Chances are, you don't even realize how often you use your shoulder muscles. Think about it: Every time you lift a grocery bag, place an object on a tall shelf, or pick up your toddler, you're putting your shoulders to work. That's why it's super-important to ensure that your shoulders remain strong and healthy. Even if you work out regularly, it's common for women as young as 30 to face issues such as tendinitis, bursitis, and even simple aches and kinks. Prevent injury—and develop a strong and sexy upper body—with this shoulder exercise circuit.