The most common shoulder injury in tennis (and every other racquet sport) is the aggravated supraspinatus, sometimes know as impingement syndrome or often simply as “a bad shoulder”.
Impingement syndrome causes pain in the shoulder when lifting the arm between 60 and 120 degrees sideways or when rotating the lifted arm inwards. It is a nagging pain that occurs because the supraspinatus tendon (the muscle under the roof of the shoulder) is pinched and aggravated when lifting and rotating the arm.
In most cases, it is an easy problem to fix requiring only some stretching (as the lady to the right is doing now) and some healing time. However, many athletes choose not to do what is required to resolve it and end up suffering needlessly for years. Some also end up quitting tennis because of it. “I had to quit because of a bad shoulder,” they will typically say.
As a coach, I encourage all of my players to include stretching of the shoulder’s posterior capsule (as in the video below) for five minutes at the end of every match or practice. Players who are diligent about this rarely suffering shoulder pain. Those who are not diligent, eventually run into shoulder problems and end up missing or losing matches because of it.
Stretches like these are what separate players in the I-play-to-get-fit category from the I-get-fit-to-play category. In other words, they are what separate dabblers from serious athletes. No truly serious athlete will allow a nagging and completely avoidable supraspinatus injury to keep them off of the court while recreational athletes will not only allow such an injury to develop and get worse, many will simply quite playing tennis because of it.
If you are a serious tennis player — or are aspiring to become one — consider adding some post match posterior capsule stretching to your post match routine. Your serve will thank you, your forehand will thank you and your backhand will thank you, too.