The knees are hinge joints that have the ability to flex and extend. Flexion takes place when you bend your knee backwards and move your heel toward your buttocks. Extension takes place when you straighten your leg. Stiffness and pain can result from numerous reasons. In some cases, this is acute and in others, it is chronic.
Arthritis
Osteoarthritis, rheumatoid arthritis and post-traumatic arthritis can all cause stiffness and pain in the knees. Osteoarthritis is usually a slowly progressive degenerative disease in which the joint cartilage gradually wears away; it is the most common type of arthritis to affect the knees, according to American Academy of Orthopaedic Surgeons. Activity levels, long periods of sitting and weather changes can all contribute to pain and stiffness from arthritis.
Bursitis
The joints contain small, fluid-filled sacs that act as cushions between the bones and tendons. These are known medically as bursae. Bursitis is a condition that causes bursa sacks to become inflamed. This can not only cause pain and stiffness in the knee, but it can also cause swelling, warmth and redness, according to the National Institutes of Health.
Gout
Gout is a form of arthritis that usually starts at the big toe and then spreads to other joints like the knees. It is related to uric acid. This is a by-product of purines, which are found in such foods and beverages as organ meats, mushrooms, scallops and alcohol. When these foods and beverages are consumed, purines create urate crystals, which then pool up at the joints, leading to pain and stiffness.
Ligament Damage
Ligaments are strong cords of tissue that connect bones to bones. A torn ligament takes place in the knee as a result of a fall or sharp twisting motion. This often happens in sports like football, skiing and baseball. Depending on the severity, this injury can need surgery to repair the damage. Once the injury takes place, pain and stiffness appear and it can also linger on during the rehabilitation process.
Meniscus Tear
The femur, patella and tibia are three bones in your legs that form the joint of the knee. In between these bones, there are rubbery pieces of cartilage that help absorb shock, which are called menisci. In similar fashion to ligament tears, the menisci are subjected to tearing when the knee gets hit or you twist at a bad angle. This is often accompanied with a "pop" feeling in your knee. The end result is pain, stiffness and weakness.
Tendinitis
The patellar tendon runs from the tibia to the triangular-shaped patella bone in the knee. Repetitive jumping, being overweight, having muscle imbalances and tight leg muscles can all lead to small tears in the patellar tendon. As the tears become more numerous, your body can't keep up, causing the inflammation in your tendon to worsen according to the Mayo Clinic's website. This in turn leads to patellar tendonitis which causes pain in the knee.
