I am a kickboxer. A few weeks back, I played an unrelated game in a field where I sprinted a lot. When I was finished, my left leg around the knee was really sore.
Now, two weeks later, my left knee hurts a bit, but only on flexion, especially with resistance. Flexion with resistance is not a motion that is commonly done, but think: pulling your foot out of a really tight shoe. That hurts. Extension doesn't hurt at all. Kickboxing and walking cause no problems.
I also noticed as a kickboxer, I almost never perform flexion with resistance at the knee (always pushing outwards -- kicking). So perhaps my muscles/tendons for this movement have simply atrophied, leading to the problem.
How can I check if my left knee flexion pain is tendinopathy or a muscular imbalance, and what exercises I can do to prevent this from re-occurring?