These miniature muscles need exercise just as every other part of you does. And although it’s hard to impress people in the gym by working muscles you can’t even see, let alone flex, look at it as health insurance. "Choose to ignore these muscles and they can easily stop you dead in your tracks," says Lyle Micheli, M.D., author of the Sports Medicine Bible and former president of the American College of Sports Medicine. 

If you don’t learn about these muscles now, the next lime you see them may be on an anatomy chart in your doctor’s office. Here’s what they can do for you, and what you can do for them.

What they do: A chain is only as strong as its weakest link. When you’re talking about your shoulders, the same theory applies. These four tiny muscles work closely with other, larger muscles to lift, lower and rotate your arms. But when you exercise, you’re primarily strengthening the larger muscles instead, leaving the rotator muscles unchallenged. You’ll never get as strong or as big as you could be unless you also concentrate on the smaller muscles.

What it does: Few people pay attention to the broad, flat muscle that lies beneath the biceps muscle and the humerus (the upper arm bone). That’s a big mistake. When this muscle grows, it has no choice but to press upward, adding size to your biceps and making it larger than it could get on its own.