The vigor with which the arches of the ribs move in an inspiratory direction is determined by the excess of inspiratory force of costal movement over the counterbalancing or restraining agencies that retard the movement of the ribs.
In inspiration the thorax is enlarged circumferentially and longitudinally by the scaleni and the intercostal muscles and the diaphragm. The muscles that increase the circumference of the thorax (the scaleni and intercostals) and the diaphragm, which increases its length, must mutually counterbalance and overcome each other. To appreciate this fundamental and important fact, one should study a patient in whom paresis of all these muscles is sufficient to preclude their action in concert but which still affords sufficient power to enable them to function separately.
Such a patient recently entered the women's ward at Lakeside Hospital. She was 40 years old and had a chronic poliomyelitis anterior that involved all the neck
HOOVER CF. THE DIAGNOSTIC AID IN EVALUATING THE VIGOR OF INSPIRATORY COSTAL EXCURSION. Arch Intern Med (Chic). 1926;37(6):773-779. doi:10.1001/archinte.1926.00120240043003