Dynamic cardiovascular response to +Gz stress in aerobically trained individuals.

Very high onset sustained +Gz stress requires rapid cardiovascular response to support human tolerance. This study was conducted following a previous study concerning +Gz tolerance in aerobically trained individuals, and was initiated to determine if intense aerobic conditioning might affect cardiovascular +Gz tolerance through reduction in heart rate response to +Gz stress. The study compared heart rate response data on 22 aerobically trained runners and 13 less-conditioned individuals. All subjects were exposed to a standard medical evaluation protocol, which consisted of a gradual-onset (0.1 G/s) acceleration exposure (GOR1), followed by a series of rapid-onset (1.0 G/s) acceleration exposures (ROR), a second gradual-onset rate exposure (GOR2), and a third gradual-onset rate exposure with the subjects performing anti-G straining maneuvers (GORS). Aerobic conditioning was not found to be associated with a reduced heart rate response to +Gz stress, compared to the response of unconditioned subjects, when the following variables were considered; heart rate change from rest to maximum exposure heart rate, heart rate change from rest to the heart rate achieved at the onset of maximum G, and the rate of change in heart rate per unit +Gz. Although enhanced parasympathetic tone, induced by long-term aerobic conditioning (running) results in a reduced heart rate at rest and during +Gz stress, it does not alter the responsiveness of the heart rate to +Gz stress.