| Abstract: | To explore the hypothesis that lower body musclemass correlates with orthostatic tolerance, 18 healthy volunteers (age18-48 yr; 10 men, 8 women) underwent a graded lower body negativepressure (LBNP) protocol consisting of six, 5-min stages of suction up to 60 mmHg in 10-mmHg increments. Forearm blood flow, heart rate, andblood pressure were measured, and forearm vascular resistance wascalculated. Leg muscle mass was assessed by dual-energy X-ray absorptiometry. All subjects received standard intravenous hydration for at least 8 h before the study. Six men and four women completed allstages of LBNP. Four men and four women developed presyncopal symptoms,including marked bradycardia and/or hypotension, at LBNP levelsof 30 mmHg (n = 2; 1 man, 1 woman), 40 mmHg (n = 2; 1 man, 1 woman), and 50 mmHg (n = 4; 2 men, 2 women). Thepresyncopal subjects had leg muscle masses ranging from 19.5 to 25.2 kgin men and from 11.7 to 16.6 kg in women. In subjects who completed allstages of LBNP, leg muscle mass ranged from 17.5 to 24.1 kg in men andfrom 10.4 to 18.0 kg in women. Leg muscle mass did not differ betweenpresyncopal subjects and those who completed the protocol. Furthermore,there were no differences in the hemodynamic responses to LBNP betweensubjects with low vs. high leg mass. These data suggest that leg musclemass is not a critical determinant of LBNP tolerance in otherwisehealthy men and women. |
