Muscular System

More than 600 skeletal muscles move your skeleton and organs, heat up your body and keep you upright. A muscle is made up of bundles of fibers cinched together by connective tissue. Muscles need to move; they weaken and shrivel when they can''''t. When you move, muscles contract around your joints, resulting in activity of those joints, such as bending an elbow.

* The two lats, short for latissimus dorsi, are your back''''s biggest muscles. A spasm in either one can lay you out like a log on your bed. (To find out how muscle spasms happen, and how to prevent them, see "Back-ercise" on page 13.)

* Women''''s necks are smaller and more vulnerable than men''''s necks. That''''s one reason why 70 percent of all whiplash cases occur among women. Whiplash happens when the neck is violently thrust backward and then forward, straining or spraining muscles and ligaments in the neck. (To treat it or to help prevent it, see "Whipping Whiplash" on page 256.) * The pectoral muscles lie under a woman''''s breasts. Firming up those muscles can give sagging breasts a nice boost--a natural breast lift. (For two exercises that tone up the pectoral muscles, see "Chest" on page 456.)

* The quadriceps are the muscles that take more verbal abuse than any other part of the body. That''''s because they form a good portion of the universally unloved thigh. (To firm up flabby quads, see the two exercises under "Thighs" on page 461.)

* Tense muscles in the head and neck are responsible for tension-type headaches--the most common kind. They feel like a tight band around your head. (To relieve them, try the acupressure exercise under "Putting the Pressure On" on page 293.)

* The vastus medialis is a strip of muscle along your inner thigh. If it''''s underdeveloped, it can cause knee pain when you first start an exercise program. (To prevent knee pain or to quell it, see "Easing the Knees" on page 194.)