The pain develops gradually over a few weeks with on specific incident that you hurt it, or fell on it. The pain is there when you are doing the activity (running), and even worse 30 minutes after you’ve stopped. One unique feature of a tendinitis is stiffness. Stiffness, especially in the morning, or then you were in a resting state (sitting down after a run). When you get up, you’ll feel stiffness over your knee and you will feel like you have to stretch it out to relieve it. DON’T!
Another feature of a tendinitis is that the pain goes away when you warm up for training, or then you are at the start of your training. This is only at the beginning of a tendinitis, later the pain will not ease up during training, and will become more intense. The pain after training takes longer to go away (2 hours instead of 20 minutes). Even walking, climbing stairs and standing will be enough to bring on the pain.
Patients usually complain of the pain as being a 3/10, more a niggle or discomfort, rather than ‘pain’. They report that their main concern is the knee feeling unstable and weak when they want to use it, land, run downhill, etc.