Comparing two methods to quantify physical resilience.

Physical resilience, conceptualized as the extent of observed recovery after a health stressor, is an important construct in research on aging and frailty. Various methods have been proposed to quantify physical resilience using repeated measures of health or function following a stressor. The purpose of this analysis is to directly compare two alternative approaches to quantifying physical resilience - the recovery trajectory (RT) and the expected recovery differential (ERD) - using data from 170 older adults in an observational study of elective knee replacement surgery. Each participant's resilience, based on repeated measures of pain interference over 6 months after surgery, was determined using both the RT and the ERD method. The 10 individuals classified as high resilience based on RT also had better-than-expected recovery based on ERD. However, ERD scores were more variable among individuals classified by RT as moderate resilience (n = 82). Of those classified as low resilience (n = 78) by RT, most (85.9%) also had worse-than-expected recovery based on ERD. In this head-to-head comparison of two conceptually distinct approaches for quantifying resilience after a health stressor, the results were most comparable in individuals at extremes of high or low recovery patterns. Each approach has merit for quantifying physical resilience after a health stressor, and factors that may influence the choice of method and interpretation of results are discussed.