Changes in lower limb volume in humans during parabolic flight

Variations in gravity [head-to-foot acceleration (G(z))] induce hemodynamic alterations as a consequence of changes in hydrostatic pressure gradients. To estimate the contribution of the lower limbs to blood pooling or shifti ng during the different gravity phases of a parabolic flight, we measured i nstantaneous thigh and calf girths by using strain-gauge plethysmography in five healthy volunteers. From these circumferential measurements, segmenta l leg volumes were calculated at 1, 1.7, and 0 G(z). During hypergravity, l eg segment volumes increased by 0.9% for the thigh (P < 0.001) and 0.5% for the calf(P < 0.001) relative to 1-G(z) conditions. After sudden exposure t o microgravity following hypergravity, leg segment volumes were reduced by 3.5% for the thigh (P < 0.001) and 2.5% for the calf (P < 0.001) relative t o 1.7-G(z) conditions. Changes were more pronounced at the upper part of th e leg. Extrapolation to the whole lower limb yielded an estimated 60-ml inc rease in leg volume at the end of the hypergravity phase and a subsequent 2 25-ml decrease during microgravity. Although quantitatively less than previ ous estimations, these blood shifts may participate in the hemodynamic alte rations observed during hypergravity and weightlessness.