Hey Kamil, welcome aboard!
Yeah the tensing up is a real issue, with the only consolation pobably being that you’re not the only one who deals with this stuff. My guess is It is one of the more common problems amongst musicians on all sorts of instruments.
You indicate that it happens in other areas for you as well, so tensing up is clearly something that your brain has trained itself to do when you point your attention at something in a certain way.
The question is, what causes it? Most of the time it happens because something is hard to play, or fast or something like that. BUT it can also be that this is what happens every time you do “intense listening”, by which I mean really zooming in with your ears on what you are playing, or the music you’re playing along to, or both.
So let’s figure that out first, but let me give some advice for both cases I mention above (if its neither of them, feel free to say so, there might be other stuff going on):
- If you’re cramping up because stuff is technically difficult to play, the answer is almost always to slow things down or to work on easier beats for a while in order to lay a more solid, effortless foundation. I think this is probably not your problem right now though, from what I’m reading.
- If it’s the intense listening thing it’s more complicated because it’s more like your ears are distracting you from paying any attention to your body
I still struggle with this in the form of forward head posture whenever I’m “in the music”. I do think I found a way to deal with the problem but it does take some time and it’s very easy to fall back into previous “bad behaviour” as soon as you stop paying attention to it.
So what I do is this:
- I determine what it is I’m going to play or practice, usually a short loop of a couple of bars plus one fill or something.
- I allow myself to play it like total crap. So it does not have to sound good at all.
- I do listen to what I’m playing but I don’t ‘really’ listen, because I mostly focus on my body and what things feel like. I then do a sort of body scan while playing, so I let my attention go to my left arm, right arm, and then my neck and then just the overall feeling of standing up straight, and then my left arm again, right arm etc.
This does work and it seems to improve things over time, but it is a nasty habit to overcome. And I must admit that when I truly give it my all in a difficult performance, my head does move forward again.
I do think this is the way though. You probably need double the amount of practice sessions, where one half of the session you focus mostly on your body and less on the music, and from there, having established and familiarised your brain with playing and relaxing, you make the shift towards listening more and really getting into the music.
One way to help with this is to set a timer. So only go “into the music” for like 1 minute or so, and then th timer goes off and you have to do 1 minute of the more body scan kind of playing. Alternating that a couple of times might slowly get you out of the cramping up habit.
Finally… note that this gets harder when the beats and fills get harder. I still deal with his. If I’m performing on the top of my game musically my head does move forward again because I have no room left So there is always a component of stretching your limits there. The thing is that stuff that is easy to me now was not easy a few years back and with the easy stuff I can focus on my posture more… so that’s probably the same for you. You will always have a tendency to cramp up, but as you get better, certain things will start to feel easier, and you will have room left to also focus on relaxing when playing those things. It’s a lifelong thing to keep paying attention to though!
When focusing on being relaxed everything seems to be off 
