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The patella, more commonly known as the kneecap, is the hard, triangular bone that protects the front of the knee joint. Seeming to float in front of the knee joint, it is actually connected by ligaments to the muscles above and below the knee. Besides its role in protection, this bone serves as a lever for the leg muscles, increasing their ability to move the leg.
The knee joint undergoes a great deal of stress and is actually rather delicate. If you consider how often a person walks, stands up, bends, jumps, runs, and caries heavy loads, you'll see that the knees get quite a workout every day. The patella hovers over the front of the knee joint, serving as a hard, physical barrier against damage. When you fall and skin your knees, the kneecap helps insure that the damage is absorbed by the skin and bone, instead of harming the more delicate muscles, cartilage, ligaments, and tendons of the knee joint. Every time you walk into something and bang your leg, bounce a soccer ball on your knee, do a somersault, or kneel down, it protects your knee joint from unnecessary damage.
The tendon of the quadriceps femoris muscle, the large muscle in the front of the upper leg, is attached to the top of the patella. When you straighten your lower leg, the quadriceps muscle contracts, pulling the bone upward. The bottom of the kneecap is connected to the lower leg by the patellar tendon. As it is pulled upward by the quadriceps muscle, it in turn pulls against the patellar tendon, moving the lower leg into alignment with the upper leg.
This movement is used every time you take a step forward, kick with your lower leg, or rise from a crouched or seated position to a standing position. Moving the entire body upward, sometimes while lifting a heavy weight, can require a great deal of force. The patella acts as a lever, extending the length of the knee joint and acting as a solid base for the tendons to attach to. This way, when the quadriceps contract, the muscle can exert more force and a person is able to stand up more easily, lift more weight, or kick with more force.
On a few occasions I have opened a wooden door, probably with a bit too much force and hit my patella. The pain is excruciating. There is really nothing much that protects it. It is mostly just covered with skin. So when opening a door toward you, be aware. Protect your patella.