"Get jacked!" "Get pumped!" "Gain 7 pounds in 7 days!" Walk into any vitamin store and you'll get the idea that you can be cut like an Abercrombie & Fitch model in no time. Protein powders compete for shelf space with muscle enhancers and testosterone boosters. You'll find energy drinks, protein bars, amino-acid packets, and even muscle-building oatmeal with 30 grams of protein. Tempting, but do they really work? We've broken down some of the most popular muscle builders and asked the experts to explain the science behind the pitch.

The first two supplements in the slideshow below -- Tribulus Terrestris and DHEA -- are considered testosterone boosters. Steroids that jack up testosterone were removed from store shelves in 2005 due to safety concerns. But supplement makers have responded with natural products purported to boost this muscle-building sex hormone.

The next three -- Creatine, HMB and L-Arginine -- are intended to help with athletic performance. Whether you want to work out harder, lift longer, or recover faster, you'll find a variety of supplements that promise to help you do just that. (With or without supplements, our new cutting-edge DVD fitness program Speed Shred will help you torch calories and build rock-hard muscle.)

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