[The respiratory muscles and the thoracic cage during respiration at rest].

Even though the diaphragm is the primary respiratory muscle in humans, resting breathing involves other muscles. The diaphragm acting alone is not able to expand the entire rib cage. The expansion of the upper rib cage results from the combined actions of the scalene and intercostal muscles, in particular the parasternals. The external intercostals and the levator costae might constitute a reserve system that is called into action when the load placed on the inspiratory pump is increased.