LEG MASS AND LOWER-BODY NEGATIVE-PRESSURE TOLERANCE IN MEN AND WOMEN

To explore the hypothesis that lower body muscle mass correlates with orthostatic tolerance, 18 healthy volunteers (age 18-48 yr; 10 men, 8 women) underwent a graded lower body negative pressure (LBNP) protocol consisting of six, 5-min stages of suction up to 60 mmHg in 10-mmHg i ncrements. Forearm blood flow, heart rate, and blood pressure were mea sured, and forearm vascular resistance was calculated. Leg muscle mass was assessed by dual-energy X-ray absorptiometry. All subjects receiv ed standard intravenous hydration for at least 8 h before the study. S ix men and four women completed all stages of LBNP. Four men and four women developed presyncopal symptoms, including marked bradycardia and /or hypotension, at LBNP levels of 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). Th e presyncopal subjects had leg muscle masses ranging from 19.5 to 25.2 kg in men and from 11.7 to 16.6 kg in women. In subjects who complete d all stages of LBNP, leg muscle mass ranged from 17.5 to 24.1 kg in m en and from 10.4 to 18.0 kg in women. Leg muscle mass did not differ b etween presyncopal subjects and those who completed the protocol. Furt hermore, there were no differences in the hemodynamic responses to LBN P between subjects with low vs, high leg mass. These data suggest that leg muscle mass is not a critical determinant of LBNP tolerance in ot herwise healthy men and women.