Effect of water immersion on human tolerance to forward and backward acceleration.

T H E physical basis of the protective effect of water immersion during acceleration has been recognized for many years. In essence, the immersed subject, rather than being exposed only to the inertial force caused by acceleration is suspended, as it were, between this f o r c e and an opposite and almost equal force due to buoyancy. Acceleration causes a proportional increase in the magnitude of both of these forces. Thus, even at large accelerations, there is little net force acting to displace the subject. For t h i s reason, the immersed subject would not be expected to experience shifts of blood like those which cause blackout. Further, the immersed subject should require no restraint during acceleration and should retain the ability to move the entire body at all accelerations. The heart and mediastinum however, are immersed in the air-filled lungs and never in water. For this reason water immersion does not alter the forces acting on the heart and