Resilience to stressors has emerged as a major gerontological concept aiming to promote more positive outcomes for older adults. Achieving this aim relies on determining mechanisms underlying capacity to respond resiliently. This paper seeks proof of principle for the hypothesis that physical aspects of said capacity are rooted in the fitness of one's physiology governing stress response, conceptualized as a dynamical system. The Study of Physical Resilience in Aging ("SPRING") leveraged stimulus-response experiments to characterize physiological fitness in older adults scheduled for one of three major stressors: Total knee replacement, incident hemodialysis, or bone marrow transplant in hematological cancer. Here we analyze Holter monitor time series, cortisol responses to adrenocorticotropic hormone (ACTH) stimulation, and repeated diurnal salivary cortisol assessment in the SPRING pilot (n=79). Principal components analysis was applied anticipating steady-state and "adaptation" mechanisms underlying the repeated physiological measures. Analytic features evidenced these mechanisms, supporting construct validity. Component scores were analyzed by major stressor, hypothesized surrogate physiologic measures (physical frailty phenotype, self-report of health), and demographic, health and behavioral characteristics. Scores differed substantially by stressor type and the surrogate physiologic measures, evidencing criterion validity. Our data support that HRV, ACTH and salivary cortisol stimulus-response data jointly assess adaptation capacity across a variety of major stressors. We believe that SPRING is the first study in humans to concurrently query multiple physiologic systems using stimulus-response tests. Our findings lay groundwork for future validation with further data and to better forecast resilience of older adults to clinical stressors.