I've just started learning the guitar about a month ago. My primary goal will be in learning rock guitar (rhythm guitar), but I have found the information regarding guitar positioning and ergonomics to be much more well-documented regarding the classical guitar. Lately, I've been experimenting with placing the guitar on my left leg in the classical position versus placing it on the right leg.
I've been studying proper positioning for a few weeks now, and I've got two books on classical guitar, a membership to mangore.com, and a dvd set on rock guitar that I reference, and for some reason, my left shoulder aches when I'm in the classical position. Since I'm a beginner rock guitar player, I'm primarily working on open chords on the first and second frets and making good progress there. I have not ventured further down the neck yet, so I practice for about 30 minutes at a time with my left hand in the first or second frets.
I've been practicing in front of a mirror to really get the classical position down, because it is supposedly the more ergonomic one when compared to the rock/jazz/folk position of having the guitar rest on the right leg. I experience no shoulder discomfort in my left shoulder at all when the guitar is on my right leg, which I have propped up with a footstool. This seems to me to be because placing the guitar on the right leg puts the guitar in a more comfortable position for the left hand to reach the lower numbered frets, whereas the classical position seems more comfortable for frets 5 and above. It seems to me that the reason for my shoulder fatigue in the classical position is due to the need of the left elbow to venture further away from the chest in order to reach the low frets. This is true for me as well as for classical guitarists I've seen on youtube. The thing with them is that their trip to the low numbered frets is brief, whereas I am spending 30 minutes in that position in order to learn the open chords. I've tried raising/lowering my chair height, raising/lowering my footstool, changing the front-to-back angle of the guitar, as well as the positioning of my legs. However, nothing seems to alleviate the discomfort in my left shoulder except for placing the guitar on the right leg, in which case, the discomfort goes away. Anyone else have experience with this?