Check out my article in Gracie Mag that came out yesterday about correcting the posture and aligning the shoulders. If you are a martial artist you may be susceptible to having rounded shoulders because of the posture you constantly have to maintain to protect yourself. For those who have a desk job and sit on a computer all day, this problem is further compounded since most people sit with poor posture with their spine rounded their shoulders slouched forward.
Over time this posture tightens and shortens the chest muscles and the front deltoids and stretches and weakens the upper back muscles such as the lats, rhomboids and the rotator cuff muscles that stabilize the shoulder. Due to incorrect alignment this posture can make you more susceptible to injuries such as impingement syndrome, tendonitis and rotator cuff injuries which can result in chronic pain and limited mobility. Having any of these conditions puts more stress on the bicep tendons and front deltoids, making  you more susceptible to bicep tendon tears and labrum tears because the front deltoids and the chest are engaged during 'pulling' work instead of just the back muscles. 

Here are a few photos that demonstrate opening up the chest and pulling back the shoulder to minimize injury. 

The stick behind your shoulders will open up your chest and pull back your shoulders.

Raising the knees will loosen up the lower back also.