The Physical Structure of the Body
One of the bad guys on Sarah Conner Chronicles wondered why the human body he was occupying didn’t have a hip joint (ball and socket) where the shoulder is located. The implication is that the body is an inferior design or perhaps an oversight. The differences between shoulder and hip, arms and legs, hands and feet are interesting. The differences are based upon use and need for the user. It would seem the hip joint is all about supporting body weight and locomotion of the body. The shoulder joint can be seen more as a servant of the hand.
Hands and feet are similar in many respects and when people lose their hand function amazing things have been done with the feet as replacements.
Hands have very few muscles within them most of the strength and movement comes from the forearms with the energy transferred through long tendons acting like cables.
The forearms are able to rotate giving the hands very fine motor control all the way into the fingers. The elbow is a hinge joint and once the forearms rotate to a point the rotation is transferred up into the shoulder. The shoulder is enormously complex as compared to the hip. It seems the shoulder is in the business of more flexibility than the hip and yet a ball and socket would seem to provide pretty good mobility, but the deeper the socket the more mobility is limited. The deep socket is much stronger than the complex shoulder joint but would seem to lack flexibility of the shoulder.
Just as the arm is the servant of the hand. Legs are the servants of the feet. Feet are different than hands. The heel provides support for walking. The muscles operating the toes are in the feet as opposed to forearms for the hands. Toes are not as flexible although quite remarkable in their own right.
The knee is a hinge joint but the calf does not rotate so all rotation of the foot goes up into the hip. You can bend your elbow and rotate your hand, you can’t do this with your knee and foot. Again we go back to the needs of the body. Legs and feet are supporting structures, hands and arms are more for manipulation.
If we still ran on all fours, if we ever did, our shoulders would be hips. We would probably loose some of our functionality of our upper body as a biped. Particularly in the hands.