Getting your seated posture sorted for playing is pivotal to physically enjoying your playing, reducing the chance of overuse or incorrect use injuries and ensuring the longevity of your playing. You really don’t want to end up 10 or 20 years down the line being forced to play only in small goes or being forced to stop playing at all. It’s worth taking the time just to slow down or stop and assess your posture.

Big to small
When looking at posture I always work from the biggest structures first (i.e. the larger body parts or muscles) before then moving thought to successively smaller elements. Getting your overall seated position needs sorting out before moving on to the movement on the hands and fingers. Your seated position and posture is the framework, the basis from which you’re building your playing and your finer movements.
Building a good solid base for yourself, a bit like practicing your daily scales, is key to much faster, or rather much easier progress, down the line.
Pointers on posture
When seated make sure your legs are bent at roughly 90 degrees, with both feet flat on the ground. Then holding the guitar (using either your preference of foot stool or rest), make sure that your back is straight, using your stomach muscles to help keep that nice straight form. Sit towards the front of your seat; don’t sit too far back on the chair or lean on the back of the chair.
Think about tucking your tail bone under your bum. Lengthen your spine by imagining you’re a puppet with a string attached to the crown of your head keeping you upright.
Legs
Whatever your choice of foot stool versus guitar rest (and I recommend rest as I believe it reduces adverse impacts to the hips and lower back) ensure that your left foot, knee and shoulder are all aligned – no knee or foot sticking out to the side. This neutral position is the most natural and ultimately comfortable position to play in. With the right leg, bending this in at a similar angle, or even slightly closer to the chair acts as a stabiliser rooting you to the ground and creating a solid playing position.
One key tip, passed on to me by the great Phillip Houghton that I in turn pass on to my students, is a non-slip mat on my right leg where the body of the guitar contacts with my leg (of course, swap that to left leg if you’re a left-handed player). This helps to stabilise the guitar, make it feel really solid and in control, particularly if your guitar is heavy (as some of the newer style lattice braced guitars are) or you’re wearing potentially slippery clothing. The grip of the mat takes a bit of the tension away from your right arm in pinning the guitar back or limiting the guitar’s movements.
Arms
Your right arm should be soft and heavy, positioned over the bell of the body, with just the minimal amount of tension through it to hold that relaxed form. The feel is almost as if you’re embracing the guitar. Just be careful though not to drop and round the shoulder; hold your form through the shoulder joint too.
With the left arm, also be careful not to drop the shoulder and ensure your wrist is nice and straight, and in line with the hand and arm most of the time. Movement to reach the strings should primarily come from the shoulder in a vertical pivot like movement – make use of this big muscle group, rather than putting undue stress and strain on the smaller muscles of the wrist and lower arm.
Similarly when moving up and down and around the fretboard, use you shoulder as a horizontal pivot. Use the bigger muscles to do the majority of the work. This is what the shoulder is designed for and you’re really going to protect yourself from carpal tunnel syndrome type symptoms this way. I should know – I learnt the hard way on this and went through a long journey of correcting my previously poor technique.
So the key, “take home” message today is to make use of the body’s natural movements. Use leverage by using the biggest muscle groups to carry out the bulk of the work – that’s your back muscles, shoulder and arms – before moving on to think about smaller movements from the hands, fingers and thumbs. Movements are not massive either, probably much less than you’d think.
And relax!
Here’s a great You Tube clip I found on setting up your posture that may also help….