Changes in end-expiratory lung volume can alter the mechanical function of the muscles of inspiration and may affect the pattern of thoracoabdominal movements. The present study examined the effect of increasing end-expiratory lung volume on the motion of the rib cage and abdomen during inspiration. In six seated subjects, end-expiratory volume was increased by expiratory threshold loading. The end-expiratory thoracoabdominal configuration shifted to the left of the relaxation curve presumably as a result of the activation of the abdominal expiratory muscles. There was outward displacement of both the rib cage and the abdomen with inspiration at the elevated volume but the relative volumetric contribution of abdominal displacement to the inspired volume was significantly less than during breathing from FRC. When at an enlarged lung volume subjects were constrained to initiate inspiration from a point on the thoracoabdominal relaxation configuration, there was inward movement of the abdomen and a decrease in abdominal dimensions during inspiration. Inward abdominal movement occurred despite large increases in diaphragm electrical activity and was associated with an inspiratory fall in gastric pressure. These results suggest that at large lung volumes, the function of the diaphragm as an agonist is lost and its function as a fixator may be impaired unless there is an activation of the abdominal muscles and the diaphragm is appropriately lengthened.