Having pain in the front of the shoulder while reaching behind is a common issue and straightforward to address. Often this is diagnosed as a bicep tendinopathy. Interesting enough, however, the bicep is not the culprit—it is only the source of the pain, but not the cause.
Let us imagine that you are holding a peeled banana at its base. The banana will move upward/outward when you squeeze your fingers together; the shoulder joint acts in the same way. The muscles in the back of the shoulder blade are put on tension when reaching behind (i.e. fingers squeezing the banana). If they are already shortened, then it causes the humerus bone (the banana) to translate forward pinching on the bicep tendon in the process leading to pain and discomfort.
If the cause of the pain on the front of the shoulder is because of tight muscles the posterior aspect (back side), is the treatment as simple as stretching them out? YES! Here is a two-step process you can easily try at home to address this issue:
Mobilize the Muscles
Do this with a lacrosse ball. Place the lacrosse ball on the back of the shoulder blade and lean up against a wall. Cradle the arm of interest with the opposing arm and gently pull across your body—this puts the muscles on stretch. Move your body up/down/around, finding sensitive points on the back of the shoulder while keeping the shoulder of interest as relaxed as possible. Keep the pressure on the ball. Perform this for 3-5 minutes.
Lying on the side of interest please raise your arm of interest to about 90 degrees. Keep your torso completely on its side. Begin rotating your forearm down towards the floor with your opposing hand giving light overpressure. You should feel a stretch in the back of the shoulder. Hold for 1 min and perform 2x.
Finally, re-assess your hand behind back discomfort. If done correctly you will likely see reduction in discomfort and improved range of motion. If so, keep up this routine to your satisfaction.