Significant abnormalities of lung volumes, ventilation, and gas exchange have been demonstrated in subjects with respiratory muscle dysfunction due to spinal cord injury. The differential loss of respiratory muscle function which exists in these subjects has provided a means of analyzing the role of the various respiratory muscles. The dual role of these muscles in providing energy for movement of gas and in maintaining a normal lung volume has been stressed. The cause of the observed increase in residual volume, gas exchange abnormalities, hyperventilation, and reduced lung compliance is believed to be the collapse of lung units at end-expiration. This, in turn, is thought to result from the altered mechanical properties of the chest cage of the subjects.