Water on the knee is known in medical terms as "knee effusion". Quite simply, it is an accumulation of fluid around the knee joint, making it appear swollen. This reduces the flexibility of the knee and makes movement or even support of the body by the joint rather painful. Overuse of the knee joint, using it to perform a type or amount of activity it's not accustomed to, can place undue strain on the joint, leading to this condition. If you have not exercised in years and experience a sudden increase in physical activity, this could result in water on the knee. When changing your level of activity, do so gradually. This gives your body a chance to become acclimated to increased use.
Injury is another cause of knee effusion. Torn ligaments, broken bones or damage to the meniscus (knee cartilage) can lead to this reaction in the joint. Other conditions such as infection, arthritis, bursitis, gout and other diseases can also lead to this kind of fluid buildup. Being overweight can add greater stress on knee joints and thus lead to water on the knee. This can prove to be rather problematic, especially if the overweight person wants to exercise in order to reduce his or her weight. Water on the knee can put plans for exercising on hold until the joint is healed.
The first line of defense is to rest the joint. Working the knee joint while in this state can lead to further damage and more swelling. Use pillows or other objects to raise the knee joint above the level of the heart. To reduce the swelling, apply ice packs. Never leave ice on the body for more than 15-20 minutes at a time. Repeat the application if necessary but allow half an hour or more between icings.
There are many sports that place a great deal of stress on the knees and should be limited or avoided if you've already had water on the knee unless a healthcare professional has helped you eliminate the underlying cause and strengthen the affected joint . These sports include soccer, football, hockey, rugby, squash, basketball, tennis, volleyball and downhill skiing. Essentially, any sport that requires strong, quick lateral movement.
Low-impact exercise can help strengthen the joint once the underlying cause of water on the knee has been eliminated. The key focus should be on gradual strengthening of the body and the joint. Give your body a chance to get used to any change of activity before ramping up the energy on any exercise program. Avoid any sudden jolt to the knees. When starting out, you might avoid running and opt for other types of exercise. Try short knee bends (not the deep kind). Walking and swimming can also prove beneficial. If you take up cycling, consider staying on flat terrain to start with, and stick with a high seat and low gear.