Sometimes girls worry about doing exercises for muscle fitness because they are afraid that they will build big muscles. It is true that muscle fitness exercises cause muscles to get bigger for both boys and girls. But it is also true that most girls will not be able to build muscles as big as boys can. During the teen years and into adulthood, the body produces more of the hormone that causes the body to build muscle than it does during childhood. But males get much more of that hormone than females. That is why males develop bigger muscles than females do. Most experts, and most girls and women who have done regular exercise, agree that building muscles helps you look your best because muscles keep your abdomen from protruding and help you to look firm and fit. Muscle fitness provides many benefits other than feeling and looking your best.