Shoulder injuries can be either acute or chronic depending on when they are diagnosed and how long the pain / disability has been felt for. An acute shoulder injury occurs suddenly either through direct impact, over stretching a muscle, tendon or ligament, overusing a muscle or tendon or twisting of the shoulder joint. The top five most common categories of shoulder injury are listed below.
Many shoulder problems are caused by the breakdown of soft tissues in the shoulder region. Using the shoulder too much can cause the soft tissue to break down faster as people get older. Doing manual labor and playing sports may cause shoulder problems.
Treatment for a shoulder injury may include first aid measures, physical therapy, medicine, and, in some cases, surgery. Treatment depends on:
The location, type, and severity of the injury.
How long ago the injury occurred.
Your age, health condition, and activities, such as work, sports, or hobbies. 
Regular strength training is one way of helping manage a shoulder injury, but also preventing injury. It is a valuable component of fitness and most injury rehabilitation.