The main problem related to the left hand when playing guitar is maintaining the arm in the air, often for many hours. In order to let the hand be entirely free, it is important to remember that each muscle must play the role for which it was designed.
First, it is necessary to ensure that the shoulder blade is in place (in coaptation) using the technique mentioned above. To maintain the arms in the air, all the muscles of the shoulder must be involved. The arm must be held in the air, even if the instrument is not there. This seems obvious, but after several minutes of instrumental practice, those tired muscles relax, and the hand tends to hang onto the neck of the instrument and thus sustain the weight of the arm.
The hand then loses a lot of agility. In addition, the small hand muscles are required to perform a function against nature. This causes unusual tiredness and multiplies the risk of injury. This is a problem that we find regularly among guitarists led by the good intention of playing relaxed. In attempting to relax the muscles above the shoulders, they place the weight of the arm on the hand.
Another issue related to the left hand is that it must be maintained in supination. I’m talking about the hand to draw your attention, because here (as in the case of the elevation of the arm), the hand is far from being the origin of the movement. If that is the case for you, learn to change your conception of things. It can be difficult to stop considering our precious hands to be the centre of the universe. As the Buddhist proverb says, “A finger points to the moon; unhappy are those who look at the finger.”
Thus, the supination of the left arm must take root in the shoulder. The rotation is done from the glenohumeral joint, which (surprise!) allows one to perform the supination of the arm from the base of the shoulder. This arm is held in place by the strong bicep muscle. Under no circumstances should the movement of supination be done through the action of muscles in the forearm. Moreover, the hand must never be the source of the movement. One must, under all circumstances, use the small radial and ulnar muscles to turn that hand and, by extension, the rest of the arm. They are not strong enough to fight against other muscles located upstream, which themselves were not involved in the road to supination. They would find themselves antagonists of the biceps, and that would harm them and cause them injury. Imagine how much the utilisation of the radial and ulnar muscles in the action of supination influences the freedom of movement of the hand. It finds itself in a state of constant contraction and effort, which inhibits the action of the fingers in the realisation of the precision work for which they were actually designed.
The forearm is mainly used to place the hand in an optimal way by extending the movement of the shoulder and biceps. It completes the positioning accuracy, because thinner muscles act with greater precision. Nevertheless, the forearm should be the engine of the full supination of the left arm.
On the topic of the optimal placement of the hand, let’s now, without going into detail, take a look at another basic principle. When the wrist is slightly inclined from the radial side (thumb), it facilitates the action of the index and middle fingers. These two fingers gain accuracy and stability when they are placed as well. However, this position is a bit more demanding on the tendons; thus, it should not be the basis of our positioning. If, on the contrary, we impose a slight flexion at the wrist on the ulnar side (little finger), the fingers will be freer and faster. Note that the change is evident especially in the auricular and ring fingers, but the middle and index fingers also gain agility. Unfortunately, they lose stability. It is also important to reinforce the elbows and shoulders in the lateral flexion of the wrist so you do not overload the muscles of the upper extremities.
Read full article at: http://brainyguitarist.com/2015/02/04/left-hand/