Maintenance of oxygen/carbon dioxide homeostasis requires continuous movement of air into and out of the alveolus. In healthy humans, inspiration occurs when the inspiratory muscles contract to generate a sub-atmospheric pressure in the thorax; in contrast, expiration is passive at rest, though it can be assisted by expiratory muscle contraction in the absence of flow limitation. The respiratory muscles are histologically and physiologically identical to peripheral skeletal muscle. The fibre-type mix of the diaphragm, the most important inspiratory muscle, is similar to that of the quadriceps − 50% type I and 50% type II. Diaphragm contraction results in downwards movement of the diaphragmatic dome (Figure 1). This generates a negative pressure change in the thorax (resulting in inspiratory airflow) and a positive pressure change in the abdomen. Unopposed diaphragm action (as in high tetraplegia) therefore results in inwards movement of the ribcage. In healthy humans, this is opposed by co-contraction of the upper thoracic inspiratory muscles (scalenes and sternomastoid) and by expansion of the lower ribcage, achieved by transmission of the abdominal pressure through the zone of apposition.