Contradicting studies is a common issue. Thing to do is look at who funded them and when they were done. You can tell a lot about the reliability of information based on the motives of the study. For example if a shoe company funded the study promoting standing I would not trust the findings. On the other hand, if the study promoting sitting was funded by a chair company, I would not trust it either. And while many studies are non biased and scientific, many are not, so it's important to do research on the researchers.
Also when a study was done plays a role. It's quite common that someone does some research and we get all excited about the findings and jump on that bandwagon. Then someone does more research and adds new information and in sometimes we find that both studies have merits.
In the case of standing versus sitting I believe it is the later. One study shows standing for prolonged periods is bad and the other that sitting for prolonged periods is. Both are correct, which is why students that practice while standing need to take frequent breaks and sit down, while students that use a sitting position need to take frequent breaks and sit down. This is precisely the approach companies are taking to the conundrum. They have desks that can raise and lower so you can alternate between sitting/standing.
Whether sitting, standing, or laying down you are putting strain on your body. That's a result of gravity and can't be avoided unless someone comes up with a way to practice in zero gravity (a whole other host of problems related to that though
). So it really comes down to personal preference and what works best for the player as long as they spend time away from sitting/standing and do other activities.
Another thing to factor in is what is best for one's hand health. In my case sitting avoids certain positions that cause me strain in my fretting hand so the obvious choice is to sit while playing and stand (or walk around) while taking breaks. What works well for me as a multi style guitar player, is to alternate between working on classical and the electric, which I play while standing.
One possible standing position that I have not seen explored would be to use Paul Galbraith's method (similar to cello posture) with a longer stand (pole thing) so that it could be done standing (as in, more like an contra bass). If one had a short and a long pole (support), they could alternate between standing and sitting while using the same basic posture and approach to holding the guitar... Could be interesting for a player that wants to alternate (standing/sitting) while playing.
One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them.