An important part of diagnosis is to determine if your knee pain is in fact quadriceps tendonitis or a tear in the quadriceps tendon or muscle. Commonly with a tear, you will feel a sudden onset of pain with swelling and bruising local to the area of pain. There might also be a palpable gap in the muscle. With tendonitis, it is unlikely to have much swelling or bruising and there will be no gap to palpate.