Muscles for flight: the sternum

The keel-shaped sternum (breastbone) is where the powerful flight muscles attach to the body. The size of the sternum and hence the breast muscles depend on various factors such as flight activity, technique and body mass. Flightless birds have a flat sternum. Physicists have calculated that humans would need a sternum that protrudes by 1.8 metres if they had to attach flight muscles.