My shakes on my first national were so bad i could barely load a pellet
I still get them now and then. I sent the first shot off by clipping the trigger in Norway. Cost me another top 10 along with the cross shot lane (another fave of mine learnt from Hungary)
It's unlikely to be your knee, and more likely to be your chest or arm contact on the butt. If you use a hook try just moving it about to see if it's contacting the chest or the inside of the arm where there's an artery.
Things like being too hot or dehydrated won't help either. Some basic fitness will also help.
Sometimes you just have to accept that it's along for the ride and work through it. Controlling the heartbeat is the stuff of bond novels and once your adrenalin kicks in and the beta waves are pinging you could end up winding yourself up even more. Once over the hill it's very hard to climb back and the expert coaches are not even universally convinced you can. Better to put yourself into a good state to start with.
Best thing i've found is to completely concentrate on your shooting technique and routine. If you accept you can't change it then the only thing left is to make sure you can maximise what you can change. Even vocalising your shot routine (which may see odd) can help. Get it right and you will shoot better, but it's a hard trick to master. The more your mind is occupied with what you are doing the less it's occupied with it's reactions.
Try and turn it into a positive. See if you can pay attention to what it's telling you. In a weak position like standing you can be all over the place, but in a strong position like sitting you should be able to relax and still stay on target. If you can't and you're struggling to then something may need working on. That's completely normal for a new shooter. But see if can keep your mental eyes open for what isn't right, and then make a note and then work on that in practice. Give yourself a pat on the back and enjoy the rest of the shoot.
One thing that will increase stress is thinking about your score, or what it means to you. Instead think about what you are doing and occupy the thinking part of your brain with that leaving far less room for the worrying part to play in.