Muscle Weakness in Older Americans: Examining the Measurement, Health Outcomes and Life Course Determinants

Abstract

Muscle weakness, as measured by hand grip strength, has been shown to be an important indicator of future disability, chronic disease status and early mortality. While muscle weakness is considered an important prognostic indicator, how best to measure weakness, the long term health outcomes associated with muscle weakness across different sub-populations and the life course determinants remain poorly understood. Indeed, there is little agreement on how best to measure muscle weakness across different groups and how to screen for muscle weakness in order to prevent disability and premature mortality. We begin by first deriving cutpoints for muscle weakness in a nationally representative sample of older adults from the Health and Retirement Study. After using Classification and Regression Tree (CART) models to identify cutpoints, we found that fifty-five percent of men (max grip strength < 39kg) and 47% of women (< 22kg) were classified as weak. Higher cutpoints were identified for Black males (< 40kg) and females (< 31kg), and the prevalence of weakness (57% and 88%, respectively), was higher compared to Whites. As an extension to these results, we examine the validity of these cutpoints in a longitudinal setting to determine whether muscle weakness is associated with disability dynamics and premature mortality. We found that clinical muscle weakness, as identified by sex/race specific population-derived cutpoints from Aim 1, is strongly associated with the onset, progression and persistence of physical disability status. The odds of experiencing an onset of ADL disability were 54% higher among weak individuals compared those who were not weak at baseline. We also found that weak individuals had a steeper decline in their survival trajectory, compared to non-weak individuals. Specifically, weak individuals were over 50% more likely to die earlier than non-weak individuals. Lastly, the third aim of this dissertation seeks to quantify and understand whether experiencing stressful and traumatic events across the life course influences trajectories of grip strength in later life. We found that life course trauma and stress experienced during emerging/early adulthood was associated with both mean grip strength at age 50 and trajectories of grip strength over time. Among Black men, stress and trauma experienced during emerging/early adulthood was not only related to higher mean grip strength at age 50, but also associated with steeper declines as individuals aged over time compare to White men. Among Black women, traumatic events during emerging/early adulthood were associated with lower mean grip strength at age 50. Taken together, the results of this dissertation chart new territory in its overall goal to improve the measurement of muscle health across the diverse, older adult population and in identifying those most at risk for future disability and premature mortality. This dissertation also demonstrates the importance of investigating how life course social exposures drive differential vulnerability to muscle weakness among older adults. The results of this dissertation can be used to inform clinical practice in screening adults for muscle weakness while also seeking to shift the distribution of muscle weakness at the population level in order to intervene among those who are most vulnerable.