On to muscles. Remember that muscles move bones at joints, which means that all shoulder muscles cross the shoulder joint and perform some movement or action at that joint. Also, muscle tissue has two very distinct characteristics. First, it has contractibility, the ability to shorten and then return to its normal. Second, muscles have extensibility, the ability to lengthen or stretch and then return to whatever that muscles normal is at the moment. The same cannot be said for ligaments, which attach bone to bone. Once you stretch a ligament, it has difficulty returning to a toned state and recreating integrity at the joint. Tendons, which attach muscle to bone, have a greater ability to retract after being stretched. They have to in order for the muscles to return to normal tone.
Why do I say that muscles return to what is normal for them at the moment? This is because the human body is highly suggestible and trainable. If you continuously do one movement or stay in one position, the body will decide that this movement or position is what you want it to support. The body will tighten muscles that support this action and loosen other muscles to allow for greater tension in the supporting muscles. For example, I am sitting here typing. My arms are out in front of me and my palms are turned down. Let’s say that this was my job and I did this six to eight hours a day. Although my elbows are laying on the desk and there is little to no tension in my shoulders, my shoulders would begin to adjust the muscle tension in all of the muscles crossing my shoulders. Muscles in front or at the anterior aspect of the shoulder and muscles that create medial rotation (twisting of the shoulder toward the chest) will tighten, while muscles in the back or at the posterior aspect of the shoulder and muscles that create lateral rotation (twisting of the shoulder toward the back) will loosen.
Now, this may sound like the muscles are naturally tightening because I am working them repeatedly in the same way. Yes, this is true, but the nervous system is designed to look for patterns and then support those patterns, so I would get extra tension in the muscles performing the action. What happens next also supports the tension in the anterior or front shoulder muscles and the medial rotation muscles. The constant, chronic position will begin to shift the position of the humerus in the glenoid cavity, creating a misalignment of the glenohumeral joint. Okay, in english, the upper arm bone will shift to accommodate the largely unvarying position, moving the head of this bone forward and toward the chest within the joint. This creates a misalignment of the joint, diminishing space in the front of the shoulder and pushing into the tissue. And, again, the nervous system picks up these cues and decides this position needs support, moving the bones further out of alignment and causing additional tension in the musculature.
The first thing you have to do if this is your issue is give your body different messages. It would be nice if you stopped putting your arms in this position for long periods of time, but some of us cannot do that. We need our arms in that position to make a living. In lieu of that, know which muscles the tone is being decreased in (i.e. the posterior shoulder muscles and muscles creating lateral rotation) and deliberately tone these muscles. Also know what muscles are being tightened and deliberately stretch these muscles. Do specific exercises to create balance in the joint, and get up as often as possible, putting your arms and shoulders in the opposite position of what they have been in.