Human Responses to Activity in Earth and Simulated Moon and Mars Gravities

PROJECT SUMMARY Models of activity in reduced gravity are needed to study physiologic responses to space mission activities on the moon or nearby planets. Current paradigms of hypo gravity, including parabolic flight, water immersion, centrifuge models, head up tilt and body suspension. Each of these methods has limits, such as restricted motion, restricted simulation time, or limitations in tasks subjects can perform. Using these models, investigators have identified several major physiological limitations for persons who experience extended space travel. One major problem is controlling stroke volume in standing and walking subjects. Lower Body Positive Pressure (LBPP) has been used to manipulate stroke volume during various activities. From previous studies, LBPP applied at >25 mmHg to upright subjects, can return stroke volume to supine levels [1-3].The return of stroke volume was accompanied by increased mean arterial pressure (MAP) and decreased heart rate, with either increased cardiac output and decreased vascular resistance [I, 2] or no change in cardiac output and increased muscle sympathetic nerve activity and vascular resistance [3]. Another study reported no increase in blood pressure [4]. Use of LBPP to control stroke volume in upright subjects has potential to reliably simulate cardiovascular responses to reduced gravity [14]. Even though it has been shown [I] that cardiovascular responses to LBPP are affected by posture (supine vs. upright) as in several studies involving head up tilt (HUT) [11] limited data are available on cardiovascular responses to increased tissue pressure as a result of wearing various antigravity suits in the supine and upright positions. For example, at rest, MAP has been shown to increase proportionally to the increases in LBPP and the increases were greater in supine than in upright position. However, each ofthe above results was obtained in a variety ofways, each with its own limitations: either using an LBPP box, or a pressure chamber, Anti-shock trousers [10], pneumatic trousers [8] or Mast garments [10]. Thus, the first goal ofthis project is to compare the cardiovascular responses ofLBPP and head up tilt. In the last few years, studies of LBPP on cardiovascular regulation have gained popUlarity, in part due to the introduction ofthe G-Trainer/Alter-G treadmill [15]. In the Alter-G, the subject, wearing special Alter-G shorts is sealed at the waist inside the LBPP enclosure and positive air pressure is applied to the subject's lower body while they stand, walk or run on an enclosed treadmill. This LBPP results in fluid shifts which in tum affects the cardiovascular responses to exercise. However, it is not clear how much of the cardiovascular effect is from the air pressure inside the chamber and how much is from the Alter-G shorts themselves. Since the shorts are tight, it is possible that much offluid shift and resulting cardiovascular changes are due to the shorts and not the LBPP system used by the Alter G. Therefore the second goal ofthis study is to separate effects ofthe compression shorts from the Alter-G LBPP. Finally, in an upcoming study, Univ. of Kentucky investigators and their collaborators from the Cardiovascular Lab at NASA Johnson Space Center will compare healthy human cardiovascular responses to standing and moving on the earth to standing on the Moon and Mars. The subjects will be studied twice, once on an "Earth" day where their body fluid will be normal and on another "Space" day where their fluid will be reduced to the amount typical ofa short space mission. The third goal of this study is to compare two models of activity in reduced gravity environments (Head up tilt to -10 degrees [Moon], -20 degrees [Mars], 80 degrees [Earth] vs Lower body positive pressure applied to subjects in an enclosed chamber (Alter_G), to reduce body weight to 20% [Moon], 40% [Mars] or 0% [Earth]. Results will be compared for their suitability to model actual space flight activities.