How does a poker player need to sit properly to stay healthy?

While professional poker players don't have a boss and can work anytime they want, the harm from a sedentary lifestyle and constant gazing at a monitor cannot be ignored.

Interesting. I am suffering from such an issue today as a matter of fact. couple of days ago, I sat awkwardly, with my dog on my lap...and injured my knee.

My suggestion....play the Big Blind standing up.
If you stand up, once per orbit, your posture gets a chance to reset.

Outside of this, most experts will agree that a regime of daily exercise is beneficial to both your health and your mental game. In the end, success is determined by habit. Once a positive behaviour becomes habitual, you are set to be impacted by it. So make sure your habits are positive.

Cheers,
ObbleeXY

You should be sitting in an ergonomic chair when you play long sessions.
And you should change your posture and also stand up once in a while.

Staying in the same posture for a long time is always bad, even if you are sitting "correct".

Having your monitor on eye level will help to reduce the stress for your neck.

Take breaks and do some yoga or gymnastic excersices that train the muscles in the
back and neck.

It is not healthy to sit and this is why the tourneys should not be as long as they are. If you enter one of those tourneys with umpteen hours of late registration at the beginning and do well in it that would be probably be as bad for your health as chainsmoking. The cardschat 100 daily freerolls on pokerstars should be changed back to the way they were before when there was 1500 starting stack..or change blind structure so money bubble is burst by 90 minute mark.