The shoulder and elbow

The primary role of both the elbow and the shoulder is to move the arm so that it can position the hand in space. They evolved this function in order to pick fruit from trees, throw spears and use a flatpick. The ability to stand erect allowed us to use our hands but required a crucial adaptation of the shoulder complex. It went from being purely a weight bearing structure to one that provides an extraordinary combination, more than any other joint complex, of both stability and mobility. The more detailed examination of the shoulder and elbow complexes that follow examine how this all happens and the consequences for guitar players.

Any attempt to position the hand correctly at the guitar, either hand, is futile
(read my lips - f . u . t . i . l . e )
unless you first account for the position of the shoulder.

Factors that influence the position of the shoulder and elbow:

The overall posture and the shape of the spine
Tension in the shoulder
Weakness in the muscles that give the shoulder it's stability

Posture

The most common postural problem for guitar players is the tendency to slump forward over their instrument. This creates an exaggerated curve in the thoracic spine, which in turn influences the position of the rib cage.

As it tries to find the best position for the arm to position the hand properly the scapula has to roll forward over the rounded rib cage causing the arm to rotate medially, pushing the (right) hand into the guitar.

Myofascial imbalances

Muscular tension tends to shorten muscles, which in turn changes the position of the structures that they attach to. It’s causes are varied, some straightforward and others we have no idea about.

Muscular atrophy, or weakness, will also change our shape but in the opposite way. Now weak muscles aren't able to hold skeletal structures in place. This is especially important in the shoulder whose muscles, more than anywhere else in the body, are responsible for stability as well as movement.

In combination these muscle imbalances tend to raise the shoulder, pull it forward and rotate the upper arm medially. This will affect both shoulders but in different ways according to how they both approach the guitar.

The left shoulder and elbow

Notice as you sit at your computer the tension in the upper part of your shoulder up to your neck, now take a deep breath and relax both of them. They probably fell down a long way. The raising of the shoulders will rotate the scapula causing the arm to wing out, moving the elbow away from the body. Lift your shoulders up again and see what happens to the rest of your arm. We all know that this is not a good position for the elbow, it rotates the wrist and makes it impossible to get it parallel to the neck of the guitar.