Change of shape of the chest wall during static respiratory efforts. During static inspiratory efforts the horizontal section of the rib cage at the xiphoid level becomes, generally, more elliptical, whereas during expiratory efforts it becomes more circular. Hence the force of the respiratory muscles is not evenly distributed, but it acts mainly on the lateral parts of the rib cage, whereas the position of the frontal part is in part related to the transthoracic pressure. In about 50% of the cases the horizontal section of the rib cage during inspiratory efforts is smaller and during expiratory efforts larger than that during relaxation at the same lung volume. The changes of shape of the chest wall during static respiratory efforts suggest that, during hyperventilation and when the resistances to breathing are increased, the work of breathing is larger than that calculated on the basis of the volume-pressure relation, since part of the force exerted by the muscles is expended in deforming the chest wall.