A rotator cuff injury, which is fairly common, involves any type of irritation or damage to your rotator cuff muscles or tendons. The risk of injury increases with age, and is particularly common in middle-aged persons.
Causes of rotator cuff injury can include:
Normal wear & tear, repetitive movements, trauma, healthy lifting or pulling, and poor posture.
Total inactivity can cause stiff joints. In addition, favouring your shoulder for a long period of time can lead to frozen shoulder, a condition in which your shoulder becomes so stiff you can barely move it.
Once your injury heals and you have good range of motion in your shoulder, continue exercising.