For years, the common knowledge has been that if you want to lose fat, you have to do aerobic exercise. But while it is in part true, that belief unfortunately led many women to think that they could skip strength training. Now a new study is changing all that.

Researchers from the University of Pennsylvania's Center for Clinical Epidemiology and Biostatistics recently found that strength training — or weight lifting — plays a key role in controlling a woman's weight. They discovered that women who lifted weights just twice a week were able to avoid the slow, one to two pound weight gain per year that's so common in middle age. While that might not sound like a lot, over the years it can really add up. Another plus? Women who lifted weights had healthier hearts.

So how does lifting weights help keep you slim? It's simple — strength training builds muscle, and muscle cells burn more calories than fat cells. The more muscle you have, the more calories you burn, 24 hours a day — even when you are asleep! So if you aren't already, make strength training a part of your fitness program. Your efforts will pay off for years to come!