The concept of "complexity", derived from the field of nonlinear dynamics, can be adapted to measure the output of physiological processes that generate highly variable "chaos-like" fluctuations. It has been proposed that physiological aging is connected with a widespread loss of such complexity in the dynamics of healthy organ system function, resulting in a decreased ability to adjust to physiological stress. This study presents the non-linear analysis of heart rate variability in response to isometric exercise in frail elderly.