: Why shin and forearms bones are both in 2 parts ?
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The elbow is a hinge joint. It is only capable of one axis of movement. The wrist is also mostly a hinge joint, it is capable of slightly rocking side to side, but mostly rotates on a single axis. The ability to “rotate” our hands comes from this two-bone plan of the lower arm.
The Ulna (large lower arm bone) hinges on the Humorous (upper arm bone) to make the elbow hinge. The Radius (small lower arm bone) forms a hinge with the tarsal bones with the wrist. The two lower arm bones are able to rotate around each other to allow the hand to completely “roll over” about 180 degrees.
The leg is similar, however the size difference between the tibia and fibula are more pronounced and the ankle is less able to rotate around the long axis of the leg, only about 90 degrees.
So we can move. Otherwise we would be stiffer.
I’ll explain further; the purpose is the ability to rotate the hand or foot, though the human foot is not terribly mobile in terms of rotation per se.
Also, different muscles are attached to the different bones to produce different movements.
The two bones are not fixed together, they are connected via ligaments and can move in relation to each other.
The ulna and radius (forearm) actually completely cross over one another when you rotate the hand (palm up, palm down). Your wrist doesn’t rotate, it’s all in the forearm. Rotate your hand. See what moves.
Thats why it is two bones. And not one big stiff one.
Along with all the other answers but also cuz fish. There is a documentary called “Your Inner Fish” that explains it really well
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