You are kneeling or squatting on the floor and suddenly you can't stand up. Or perhaps you are walking or playing sports and one leg becomes frozen in position. Or you try to rise out of your seat...but you can't.

It is almost as if a vice has clamped noto your knee and is preventing from moving. Well...almost. Something very real is probably preventing your knee from moving.

In normal situations, the knee is a smoothly operating mechanism, like a door hinge swinging open and shut. But if you put a jam in the door or an object in the hinge, that smooth movement may be obstructed or frozen in position.

This is what orthopedics call a true locking of the knee: something that physically prevents the knee from fully straitening out or bending and holds the knee rigidly in place. It is often painful, and since it can occur while climbing stairs, sitting on the ground, or being in some other vulnerable position, it can be quite frightening.

Usually, the locking is caused by a torn piece of cartilage, or possibly a loose bone fragment resultibg from a bone disorder called osteochondritis dissecans. The cartilage fragment or bone chip moves freely in the knee cavity until it gets trapped between two joint surfaces, impending your ability to straighten the knee.

Sometimes the obstruction is from a imsaligment of the bones and muscles around the knee. Weakness of the muscles on the inside of the thihg or tightness of the outer muscle can throw off the aligment of the kneecap. The kneecap comes out of groove with the thigh bone, derails and gets stuck, not allowing you to bend or extend. When the muscle returns to normal, the kneecap gets back on track and movement proceeds normally.

True locking is fairly rare. But many times, say following an injury, a person will experience an inability to move the knee. Doctors call this a pseudo-lock.

A pseudo-lock is simply a reaction to pain. The pain mechanism won't allow the joint to extend or flex fully. Sometimes the pain that creates a speudo-lock is brought on by a twist or a bump to the knee. Or it could just as easily be a bad case of stiffness from sitting too long.

Although a locked knee may simply unlock with a little rest, don't assume you have been cured. Serious knee damage can occur if your lock was brought on by a loose piece of bone or cartilage. Any restriction of knee movements should be treated by a doctor. The problem may have to be corrected surgically.

A fall, twist or bang on the knee can obviously cause knee pain. So can an old injury that never healed properly and now acts in response to changes in the barometer. But most recurrent or unexplained knee pains comes from over-use.

The knee is held by tough ligaments that connect, protect and stabilize the joint; cartilage that cushions the bones; and tendons that join muscles to bone. But even these resilient tissues have their limits. Too much bending and twisting, too much running or too much jumping can cause them to rupture or become inflamed.

Damage to these tissues also takes its tall on the sensitive surfaces of the knees' bones. The tissues in the knees work like the shock absorbers in a car. If you make a lot of sudden stops or subject these components to stresses they were not designed to handle, they wear out. In a car you will hear the sound of metal on metal; in a knee you will feel the pain of bone rubbing on bone.

Many over-use pains are lumped under the umbrella term chondromalacia patellae, a fancy way of saying pain in, around and under kneecap.

But not all over-use pains belong to this category. Suppose you have an active teen in your house complaining of pain just south of the kneecap. It could be Osgood-Schlatter disese, commonly called growing pains, a condition arising from excess stress on the tendons of the lower keg bone. Combines lots of physical activity with the rapid muscle and bone growth of puberty, and you get one misserable kid with a painful, bony enlargement or bump on the upper part of his leg, just in front of and below the kneecap.

When the knee is subjected to misuse as well as over-use, it can develop a condition called synovitis. Your mother may have called it water on the knee, because the achy joint now bears a striking resemblance to a water balloon. Synovitis is from a bang or twist that causes certain tissues in the knee to fill with blood or other fluids.

Another knee condition realted to over-use is sometimes called housemaid's knee. It is really a form of bursitis, irrigation to the front of the knee causes a bursal sac in front of the kneecap to fill up with fluid. It's most common cause: prolonged kneeling on hard surfaces.

The knee is also a potential site for a painful condition called osteochondritis dissecans, a necrosis, or deth of a segment of bone or catilage. Its cause is unknown. Eventually, the dead cartilage or bone chip can break off and produce even more pain as well as a locked knee.

In some instances, nasty knee pain can actually originate elsewhere in the body, such as the toe, foot, spine or hip. Fallen arches or weak ankles can cause your foot to ovre-pronate (rotate too far inward), putting too much force on the knee.

And if you have poor posture or an improper gait, it can focus pain directly to your knee.

The knee is also a favourite target for arthritis in its many forms. Osteoarthritis results from the breakdown of cartilage and other joint tissues after years of wear and tear. Rheumatoid arthritis is characterised by a progressive development of pain and swelling in joints and their connecting tissues, which can be coupled with other symptoms like fatigue, weight loss and low-grade fever. And gout is a metabolic condition in which severe arthritis develops when uric acid is deposited in joints and other tissues.

In addition, the knee is also a potential site for tumors and cysts as well as painful bacterial infections.

The key to silencing most angry knees is not to subject them to the kind of activities that ticked them off in the first place. Repeated abuse is a sure-fire means of provoking a ferocious response.

Treat pain knees with kindness and they'll behave. Let these tips show you how.

Use RICE on Injuries. Whether from an acute trauma like a sprain or simple over-use, injuries respond best to RICE; an acronym for rest, ice, compression and elevation. Rest is the key component. Then after a period of limited activity, doctors like to see a gradual resumption of activity and exercise.

Supplement several days of rest with applications of ice: 15 minutes at a time, several times a day to reduce swelling. Compress the knee by wrapping it snugly, but not too tightly, in an elastic bandage to limit movement. And elevate the knee with pillows to drain fluids from the joint.

Take an analgesic. Aspirin and ibuprofen are powerful pain and inflamation fighters that will help sore, swollen knees. Accetaminophen will help pain, but won't do anything for swelling.

Warm your wbbly knees. Cold is fine for injuries after they first occur, but most lingering pain responds best to moist warmth. We recommend a warm, moist towel, a hot-water bottle, a moist heating pad, a warm bath or a whirlpool.

Drop a few pounds If you're overweight, losing weight is a good way to reduce some of the painful forces acting on your knees with each step. Contact forces on joint surfaces in the knee can range up to eight times body weight. Lose 10 pounds, and you will reduce those forces by 80 pounds, which is a lot.

Support your arches A simple over-the-counter arch support in your shoe can prevent overpronation. People who pronate severely may require a professionally fitted arch device.

Cushion you kneeIf you must spend periods of time on your knees, take some of the stress off your kneecaps by wearing cushioned knee pads. And take frequent rest breaks so that the stress isn't applied constantly.

Avoid sqatting. Squatting and deep knee bends put enormous stress on the knee and can cause cartilage tears or possible rupture of the quadriceps tendon. Repetitive squatting can also produce prolonged episods of knee pain in certain individuals.

Find alternatives to running. Nonpounding activities like biking, walking and swimming can provide the same benifits as running, but are much kinder to your knees. If you must run, increase your warm-up time, cut abck you milage, run on softer surfaces and always wer quality running shoes.

Send your knees to the gym. Poor muscle tone is often the real culprit underlying most chronic knee problems. "That's why in 80 percent of all cases, painful knees will respond to a sensible exercise program focusing on building flexibility and strength, particularly in the quadriceps and hamstrings(the large muscles at the front and back of the thighs).