TOLERANCE TO TRANSVERSE (+GX) AND HEADWARD (+GZ) ACCELERATION AFTER PROLONGED BED REST.

Tolerance to the transverse ( + G x ) acceleration of a simulated Gemini re-entry profile was determined before and after 4 weeks of absolute bed rest. Tolerance to headward ( + G z ) acceleration was studied before and after 4 weeks of absolute bed rest and 2 weeks of modified bed rest. As judged by the degree of physical discomfort, the ability to respond to a central light, or the presence of electrocardiographic abnormalities, tolerance to + G x was unaffected by 4 weeks of absolute bed rest. In each subject studied, heart rates during peak acceleration were higher after bed rest than. before. As judged by the level of acceleration at which central vision was lost, no significant change in tolerance to headward ( + G z ) acceleration of rapid onset was observed after 2 weeks of modified bed rest or after 4 weeks of absolute bed rest. After each type of bed rest, the majori ty of the subjects had decreased tolerance to headward ( + G z ) acceleration of gradual onset, but the mean decrease was not statistically significant. Mean heart rates at equivalent levels of + Gz were significantly higher after both periods of bed rests. The only ar rhythmia of clinical importance noted was the appearance of bursts of premature atrial contractions during G.O.R. + Gz in 1 subject after 2 weeks of bed rest. I T IS GENERALLY BELIEVED that tolerance to transverse ( + Gx) acceleration will be unaffected by a 30-day flight in the Manned Orbiting Laboratory. No studies seeking objective support for this opinion have been performed, however. The use of prolonged bed rest to simulate the cardiovascular effects of prolonged space flight has been discussed previouslyY This paper presents an investigation of the effects of 4 weeks of absolute bed rest on tolerance to transverse (+Gx) acceleration. In addition, the effects of 4 weeks of absolute bed rest and 2 weeks of modified bed rest on tolerance to headward ( + Gz) acceleration are described.