Strange habit I have and neck pain
 

When I do long stretches with some power (solo speeds >15 on gravel for example), I have a habit of raising my right arm and shoulder, tilting my head slightly to the right and my right arm sticks out. Its like I lock hard in that position on my upper body, get in the zone, and pedal away. I do this on my flat bar and my drop bar. On my flat bar I can see my hand up and further around the front of the grip than my left hand. On the drop bar, my right arm would be sticking out further and higher than my left arm. I have been doing this for a while, probably forever. I didn't really pay attention to it until recently when I got some serious neck pain after a gravel century and some shorter rides between another century a week later. Now that the neck pain is there and not going away very fast I can feel the pressure on the right side of my neck on any ride more than 10 miles as soon as I start putting some power down. The obvious answer is don't do that with my right arm but apparently it's not that easy. I always default to that locked up tense position when I start going up a hill. There has to be a way I can learn around that or figure out why I am doing that.. Am I twisting myself to bring my right side more forward for some reason? If saddle sores are any indicator, I seem to always get them on my right side and rarely the left. I had a fitting recently on my gravel drop bar and I didnt think to bring it up, maybe they were not looking for that or I wasn't putting down any real speed at the time. At the fitting he noted my arms and legs were the roughly same length and I had a slight difference in shoulder height. I am bow legged and a wedge was put in my left cleat to help with any potential knee pain. I'm a lefty if that matters.

Most of us have some sort of left or right asymmetry where one side or the other will be more flexible or stronger. There may well be something about your bike fit that is causing your discomfort but it is just as likely the problem is related to your personal asymmetry not something induced by bike fit. I find stretching helpful after a hard ride. Perhaps stretching would help you too.

I find myself pedaling squares when my legs get tired and need to pay attention to remaining loose and relaxed. In the video, although it is about road riding, what strikes me most about it is the cyclist remains relaxed. I'm an old man and asthmatic besides so I need to pay attention to pedaling technique and remaining relaxed everywhere else so that what effort I'm able to put out is directed solely to moving the bike forward. .

I used to have a problem something like that. Our bodies are seldom symmetrical because handedness. I fixed my issue very simply by hanging from my hands from a bar as long as possible a few times every week. Monkey bars would be even better. Stretched both shoulders out symmetrically. Plus it's good for you.

And yes, paying attention is everything, including to balance and being relaxed. Sight your top tube against your downtube, stem against your front tire, make sure you're centered over your bike. Drop your shoulders, pull your elbows in parallel to centerline.