Are you Sitting comfortably?
If you play guitar for any length of time it makes sense to be comfortable.There are two broad
possibillites, the classical position and the casual position then additionally using supports such as
straps and stands.
1 Classical position.




This is good for classical guitar. The one postural negative is the foot stool which is a default with classical posture gives an uneven posture. The gluteals are engaged differently on the left and right and there is more
imbalance on the spine. I suggest both feet as flat as possible also the neck angle of the guitar can be quite be
high possibly to facilitate the angle of the plucking hand.
"Classical Technique and Position of the Guitar
There is a wide range of sitting positions, however, the vast majority of guitarists would agree that “the position of
the player should be fixed, while the instrument is arranged to suit that fixed position”(Glise, 1997, p.4). In a
detailed analysis of the sitting position, Russell (1988) suggested the use of an armless chair with a flat seat and
without a backward inclination. While sitting, the footstool should be placed in such a way that the left knee
would be at an angle of approximately 90 degrees to the left thigh. According to Carlevaro (1978), the right foot
should be placed “slightly behind the performer” while holding the guitar in order to create a balance in relation to
the rest of the body, as shown in figure 4:


After a good sitting position is achieved, the guitar should be placed between the legs and rest mainly on the left leg while its lower bout touches the right thigh. The upper bout should be resting lightly on the middle of the chest and the head of the guitar must be set somewhat above the height of the left shoulder.

The final contact  point that stabilises the instrument is the right arm which rests over the upper side of the guitar; the left hand
remains as a more neutral contact point. Nevertheless, in cases where the right hand needs to be lifted the left becomes essential.

 

Following the above principles the instrument will be centered to the body, allowing both hands to work equally
with a balanced posture. Quoting Quine (1995) “A comfortably balanced sitting position is of a fundamental
importance to the development of the sound technique; arms, hands, and fingers can only work efficiently from a
stable base, which a sitting position free from unnecessary tensions provides”.


There have been some attempts to use supports for guitar. Here is  a table


here a tripod.


Applied to a non classical approach .The image below gives a good compromise. Note the hips are lower than
the knees.


With the image above the seat doesnt look too comfortable. Here is some good chair advice.
"The first thing you need for your ultimate playing position is the ultimate chair. Many people just grab a chair and go for it. The ultimate chair however has to have a few key characteristics. The first is that the chair should have no arms, the last thing you need is the arms of the chair interfering with the movement of you left or right arms.

 

The second is the height of the chair. When choosing a chair stand in front of it so you are to the side. The seat of
the chair should fall right in the middle of your knees. Another way to test it would be to sit on it, if you feet are
flat and your lap is flat, then it is the right height. However, if your lap angles to the floor it is too high, or if your
knees end up pointed to the ceiling it is too short. Your personal chair will depend on your height, and vary
person to person.

 

Sit on the edge of the chair and do not slouch when you sit. Sit with your back straight, it is okay to look at the
guitar and your hands, it is not okay to lean over and look at your hands. Keep your shoulders level, and don’t
lean to the left or to the right. However, there will be times when you will lean to the left in order to reach those
notes above the twelfth." from http://www.guitarnoise.com/lessons/posture


2.The casual position


Two great guitarist John Lee hooker / Tommy Emmanuel.The guitar is balanced on the right leg and the neck
angle is almost parallel to the floor.The neck also points away from the body by about 45 degrees.
It is also possible to sit and use a strap to position the guitar.


Conclusions
It is worth spending time experimenting and finding an optimal position you may even have a couple of ways to sit and play.