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.

"[A larger heart] has the capability of pumping out more blood to the skeletal muscles," says Erickson. That blood delivers oxygen to the muscles that are attached to the home's bones, which enable the animal to move. A horse that has an extra-large heart may be able to pump more than 500 liters (132 gallons) of blood through its body each minute--powering its muscles at a winning pace.

Even if a racehorse owner studies each of these features in a home, no amount of science can guarantee a champion. "There's another important factor that goes into a good racehorse," says Erickson. "It's the desire and the will to run and win. That factor is the hardest one to measure."

WEB EXTRA

For more on the different body systems: http://sin.fi.edu/biosci/ systems/systems.html

Nuts & Bolts BUILT FOR SPEED

NECKLINE: Some horse owners search for racehorses with a Wider head and throat. That indicates larger airways, which help the horse to inhale oxygen into its lungs more rapidly.

IN SYNC: Elite racehorses synchronize their breathing with their running stride--exhaling when their legs hit the ground. Scientists think this improves the horse's efficiency.

HEARTBEAT: When a horse starts running, its heart rate may jump from 30 to 250 heats per minute. That helps to circulate blood swiftly through the body.

BACK UP: The horse's powerful hind muscles help to power it forward in a sprint, Nine different skeletal muscles work on each side of the horse's hind end.

LEG POWER: Thoroughbreds are taller than most other horse breeds, Their long legs help them sprint with lengthy strides.

It's Your Choice:

1 Adrenaline prepares a horse to flee by

(A) causing the horse to inhale rapidly.

(B) increasing its heart rate.

(C) contracting its leg muscles.

(D) increasing the size of the horse's heart.

2 Fast-twitch muscle fibers

(A) are designed for endurance.

(B) do not require oxygen.

(C) are not found in humans.

(D) contract more rapidly than other muscles.

3 The horse's purplish organ, called a--, stores--.

(A) spleen, red blood cells

(B) spleen, adrenaline

(C) spleen, slow-twitch fibers

(D) heart, adrenaline

ANSWERS

1. b 2, d 3. a

PHYSICAL: Body Systems

And, They're Off!

DID YOU KNOW?

* Only Thoroughbreds race in the Triple Crown, but they are not the fastest horse breed. Quarter Horses have been clocked at speeds of up to 89 kilometers (55 miles) per hour. But Quarter Horses cannot maintain their top speeds for very long. That's why Quarter Horse races are shorter--around 400 meters (1,312 feet) long.

* Most racehorse owners search for taller horses, hoping that these larger racers will have a longer stride. But some famous racehorses were relatively small, including War Admiral, the 1937 Triple Crown winner.

CRITICAL THINKING:

* Some potential horse owners use costly scientific testing, such as scanning for heart size, to help them select a "winning" racehorse. What are some advantages and disadvantages of using technology to help select a racehorse?

CROSS-CURRICULAR CONNECTIONS:

ART: Research the anatomy of a horse. Then, create a poster that shows a horse, along with the position and relative size of its main organs.

RESOURCES

* The Web site from the International Museum of the Horse has online exhibits about the history of the animal: www.imh.org/imh/exh1.html

* This American Quarter Horse Association site has diagrams showing the external and internal features of the home: www.aqha.com/association/who/thehorse.html

DIRECTIONS: Answer the following questions in complete sentences.

1. Horses are described as flight animals. Describe how a frightened or excited horse's physiological response helps it to run away as fast as possible.

2. What are muscle tissues made of?

3. What is the difference between fast-twitch and slow-twitch fibers? Compared with an average racehorse, a speedy racehorse may have a higher percentage of which type of fiber?

4. What is the function of a horse's spleen?

5. Why do some potential racehorse owners study a racehorse's heart size?

ANSWERS

1. 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.

2. Muscle tissues are made of elongated cells, called fibers, that contract and relax to produce movement.

3. Fast-twitch fibers contract faster and more explosively than other muscle fibers, giving a racehorse power to gain speed quickly. The muscle's slow-twitch fibers are designed for endurance. Scientists believe that a speedy racehorse may have a higher percentage of fast-twitch fibers than the average racehorse.

4. The spleen is a purplish organ situated in the horse's abdomen. It produces and stores red blood cells, which carry oxygen through the body.

5. A winning horse has to be able to maintain its breakneck pace, so it needs to continuously power its muscles with fuel in the form of oxygen. A larger heart has the capability of pumping out more blood to the skeletal muscles. Blood delivers added oxygen to the muscles that are attached to the horse's bones, which enable the animal to move. A horse that has an extralarge heart may be able to pump more than 500 liters (132 gallons) of blood through its body each minute. This helps get the muscles moving at a winning pace.