Cardiovascular response to orthostatic stress: effects of exercise training modality

The effects of exercise training posture on cardiovascular and baroreflex responses to orthostatic challenge were assessed in highly trained cyclists (CT, n = 8) and swimmers (ST, n = 8), and in untrained men (UT, n = 8). CT demonstrated the lowest orthostatic tolerance to lower body negative pressure (LBNP, 0 to -50 mmHg), with only 3 subjects completing the full LBNP procedures; 5 UT and all ST completed the testing. During LBNP, stroke volume (SV) decreases were similar in CT and ST, but greater than in UT. Mean pulse pressure and systemic vascular resistance (SVR) were reduced in CT relative to ST and UT at the highest levels of LBNP; the slope of the delta SVR/delta Zo and delta SVR/delta SV relationships in CT, used to assess peripheral vascular baroreflex function, were attenuated relative to the other groups. There were no between-group differences in the heart rate response to LBNP. The greater incidence of orthostatic intolerance observed in upright-versus supine-trained athletes during passive LBNP was linked to attenuated baroreflex control of peripheral vascular resistance.