Super Belgian Bull..

Mooooove over Incredible Hulk! This Belgian blue bull weighs around 640 kilograms (1,400 pounds). That's 25 percent more muscle mass than is found in other breeds of bulls.

What makes this breed so bulky? Like you--or any vertebrate (animal with a backbone)--this male cow sports skeletal muscles (tissues that attach to bones and shorten to produce movement). Inside each muscle are muscle cells stuffed with stringy filaments called myofibrils (my-uh-FY-bruls). Everyday activity causes tiny tears in these cells. In response, the body makes new myofibrils, and stitches up the tears. It's the added myofibrils that make for larger muscles.

To keep muscles from ballooning, most vertebrates have a natural brake--a protein called myostatin (MY-uh-STATin). The Belgian bull's bulk-up secret? The super bull has a genetic mutation (change to DNA) that prevents it from making myostatin. This out-of-whack gene let's the bull's muscles grow larger than usual. And certain muscles--such as those toward its hind area--bulk up more than others, says Eduardo Casas, a geneticist with the U.S. Department of Agriculture.

Now scientists hope to tap into this mighty mutation to help people. "If you could find ways to block myostatin [in humans], you might improve muscle mass in people who have muscle-wasting diseases [such as muscular dystrophy]," says Se-Jin Lee, a geneticist at Johns Hopkins School of Medicine.