I have found the difference in ergonomics between playing standing or seated to be very frustrating.
The basic advice I was given by my teacher when I first started was to set my strap length so that the guitar was the same height standing or seated. Most players sit with the guitar resting on their right leg, so we get this:
Notice that the when the guitar hangs, it shifts slightly more toward my picking arm.
Honestly, this is probably fine for most people playing most things. However, notice the external rotation of my left shoulder and the internal rotation of my right shoulder. If I try to stretch a major third interval on the low strings, this is likely to happen:
Not good, and with the guitar in this position there isn’t much I can do about it.
Some would say I started from the wrong place, and that we should sit with the guitar on left leg and use a footstool. So, we get this
Notice that the position of the guitar has drifted again. The major third can be approached much more comfortably, however, this isn’t without problems either.
Now, stretching high in the neck requires me to retract my shoulder and pull my elbow hard behind my back. I’m applying presssure with the forearm of my picking arm to lever the neck away from my body. This isn’t ideal either.
When I sit in classical position, I am able to maintain a more comfortable distance from my body to the neck so I can navigate the whole neck comfortably. When I stand, gravity makes the guitar lie fush against my body and takes that space away.
I’m 6’ and about 150lbs. The guitar just doesn’t hang on me in such a way to allow me to maintain comfortable distance.
However, if I were heavier, this would happen:
This is much, much more comfortable. I first tried this after watching Shawn Lane playing standing. I agree with @JakeEstner, being heavier naturally positions the guitar in a more ergonomic position.
So, maybe I could solve my problems by eating more. However, there are other (healthier) ways to change how a guitar hangs.
When watching footage of Allan Holdsworth playing his Charvel guitars, I noticed something
Despite having a strap pin on the upper horn, his strap was attached to a strap pin behind the neck. A few minutes with a scredriver later, and this
becomes this
And now, the guitar hangs like this
Notice my elbow isn’t pulled back so far behind me. Also, here I don’t need to lever the guitar into position by applying pressure with my picking arm.
Unfortunately, this isn’t without problems either. The neck screw doesn’t make a great connection with the neck, and this could result in damage to the threading in the neck. I used to have the pin in this position but I moved it when it started feeling a little loose. So, my plan with my Strat syle guitars is to buy more strap pins and install them here
by drilling through the neck plate and having the strap pin screw secure in the body.
However, I keep putting that job off for later, because I have a guitar which is just inherently more comfortable to play.
I had this guitar built with ergonomics in mind. To me, this is basically the ideal. It feels eactly the same sitting down or standing up and hangs exactly where I want it to. It balances perfectly and I don’t need to support it with either arm while playing.