Sangeetha Pulapaka

Birds have many physical features, besides wings, that work together to enable them to fly. They need lightweight, streamlined, rigid structures for flight. The four forces of flight – weight, lift, drag and thrust – affect the flight of birds.. They have an enlarged breastbone called a sternum for flight muscle attachment – this helps with the force of thrust.

We have a large, prominent breast muscle, called the pectoralis major, that originates along the breastbone, or sternum, and inserts near the head of the upper arm bone (the humerus). When the breast muscle contracts, it brings the arm close to the body. (The motion is like a bird’s downstroke.) Bodybuilders can bulk up their breast muscle, but a flat vertebrate sternum doesn’t have enough surface area to accommodate attachment of the enlarged muscles required by birds.

To raise an arm, we use a smaller muscle, known as the deltoid, on the top of the shoulder. Place your hand on the edge of your shoulder and raise your arm. You will feel the deltoid muscle bulge slightly. The anatomy of the vertebrate shoulder doesn’t provide a site for attaching a larger arm-raising muscle, and this location is woefully inadequate for the requirements of a bird. By the way, because the pectoralis major is large and the deltoid is small, you can bring your arm down with much greater force than you can raise it.

The first avian solution was to add a vertical keel to the sternum. The keel dramatically increases the surface area for muscle attachment. As you can see in the diagram above, the horizontal sternum forms a T with the vertical keel. In the top illustration, the pectoralis major, the lower muscle on the keel, is shown in red. It inserts in the humerus and is shown contracting, pulling the wing down.

Because muscles function only by shortening, conventional wisdom holds that a muscle must be located above the wing in order to raise it. But birds lift their wings using a large muscle located beneath the wing. Attached to the keel of the sternum, the muscle, known as the supracoracoideus, connects to the top of the humerus by way of a pulley, an ingenious mechanism found nowhere else among vertebrates.

The supracoracoideus, shown in white in the top illustration, is found just above the pectoralis. Its tendon loops over the shoulder, inserting on the top of the humerus. The supracoracoideus is shown in red in the middle illustration. It’s contracting and, because of the pulley, lifting the wing.

As you can see in the above diagram, the pulley is located where three bones — the coracoid, scapula, and clavicle (not shown) — come together to form the shoulder joint. The tendon slides over the grooved head of the coracoid.

The coracoids are large, stout bones that connect the sternum with the shoulder. The shoulder joint is much like the cupped palm of your hand. To visualize this, pretend the fist of your right hand is the head of the humerus, and place it into the cupped left palm. Now imagine that the tendon of the supracoracoideus passes over your left hand and inserts on the top of your right wrist, and that the tendon of the pectoralis major inserts underneath the wrist. Holding your wrist rigid, move your elbow up and down to simulate the alternating contractions of the supracoracoideus (upstroke) and pectoralis major (downstroke).

Birds have undergone many adaptive changes for flight. Among the most dramatic are the extreme enlargement of the breast muscles and the skeletal modifications that accommodate them, and the development of a unique pulley system that allows a muscle located under the wing to raise it. All of this points again to the amazing structure and function of birds. 

To summarize humans cannot fly because our bodies unlike birds have massive bones.