Cautionary tails of grip strength in health inequality studies: An analysis from the Canadian longitudinal study on aging.

Self-rated health is widely used in studies of the socioeconomic gradient of health in community-based populations. Its subjectivity may lead to under- or over-estimation of a true underlying socioeconomic gradient and has increased interest in searching for alternative, objective measures of health. Grip strength has emerged as one such alternative for community-based older populations, yet no study has directly assessed the relationship between these two measures and compared their associations with socioeconomic status and health behaviours. Using 26,754 participants aged 45-85 years in the baseline data of the Canadian Longitudinal Study on Aging Comprehensive Cohort, we estimated adjusted-grip strength through indirect standardization using age, sex, height, weight, and their square terms and used ANOVA to assess the variance of adjusted-grip strength within and between each self-rated health category. We ran four separate logistic regression models, examining unhealthy tails (those reporting poor health vs. not and those at the bottom 8th percentile of adjusted-grip strength vs. above) and healthy tails (those reporting excellent health vs. not and those at the top 20th percentile of adjusted-grip strength vs. below). Stronger adjusted-grip strength correlated with better self-rated health, but only 2% of the total variance of adjusted-grip strength was explained by variance between the self-rated health categories. While self-rated health largely showed the expected socioeconomic gradients and positive relationships with health enhancing behaviours, adjusted-grip strength showed no clear, consistent associations with either socioeconomic or health behaviour variables. The results give caution about using grip strength as an objective alternative to self-rated health in studies of social inequalities in health. Empirical approaches demand careful considerations as to which dimensions of health and corresponding measures of health are most relevant to the context being studied.