Lower body muscle mass and lower body negative pressure tolerance in men and women

To explore the hypothesis that lower body muscle mass might be associated with orthostatic intolerance, 18 healthy volunteers (18-48 yrs, 10 ♂, 8 ♀) underwent graded lower body negative pressure (LBNP) consisting of six, 5-minute stages of suction up to 60 mmHg in 10mmHg increments. Leg muscle mass was assessed by dual energy X-ray absorptiometry. All subjects received standard intravenous hydration for 12-16 hours prior to the study. Six men and 4 women completed each stage of LBNP. Four men and 4 women developed pre-syncopal symptoms, including marked bradycardia, at LBNP levels of 30 mmHg (n=2; 1 ♂, 1 ♀), 40 mmHg (n=2; 1 ♂, 1 female;), and 50 mmHg (n=4; 2 ♂, 2 female;). The pre-syncopal subjects had leg muscle masses ranging from 19.5 kg to 25.2 kg in men (SEM=1.04), and 11.7 kg to 16.6 kg in women (SEM=1.04). In subjects that completed all stages of LBNP, leg muscle mass ranged from 17.5 kg to 24.1 kg in men (SEM=1.04), and 10.4 kg to 18.0 kg in women (SEM =1.61). These data suggest that leg muscle mass is not a critical determinant of LBNP tolerance in otherwise healthy men and women.