The shoulder is prone to muscle strain, particularly of the rotator cuff (especially supraspinatus), serratus anterior and upper trapezius muscles. The position of the right shoulder is a problematic one for guitar players as is the myofascial tension usually present on both sides. Many technical difficulties with the position of the hands can be traced to shoulder tension which again has a significant impact on the fluency of our playing. Refer to the section on shoulder biomechanics.
Two groups of muscles articulate the hand. Muscles that originate on the arm and cross the wrist are called extrinsic muscles while those that originate on the hand itself are called intrinsic muscles.
It's not as complicated as it may look.