Pain is the first symptom of patellar tendinitis, usually between your kneecap and where the tendon attaches to your shinbone (tibia).

Initially, you may only feel pain in your knee as you begin physical activity or just after an intense workout. Over time, the pain worsens and starts to interfere with playing your sport. Eventually, the pain interferes with daily movements such as climbing stairs or rising from a chair.

Patellar tendinitis is a common overuse injury, caused by repeated stress on your patellar tendon. The stress results in tiny tears in the tendon, which your body attempts to repair.

But as the tears in the tendon multiply, they cause pain from inflammation and weakening of the tendon. When this tendon damage persists for more than a few weeks, it's called tendinopathy.

A combination of factors may contribute to the development of patellar tendinitis, including:

• Physical activity. Running and jumping are most commonly associated with patellar tendinitis. Sudden increases in how hard or how often you engage in the activity also add stress to the tendon, as can changing your running shoes.
• Tight leg muscles. Tight thigh muscles (quadriceps) and hamstrings, which run up the back of your thighs, can increase strain on your patellar tendon.
• Muscular imbalance. If some muscles in your legs are much stronger than others, the stronger muscles could pull harder on your patellar tendon. This uneven pull could cause tendinitis.
• Chronic illness. Some illnesses disrupt blood flow to the knee, which weakens the tendon. Examples include kidney failure, autoimmune diseases such as lupus or rheumatoid arthritis and metabolic diseases such as diabetes.

If you try to work through your pain, ignoring your body's warning signs, you could cause increasingly larger tears in the patellar tendon. Knee pain and reduced function can persist if you don't tend to the problem, and you may progress to the more serious patellar tendinopathy.