Elbow biomechanics for guitar players

With the left arm hanging from the shoulder and the elbow tucked in to your side there is between 110˚ and around 40˚ of elbow flexion and no pronation. This is a relatively simple job for the elbow and a good one to demonstrate the basic forces at work in any static posture. The agonist muscles in this position are the brachialis and biceps brachii. There insertions high up on the forearm make them suitable for producing large movements of the distal limb such as this one. The brachioradialis muscle produces a force that compresses the joint, pulling the distal end of the radius in a straight line toward the humerus. The tricep brachii is the antagonist having to lengthen to allow the elbow to flex. Tricep also acts as a synergist muscle with a low level isometric contraction contributing to the stability of the elbow joint.

The right arm is flexed anywhere from 120˚ to close to straight depending on the size and position of the guitar. The description of elbow flexion for the left arm is just as applicable here. The different position does however alter how some of the muscles are used.
The slight flexion of the shoulder increases the load on the tricep, it's long head being subject to some passive tension, but this should only be a factor if it was already limiting elbow flexion.
There is some pronation of the right forearm which can contribute to strain at the medial epicondyle. Its orientation almost parallel to the ground means the the radial deviators are working against gravity. Overuse of one of these: flexor carpi radialis, is also a factor in medial epicondylitis.