Life in the fast lane.

No need for expensive running shoes: These animals are adapted for speed.

You're halfway through a race. With muscles aching and chest heaving, you huff and puff toward the finish line. Is there any way to run faster and longer--and feel better while doing it?

You could get some tips by reading every article and watching every program on running stars such as Arturo Barrios and Greta Waitz. But even their great speeds pale in comparison to some other athletes of the animal world.

Which critter should be your role model? The cheetah, the fastest springer in the world? The racehorse, bref for speed for almost 400 years?

Guess again. To learn the most about long-distance running, animal physiologist Stan Lindstedt says, check out the pronghorn antelope.

Never heard of it? This three-foot tall mammal is the fastest distance runner on Earth. Though the cheetah might outsprint the pronghorn on the short track, the big cat would conk out after 30 seconds. The pronghorn would keep going and going. Pronghorns, in fact, can keep up with a car traveling 55 miles per hour for half an hour.

The secret of the pronghorn's success: Compared with cheetahs, racehorses, and humans, pronghorns, are more efficient at taking in an using the oxygen that fuels their bodies, Lindstedt says.

OXYGEN TANK

If you've ever worked out, you know your body needs oxygen. You breathe hard to get more of this gas into your lungs. Your heart beats fast as it tries to quickly move the oxygen from your lungs, though your bloodstream, to your muscle cells. That's where the oxygen is ultimately used--in the chemical reactions that give muscle cells the energy they need to contract (and keep you running).

The pronghorn faces the same challenge of getting oxygen to its cells, says Lindstedt. But compared to animals of the same size, thB pronghorn has:

* a bigger windpipe and bigger lungs to take in oxygen

* more alveoli, the tiny air sacs in lungs through which oxygen enters the blood

* a bigger heart to pump more blood through the lungs and to the muscles

* More red blood cells--the components of blood that carry oxygen, and

* more mitochondria--the structures inside muscle (and other) cells where oxygen-dependent, energy-releasing reactions take place.

These traits, says Lindstedt, are examples of adaptations, characteristics that help an animal survive in its environment. Pronghorns with speed-enhancing adaptations are able to outrun predators and survive. These animals then pass on their traits as the species evolves over time, says Lindstedt.

THE NEED FOR SPEED

Evolution might also explain why cheetahs, racehorses, and humans do not keep up with the antelopes.

To survive, cheetahs only need speed for short distances, says physiologist James Jones. They are adapted to sneak up on prey and pounce with a burst of energy from their powerful hind legs.

Racehorses don't even need their great speed for survival. For the past 400 years or so, their "evolution" has been driven by breeders who want them to win races, not ny the need to survive. Some of the speed-enhancing traits they've developed may even turn out to be harmful to their health (see opposite page).

Finally, says Jones, humans are much slower than these animals because we can afford to be. "Our method of survival is not dependent on being very fast," he says. Instead of running from predators and scampering after prey, we've designed tools and strategies for defense and hunting.

That's not to say individual humans can't get faster, Jones adds. Working out for 20 minutes three times a week will strengthen your heart and muscles, he says. That won't make you as fast as the animals on these pages, but it will make you a more fit human athlete.