Muscle mechanoreflex of lower-extremity antigravity muscles is small during upright standing in young females with orthostatic hypotension who have not undergone exercise training

Lower-extremity antigravity muscle contraction (the static muscle pump and muscle mechanoreflex) and the baroreflex are involved in regulating blood pressure (BP) during upright standing. We hypothesized that in females with orthostatic hypotension (showing a decrease > 20/10 mmHg within 3 min upon standing from supine), the muscle pump and mechanoreflex of the antigravity muscles is small during upright standing in those who have not undergone exercise training. To test this hypothesis, we compared the cardiovascular responses to head-up tilt, head-up suspension, head-up tilt + venous occlusion (which stops venous return from the lower extremities by static muscle pump), and head-up suspension + venous occlusion of 14 young females with orthostatic hypotension, of which seven had undergone exercise training (tOH) and seven had not (uOH), with those of eight females with orthostatic normotension (control) who had not undergone exercise training. Changes in BP, heart rate (HR), and stroke volume (SV) during both head-up tilt and suspension were measured. Although the increase in HR was larger and the decrease in SV tended to be smaller during head-up tilt than during suspension in the tOH, the response patterns of both were the same in the uOH. In occlusion trials, the increase in HR was larger during head-up tilt than during suspension in the tOH, whereas the increases in HR in both scenarios were the same in the uOH. These findings suggest a small muscle mechanoreflex of the antigravity muscles during upright standing in females with orthostatic hypotension who have not undergone exercise training.