at age 70 years and declined an average of 1.08 kg/y between age 70 and 75 years (P < .001) and 0.52 kg/y thereafter (P < .001). Independent of baseline grip strength, a greater rate of decline in grip strength over time, was significantly associated with higher risk for all outcomes except ADL disability. For example, the risk of developing an IADL disability was 1.32 times higher for every 0.5-SD unit (SD = 0.07 kg/y) increase in the rate of decline in grip strength (P < .001) after adjusting for age, race, education, BMI, and baseline strength. All results remained essentially unchanged after further adjustment for other covariates (Table).
Considering baseline grip strength, our analysis showed that higher baseline strength was significantly associated with a lower risk of IADL disability and frailty, but not the other outcomes, after adjusting for change in grip, age, race, education, BMI at baseline. Specifically, the risk of developing an IADL disability and becoming frail were 1.35 (P < .001) and 1.47 (P < .001) times higher, respectively, for every 0.5-SD unit (SD = 1.9 kg) decrease in baseline strength (Table).
We observed that a decline in grip strength over time is a stronger predictor of a greater variety of subsequent adverse outcomes compared with a single observation of grip strength, suggesting that “becoming weaker” is important in addition to “being weak.” These associations were independent of age, disease burden, life style, nutritional status, inflammation, and mental well-being. These results are congruent with the findings of prior cross-sectional studies. However, they offer further understanding of why weak grip is predictive of poor outcome, implying that at least some of the “risk attributable to weakness” seen in prior studies can be restated as “risk attributable to rate of losing strength” or perhaps “risk attributable to being in a strength-losing state.”
This study suggests that measuring grip strength over repeated clinic visits may provide useful risk assessment information to patients, families, and clinicians. What most needs to be demonstrated to make grip strength more useful clinically, however, is whether it should trigger any specific interventions or diagnostic efforts. Until we know much more about the clinical relevance and underlying causes of change, it may be premature to promote grip strength—even its trajectory—as a “vital sign.”
Correspondence: Dr Beamer, Baltimore VA Medical Center, 10 N Greene St, GRECC (BT/18/GR), Baltimore, MD 21201 (email@example.com).
Author Contributions:Study concept and design: Xue, Fried, and Beamer. Analysis and interpretation of data: Xue, Walston, and Beamer. Drafting of the manuscript: Xue, Walston, and Beamer. Critical revision of the manuscript for important intellectual content: Xue, Fried, and Beamer. Statistical analysis: Xue. Obtained funding: Walston, Fried, and Beamer. Administrative, technical, and material support: Fried and Beamer. Study supervision: Walston and Beamer.
Financial Disclosure: None reported.
Funding/Support: The project described was supported by Awards R01AG023701, P30AG02133, and R01AG11703 from the National Institute on Aging.
Role of the Sponsor: The funding agency played no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; and preparation, review, or approval of the manuscript.
Disclaimer: The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Aging or the National Institutes of Health.
Previous Presentation: Preliminary results of this study were presented at the Gerontological Society of America 62nd Annual Scientific Meeting; November 19, 2009; Atlanta, Georgia.
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