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The form of the penguins body is streamlined. Muscles and bones allow them move under water and use their wings like screws. Unlike other flightless birds, penguins have a sternum with a distinct keel, which is attached to powerful muscles. Underwater swimming is different from flying in the air, that wing is expended the same energy on lowering, because water resistance is greater than air resistance, so the blades of the penguins have compared to other birds a large surface which supports the muscles responsible for lifting the wing. The humerus and forearm bone connected to the elbow straight and stiff, which increases the stability of the wing. The femur is very short, the knee joint is stationary, and the legs shifted back, which is a direct cause unusual gait. Large feet with flipper relatively short - while they stand on the land, they often rest standing on their heels, with a hard tail serves them additional support. The penguins tail is much shortened. The second difference of penguins from other birds is a density of the bones. All birds has a tubular bone that makes the skeleton lighter and allows you to fly or to run fast.