Mechanics of quiet respiration
The Diaphragm is primary respiratory muscle. The Diaphragm contracts, pulling the central tendon down until the abdominal wall prevents the abdominal viscera from moving any further. This increases the vertical diameter. The 12th rib is fixed by quadratus lumborum.
Central tendon is fixed so the diaphragm now elevates the lower 6 ribs, which increases the infrasternal angle with a bucket handle movement. This increases the transverse diameter of the thorax.
Lower 6 ribs push upward thereby elevating the sternum and the upper 6 ribs thrust their anterior ends upward and forward until all motion at the manubriosternal symphysis taken up. This increases the antero-posterior diameter.
The elevation force then produces a bucket handle movement at ribs 3-6 while ribs 1 and 2 are fixed by scalenes. This increases the transverse diameter. The intercostals contract to prevent tissues from being drawn in. The scalenes maintain vertical diameter by preventing 1st and 2nd ribs from being drawn in while fascia suspends the thorax to the axial skeleton
What is missing here is the manner in which spinal action organizes movement in all vertebrates. The case of breathing, spinal extension precedes the action of the muscles of respiration. This means that the extension of the spine, and the action of the scalenes and sternocleidomastiods draw up the aponeurosis or central tendon of the diaphragm, effectively limiting its excursion and allowong it to lift the ribs, its evolved purpose. The diaphragm has little descent when posture if good: