Shoulder biomechanics for guitar players
The two shoulders have to hold the arms in different positions and are subject to very different loads.
The right shoulder
The muscles in the shoulder, more than anywhere else, have a dual role of creating movement and keeping the structure stable. If a muscle attached to the scapula wants to move the arm then the scapula needs to stay still. Because it's only skeletal attachment to the rest of the body is through it's connection with the clavicle the muscles of the pectoral girdle must adjust to maintain the position of the scapula as the forces moving the arm are also acting on it.
Look at a room full of guitar players, they all look the same, hunched over their instruments with their right shoulders reaching forward to the strings and scapula almost horizontal.
This movement of the scapula takes place relative to the posterior thoracic wall, the back of the rib cage. As the scapula elevates (upper traps and levator scapulae) it rolls forward on the curve of the rib cage so that it also tilts forward. As it moves laterally (serratus anterior) it rolls around toward the side so that the glenoid fossa is facing the front. These two movements, tilting and winging respectively, occur at the AC joint as the scapula articulates with the clavicle. They are common, and to a degree necessary, as the right arm reaches forward with medial rotation to attack the strings but are all too often exaggerated and cause problems with the shoulder and from there with the rest of the arm.
In the illustration on the far left you can see how the scapula can not possibly lift straight up, it would have to come away from the thoracic wall. Instead it rolls forward a little as in the the second illustration. If however we lean forward it has to roll even further. This effectively adds up to 30˚ of medial rotation to the humerus, pushing the arm into the guitar.
- Sit up straight with your hand on your lower abdomen about the same height that it would be if you were playing your guitar
- Bend your upper back forward as if you were bending over your guitar. Feel the extra pressure you're using to push into your body and how the shoulder has to lift to accommodate it.
The right shoulder has to position the arm to take the hand to the strings. To do this it must, to varying degrees depending on the size of the guitar and the length of the arm abduct, medially rotate and flex. This places the scapula in an elevated, abducted position, importantly also the degree of winging and tipping is significantly exaggerated. Detailed explanations of these positions in relation to the scapula can be found at the explanation of the acromioclavicular joint (coming soon).
The position of the scapula needs to be maintained by the rhomboid, serratus anterior and upper trapezius muscles posteriorly and pectoralis minor and the deep shoulder flexors anteriorly.
The excessive winging of the scapula that is generally a part of reaching forward to the strings seperates the medial border of the scapula from the rib cage and, combined with scapula abduction, puts a lot of strain on the serratus anterior muscle. Overuse of serratus anterior will eventually cause pain around the bottom and inside of the scapula.
The scapula follows the contour of the rib cage as it finds it's position. When the thoracic curve is increased, such as when we lean forward, the winging and particularly the tipping of the scapula is exaggerated. As well as increasing the risk of injuring the strained shoulder muscles this position tends to pull the forearm into the body of the guitar making it more difficult to keep your hand away from the bridge as you pick.
The left shoulder
The left shoulder presents less of a problem than the right. With the shoulder relaxed and the guitar in the right position the left arm simply needs to hang vertically from the shoulder joint.
The arm is held in a neutral position by the superior ligaments of the glenohumeral joint and passive tension in the tendon of the supraspinatus muscle, pulling the humeral head medially into the glenoid fossa.
The weight of the arm alone probably doesn't rely on an active suprapsinatus but when the arm is loaded in this position, for example when you carry a guitar in its case, the muscle contracts to protect the joint capsule and help pull the humerus into the scapula.
Most of the time the left shoulder should support the arm in a neutral position, that is hanging loosely by the side with the elbow resting tucked in to the waist. Movement of the hand up and down the fretboard should be accomplished with shoulder rotation and minimal adduction/abduction.
The position of the scapula determines the position of the upper arm and therefore the hand. The more that your left shoulder varies from this neutral position the more difficult it will be to place your hand efficiently at the fretboard. Conversely where you put your hand will effect the shoulder.
The most common problems with the left shoulder relate to hand positioning. In the photo on the left my guitar is angled up so that I can look down at it forcing me to flex my wrist more than I should have to. Apart from what this does to my wrist and hand it also moves my elbow forward which I can only do by extending my shoulder and depressing my scapula, probably in combination with lateral deviation of the thoracic spine. This requires sustained contraction of at least the anterior deltoid, lattisimus dorsi and lower trapezius as well as all of the compensatory stabilisation that is inherent in shoulder biomechanics.
Elevation of the scapula, either holding a strap on or positioning the hand, is another one to watch, it significantly restricts the freedom of humeral rotation and will strain the trapezius and levator scapulae muscles causing shoulder and neck pain.
The shoulder elevators often respond to stress by shortening, bringing your shoulders up toward your ears. Any elevation of the shoulder will cause the elbow to wing out to the side and bring the thumb over the neck.