The Skeleton - The appendicular skeleton
The bones of the appendages—arms, hands, and fingers; legs, feet, and toes—allow human beings to perform an astonishing array of complex movements, from pushing themselves through the physical rigors of the Olympic decathlon to creating an elaborate piece of needlework. The key points in the appendicular skeleton, as indeed, in the axial skeleton, are where the ends or edges of bones lie close together and must work with or against one another in order to achieve coordinated movement. These key points are the joints —not really bones at all but the non-bony spaces between bones.
A typical joint consists of several different structures. First, there are the bones themselves—two, three, four, or more almost touching in the area of the joint—with their ends or edges shaped to fit in their respective niches. Between the bones of an appendage joint (as between the vertebrae of the back) is the smooth, resilient material called cartilage that allows the bones to move over one another without scraping or catching. At the joint, the bones, with their layer of cartilage between them, are held together by tough bonds of muscle. Bursas , tiny sacs containing a lubricating fluid, are also found at joints; they help to reduce the friction between a joint's moving parts.
The Hip and Knee
The hip joint must not only support the weight of the head and trunk, but must allow for movement of the leg and also play a part in the constant balancing required to maintain upright posture. Similar stresses and strains, often literally tending to tear the joint apart, are placed on every joint in the body.
The notoriety of athletes’ bad knees attests to the forces battering at the knee joint, the largest in the human body. Sports involving leaping or sudden changes in direction, such as basketball, are especially hard on the knee. However, the fact that there are not more disabled sports heroes speaks well for the design of the knee joint. The same can be said of the ankle joint and the joints of the foot and toes.
The Shoulder, Elbow, and Wrist
The counterpart of the hip joint in the upper trunk is the shoulder joint. Free of weight-bearing responsibilities, the shoulder has a system of three interconnected joints that allow it and the arm far more versatile movements than the hip and leg.
The elbow connects the upper arm bone ( humerus ) with the two bones of the lower arm ( radius and ulna ). Like the knee, it is basically a hinge joint, which allows the lower arm to be raised and lowered. The elbow is also constructed to allow some rotation by the hand; likewise, the knee joint allows us to waggle the foot.
Of all our body parts, the wrist, hand, and fingers are perhaps the most elegantly and finely jointed—witness the performance of a concert pianist—and our ankles, feet, and toes probably the most subject to everyday misery.