3. Muscle Injury
A simple muscle injury such as a tear, sprain, or pull can be enough to cause significant knee pain. The best solution is to consult with a physiotherapist.
Usually what your physiotherapist will then do is to recommend a course of exercises and stretches to help ease the discomfort and get your knee back to full working health. This will depend on the nature of the damage, for instance, if you have damaged your vastus medialis obliquus (VMO – one of the muscle bellies in the quadriceps) then they will recommend some exercises designed to strengthen these which will include exercises like light leg extensions and squats.
They might also recommend in the build-up to this some exercises such as the static inner quadriceps contraction which requires you to place a rolled-up towel under your knee while lying down flat on a bed or on the floor and then to try pushing down with the inside of your knee as hard as you can for reps of five seconds. Repeat several times.