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One of the most common causes of knee tightness is a condition known as chondromalacia patella or "Runner's Knee," which arises from errors in the movement of the kneecap. The kneecap can slide off its normal track during movement if supporting muscles such as the quadriceps are too weak to cushion the impact. This greatly increases the risk of knee tightness for individuals involved in highly-strenuous physical activities, as the constant pressure fatigues leg muscles to a great degree. It is also possible to worsen Runner's Knee through inactivity. Other causes of knee tightness include joint inflammation, muscle cramps, and poor blood circulation.
Runner's Knee occurs when the kneecap slides off to either side, rather than smoothly over a groove located near the femur. This causes pressure to build up on the area surrounding the knee, making it difficult to move the joint. In addition, the friction created by the abnormal movement wears down the cartilage found underneath the kneecap, making it even harder to bend the knee. Misaligned or deformed joints can increase the risk of Runner's Knee.
Sitting in a position with knees bent a certain way can cut off circulation to the knee, aggravating the pain and tightness caused by Runner's Knee. Reduced circulation can lead to numbness in the knee or the sensation of swollen joints. These two symptoms can also cause the knee to feel tight, although they aren't indicative of any actual knee tightness.
Several conditions, such as rheumatoid arthritis, can cause inflammation in the knee. The increased pressure on the cartilage and ligaments can lead to both knee tightness and pain. Dehydration, on the other hand, can contribute to muscle cramps, which then make moving the knee difficult. Although the tightness is often limited to the muscles surrounding the joint, the sensation can sometimes extend to the knee itself.
Individuals can prevent knee tightness by developing strength and endurance in the muscle groups involved in knee bending and straightening. Regular leg exercises, such as jogging or cycling, can significantly reduce the risk of knee tightness. Improving the flexibility of these muscles can also help prevent Runner's Knee and cramps, making stretching an important routine before and after any form of strenuous physical activity. Individuals with poorly-formed joints or muscle deficiencies can choose to wear knee supports or orthopedic footwear to alleviate the strain on the knee. In cases of serious abnormalities, patients can opt for corrective surgery.
@Mor - You shouldn't take for granted that your weight is the only thing that contributes to your knee pain. It could be a number of things, some of which have been described in this article. Knee injuries can happen without people even realizing it and the knees are very reactive to problems with the muscles of the legs as well.
I had a friend (who was quite skinny) who had knee problems and he ended up going to a physiotherapist. It turned out that the muscles on one side of his knee were slightly too tight and were pulling it out of whack. He was given stretching exercises and within a few weeks the problem was solved.
You might be just suffering from excess weight, but it might be something else as well. You should get it checked out.
Knees seem to be especially vulnerable if you are carrying extra weight. It's not surprising considering how much impact they have to absorb when someone is just walking. Both my mother and I have knee problems and we both find that they magically vanish when we are under a certain amount of weight (which tends to not even be that little). The problem is that when you are trying to lose weight it can be difficult to find low impact activities that won't make the knee problem worse, but will help to burn calories.
We found that water aerobics can help a lot. Particularly since it can help to strength the leg muscles. Swimming in general is also good, as is walking a lot with very good, supportive shoes.
On top of that you also have to make sure your diet isn't too bad. But, being able to walk up stairs without worrying about knee pain is worth it.