Functional importance of a highly elastic ligament on the mammalian diaphragm

The diaphragm of mammals is a musculotendinous dome separating the thoracic and abdominal cavities. With no skeletal elements to stretch it, the diaphragm has the problem of positioning its muscle fibres at a length appropriate for the onset of an inspiratory contraction. This is achieved through a negative intrapleural pressure, resulting from the opposing elastic recoil of the ribcage and lungs, which sucks the diaphragm into the thorax and extends the muscle fibres. A consequence of this negative pressure is that the diaphragm muscle is under tension when inactive during expiration. This is an unusual condition for skeletal muscles, which can suffer irreversible changes when stretched to long length, or they may respond by growing longer. We now describe a highly elastic and resilient diaphragmatic ligament which sets a sarcomere length enabling the muscle to use its full operating range, reduces stress on the diaphragm muscle fibres, and assists shortening of the diaphragm muscle at the onset of inspiration by means of elastic recoil.