This stretch uses the anterior deltiod which is at the top of your shoulder and both the brachialis and the biceps brachii. They all will pull along your chest, stretching from the top of your shoulder down the top half of your arm.
These stretches are fairly easy to do and they will increase you flexibility which is a requirement just to do simple exercise. Whilst doing the behind the back chest stretch you should feel the pull from the middle of your upper chest towards your shoulders
Don't ever under estimate the importance of stretching and the benefits it holds.
The Behind the back chest stretch can help with dislocation, subluxation, acromioclavicular separation, sternoclavicular separation, impingement syndrome, rotator cuff tendonitis, shoulder bursitis, frozen shoulder, chest strain, pectoral muscle insertion inflammation.
Always remember that while stretching may be recommended to aid recovery out of injury, you should always be very careful when stretching through rehabilitation. Your soft tissues are likely to be more vulnerable to re-injury if you stretch too much or too hard during this time. Always check with your Health Care Professional who is guiding you through your recovery to make sure it is appropriate to start stretching, and please - ALWAYS FOLLOW THE RULES FOR SAFE STRETCHING (The first posting on this stretching blog).
Image taken from "Upper Body Stretches" wall chart by B. Walker.