And, they're off! How champion racehorses are built to blast from the starting gate and gallop to the finish line

Each spring, racehorse owners eagerly await the greatest racing competitions: The Triple Crown of Thoroughbred Racing. Over a five-week period during May and June, the world's greatest gallopers compete in a series of three races in the United States: the Kentucky Derby, the Preakness Stakes, and the Belmont Stakes.

As the moment of the starting bell approaches, owners wonder if their horses stand a chance at being the first to nose over the finish line in all three races--taking home the coveted Triple Crown. But in the more than 125 years of this trio of races, only 11 horses have succeeded in that task. The last Triple Crown winner, a horse named Affirmed, defeated the competition in all three races in 1978.

Winning a Triple Crown means fame and glory, so racehorse owners invest lots of time and money searching for the country's fastest Thoroughbred, a breed of horses that are known for their speed and endurance. The owners even work with scientists to learn what characteristics make for a fast horse. Then, they try to harness that knowledge to weed out the slowpokes and saddle only the speediest.

TAKE OFF

When the starting bell sounds, a winning racehorse steeds to explode from the gate. That's because a victorious horse needs about two minutes to gallop 2.1 kilometers (1.25 miles) to cross the finish line.

Horses are born racers (see Nuts & Bolts, p. 24). So the sound of the bell and the sudden opening of the starting gate trigger the animals' natural fight-or-flight response. When a horse is frightened or excited, its body releases the hormone adrenaline. This natural chemical increases the horse's heart rate, preparing it to flee. "Horses are flight animals, so they run away," says Scot Waterman, a veterinarian (animal doctor), who is the executive director of the Racing Medication and Testing Consortium. "Because of that, the horse's basic physiology (functions of a living organism) is adapted to help it run away as fast as possible."

MUSCLE POWER

A Thoroughbred racehorse can gallop at speeds of up to 58 km (36 mi) an hour. One adaptation that allows for such speed is the horse's type of muscles. Muscle tissue is made of elongated cells, called fibers, that contract and relax to produce movement. Compared with human athletes, horses--particularly racehorses--have a much higher percentage of "fast-twitch" muscle fibers. This tissue contracts faster and more explosively than other muscle fibers, helping the racer to gain speed quickly. The muscle's "slow-twitch" fibers are designed for endurance. They contract and relax steadily--without tiring--for longer periods of time.

Scientists believe that the speediest sprinters may have an even higher percentage of fast-twitch fibers than the average racehorse. So ambitious horse owners may have a veterinarian extract a small sample of a horse's muscle to determine the percentage of both types of muscle fiber. "It's certainly a factor in the horse's performance," says Howard Erickson, a physiologist at Kansas State College of Veterinary Medicine.

TAKE IT IN STRIDE

A horse's stride can also determine whether the racer blazes a trail or lags behind on the track. "The longer each step the horse takes, the more distance it will cover and the faster the horse will run," says Erickson.

Many champion racehorses have oversize strides. Man O'War, a 1920s horse that lost only one race during his entire career and is considered one of the world's greatest racehorses, galloped to glory with a stride length of approximately 8.5 meters (28 feet). That's much longer than the average racehorse's 6 to 6.4 m (20 to 21 ft)-long stride. "Top racehorses repeat this stride around two-and-a-half times a second. That's 150 strides per minute," says Erickson.

LIQUID FUEL

At the Belmont Stakes--the Triple Crown's longest race--the winning racehorse has to maintain this breakneck pace for 2.4 km (1.5 mi). To run its way to the winner's circle, a horse needs to continuously power its muscles with fuel in the form of oxygen.

Prized racehorses have massive spleens. This purplish organ situated in the home's abdomen produces and stores red blood cells, or the cells that carry oxygen through the body. Compared with other animals, racehorses have bigger spleens, which means they have a larger storage of red blood cells. "When a horse gets a jolt of adrenaline, the spleen squeezes all of these extra blood cells into circulation," says Waterman. The cells ferry oxygen to the muscles, powering them so the horse can finish the race at winning speeds.

HEART OF GOLD

To help pump the oxygen-rich blood to its muscles, a champion horse needs a powerful heart. After Secretariat, the 1977 Triple Crown winner, died in 1989, a veterinarian examined the animal's body. He found that Secretariat's heart was more than twice the size of an average home's heart. Since then, scientists have found that many champion racers have an oversize heart. Today, some horse owners have a veterinarian study a potential racehorse's heart using an ultrasound machine. This device uses sound waves to produce an image of organs inside the body. With the snapshots, a vet can determine the heart's size.