EFFECT OF ACCELERATION ON THE CHEST-WALL

The gravitational force on the rib cage has been found to be an expiratory force of similar to 8 cmH(2)O. The gravitational force on the abdomen is an inspiratory force of the same magnitude. Because the compliance of the rib cage is greater than the compliance of the abdomen, it follows that gravity has a net expiratory effect on lung volume and that upward accelerations augmenting the gravitational force would have an additional expiratory effect. This conclusion is contrary to observations that functional residual capacity increases during headward accelerations in centrifuges and during intervals of upward acceleration in airplanes. We report the results of two studies of the effects of accelerations that are smaller in magnitude and of shorter duration than those studied in centrifuges and airplanes. The first was an experimental study of the effect of acceleration in an elevator. In subjects who relaxed against an occluded airway, airway pressure increased during upward accelerations and decreased during downward accelerations. The second was the modeling and analysis of the effects of the accelerations that occur during walking. The analysis predicted an initial expiratory response to the acceleration spike that occurs during footfall. The prediction agreed with data in the literature on the respiratory effect of walking. In both of these studies upward accelerations had an expiratory effect.