Figure 1. Functional decline after a stressor.

In patient A, the stressor results in a decline in function which does not cross the threshold of independent function. The patient is likely non-frail, because the functional level returns to the baseline, indicating that the patient was able to fully compensate to the stressor. In patient B, a similar stressor causes a decline in function which transiently results in dependence. While the patient subsequently recovers independence, it is at a lower level of functioning than prior to the stressor. The patient is likely frail because she does not return to her baseline level of function, indicating that she was unable to fully compensate to the stressor. This patient has a constricted functional reserve compared with the patient above.

Note: Day-to-day functional variation is depicted by a sine wave.