Cognitive status, muscle strength, and subsequent disability in older Mexican Americans

OBJECTIVES: To examine the association between Mini-Mental State Examination (MMSE) score and subsequent muscle strength (measured using handgrip strength) and to test the hypothesis that muscle strength will mediate any association between impaired cognition and incident activity of daily living (ADL) disability over a 7-year period in elderly Mexican Americans who were initially not disabled. DESIGN: A 7-year prospective cohort study (1993-2001). SETTING: Five southwestern states (Texas, New Mexico, Colorado, Arizona, and California). PARTICIPANTS: Two thousand three hundred eighty-one noninstitutionalized Mexican-American men and women aged 65 and older with no ADL disability at baseline. MEASUREMENTS: In-home interviews in 1993/1994, 1995/1996, 1998/1999, and 2000/2001 assessed social and demographic factors, medical conditions (diabetes mellitus, stroke, heart attack, and arthritis), body mass index (BMI), depressive symptomatology, handgrip muscle strength, and ADLs. MMSE score was dichotomized as less than 21 for poor cognition and 21 or greater for good cognition. Main outcomes measures were mean and slope of handgrip muscle strength over the 7-year period and incident disability, defined as new onset of any ADL limitation at the 2-, 5-, or 7-year follow-up interview periods. RESULTS: In mixed model analyses, there was a significant cross-sectional association between having poor cognition (MMSE<21) and lower handgrip strength, independent of age, sex, and time of interview (estimate=-1.41, standard error (SE)=0.18; P<.001). With the introduction of a cognition-by-time interaction term into the model, there was also a longitudinal association between poor cognition and change in handgrip strength over time (estimate=-0.25, SE=0.06; P<.001), indicating that subjects with poor cognition had a significantly greater decline in handgrip strength over 7 years than those with good cognition, independent of age, sex, and time. This longitudinal association between poor cognition and greater muscle decline remained significant (P<.001) after controlling for age, sex, education, and time-dependent variables of depression, BMI, and medical conditions. In general estimation equation models, having poor cognition was associated with greater risk of 7-year incident ADL disability (odds ratio=2.01, 95% confidence interval (CI)=1.60-2.52); the magnitude of the association decreased to 1.66 (95% CI=1.31-2.10) when adjustment was made for handgrip strength. CONCLUSION: Older Mexican Americans with poor cognition had steeper decline in handgrip muscle strength over 7 years than those with good cognition, independent of other demographic and health factors. A possible mediating effect of muscle strength on the association between poor cognition and subsequent ADL disability was also indicated.