The dash, in description, is simplicity itself. ““You really don’t have time to do anything except get out of the blocks, and the next thing you know you’re at the finish,’’ says Erv Hunt, coach of the U.S. men’s Olympic track team. But in deconstruction, the race is a set of separate parts. Some coaches break the race down into as many as seven distinct phases: start, acceleration, transition, the first maximum-velocity phase, second velocity phase, speed maintenance and finish. The start can lose a race, but seldom wins it. ““Nobody kills anybody in the first 50 or 60 meters,’’ says Hunt. ““The key is not to decelerate as much as the other guy.''

The concept of deceleration seems odd, since it appears that the runners are speeding up continually right through the finish. In fact, the sprinter usually peaks between 50 and 60 meters. During Carl Lewis’s 1988 gold-medal performance in Seoul, he ran that 10-meter segment in .83 seconds (or 27 miles per hour). He ran the last four 10-meter segments in .85, .85, .87 and .88. ““What separated Lewis from most sprinters was his ability to decelerate the least,’’ says Cliff Rovelto, a sprint specialist and head coach at Kansas State University. For some, the ability to maintain speed is mainly physical: powerfully developed chests and legs fighting through air and the pull of gravity. For others, it’s largely mental. Former Stanford University track coach Brooks Johnson says that at the midway point, runners feel ““like they’re stepping out in space. Many slow down because the phenomenon is so scary.''

Most sprinters aspire not to run hard but to run relaxed. Unusual effort – motivated, for example, by a runner finding himself trailing the field – can sabotage the natural rhythm and ultimately the race. ““When you try to go harder, you can’t,’’ says Olympic coach Hunt. A sprinter’s speed depends on length and speed – or turnover rate – of stride. Most sprinters take about 45 strides to complete the 100. Taller runners have the advantage of longer strides. But too long isn’t good, either. It can throw a runner’s balance, propelling him too upright. ““You want your power under your center of gravity,’’ says Temple University track coach Chuck Alexander. (American sprinting star Michael Johnson may appear completely upright, but there is lean in his legs.)

Theoretically, much as in boxing, the good big man should always beat the good little man. But shorter runners, like 5-foot-10-inch Frank Fredericks, a gold-medal contender from Namibia, compensate with more rapid turnover. Fredericks also boasts ““an economy of movement,’’ says his coach Willard Hirschi.

The difference between 10-flat and breaking Leroy Burrell’s 1994 world record of 9.85 seconds is one less step. (The extremely hard track surface at the new Centennial Olympic stadium in Atlanta improves turnover and should ensure record-challenging times in the sprints.) While the race may seem a blur (and literally is to most competitors), runners do process information throughout and make minute adjustments. ““It’s amazing how much you can think in a 10-second race,’’ says Brooks Johnson. Right up to the finish line, where many sprinters think they should accentuate their lean. Most runners are already leaning slightly, and a perfectly timed head or torso thrust can steal a race at the wire. But it also can be costly. Track experts call it ““last-hurdle syndrome,’’ a tendency to anticipate the finish line and slow up. ““Too often, athletes panic and lean early,’’ says Fredericks’s coach Hirschi. ““You’re better off running through the tape.’’ Loren Seagrave, a consultant on speed who has worked with some of the world’s best-known sprinters, says the mistiming results from the fact that ““the brain always underestimates time. Runners feel the time passed should have been enough to cover 100 meters.''

By contrast with running’s 100 meters, swimming’s 50-meter sprint is less a blur than a series of furious splashes. When American Tom Jager set his world record of 21.81 in 1990, he was churning through the pool at a little more than five miles an hour, a little faster than a brisk walk. Still, America’s gold-medal hope Gary Hall Jr. sounds very much like the runners when he talks about ““euphoria, a release, definitely a high. There’s a feeling where you actually lift up on top of the water.’’ But the swimming establishment doesn’t really share his euphoria about the 50 – it wasn’t even an Olympic event between 1904 and 1988 – because it doesn’t seem to require serious training.

If the nonserious approach has a poster boy, it’s Hall. His most recent coach, Troy Dalbey, who quit shortly before the Olympics, decried Hall’s ““completely hedonistic’’ training regimen. Hall spends less than half the time in the pool that America’s top distance swimmers do; some weeks he trains only about 15 hours.Desperate, his former coach Dalbey would substitute basketball workouts for pool time to keep Hall interested; he’d reward Hall’s occasional diligence with three-day weekends in Las Vegas. (Hall lives and trains in Phoenix.)

Though some coaches believe there is no substitute for training, others view the 50 as strictly a genetic race. And Hall has designer genes. His father, Gary, a butterfly, backstroke and medley swimmer, won three medals in three Olympics, from ‘68 to ‘76. His grandfather Charles Keating was an NCAA swim champ (but is better known for going to prison in the nation’s savings and loan scandal). At 6 feet 6 inches, Gary Jr. is a perfect 50-meter specimen. Height maximizes propulsion. Lanky height is even better. Hall’s body steadily narrows from his powerful shoulders, thus cutting down on the body’s drag in the water.

His muscles also have an abnormally high number of fast-twitch fibers, which contract rapidly without requiring oxygen. That means Hall can swim the course on fewer breaths. The more times a swimmer has to swivel his head to take a breath, the slower he goes. Virtually anything can make the difference. ““One false move and you’re dead,’’ says Hall. A false move can even come before the race. ““If I miss [shaving] the hair on my knee, it could cost me a hundredth of a second,’’ says Amy Van Dyken, the United States’ top contender.

It’s not just the hair; it’s the swimsuits, too. Van Dyken and her teammates will wear the latest in racing gear, suits made from a new Speedo fabric called Aquablade, which is faintly striped. Each stripe is treated with a resin that catches water, briefly slows it down and – at least theoretically – reduces the drag on the swimmer. ““It’s better than skin,’’ says Speedo’s Stuart Isaac. Some male swimmers may wear a tight shorts version rather than the traditional brief because it covers up more skin. Van Dyken may choose from a unitard with shorts and a high neck or a suit with 12 rubber upside-down V’s – ““gummy bears,’’ she calls them – across her rear end. The gummy bears break up the eddy that drags her from behind.

What isn’t genetic or synthetic, Hall says, is ““a mental thing.’’ And he has the mental part down pat. ““In the 50 you need to know you’re going to win,’’ says Jager. ““Gary comes in with that aura.’’ So does Alexander Popov, who has dominated the event since winning the gold medal in Barcelona. ““The Big Dog,’’ as Hall calls his 6-foot-5-inch Russian rival, likes to turn in the starting blocks and drill his opponents with a steely stare. ““Doesn’t faze me,’’ says Hall. ““I think he’s showing a bit of fear.’’

It’s in those stares and trash-talks that sprinters and swimmers find common ground. ““Sprinters are definitely more macho,’’ says Van Dyken. ““I like to be known as the tough girl.’’ For Dennis Mitchell, America’s 100-meter champion, Saturday night’s gold-medal showdown ““is not about times; it’s about being a man.’’ For the rest of us, it’s all about thrills. Speed thrills.

In the swimming phase, about 38 strokes, the pace slows to 4.7 mph. The flutter kick, 3 times per stroke, provides 20 percent of the total power.

Stroking form is crucial: the head stays down, the hands pull down in an S-curve pattern and hip rotation adds power through to the finish.

SOURCE: SWIMMING WORLD.

Exploding out of the blocks, he hits 20 mph near the 10-meter mark

About 60 meters down the track, a racer tops out at 27 mph

For the next 40 meters, a runner struggles not to slow down

A lunge at the tape - chest forward, arms back-can save 0.03 second

SOURCE: COMPUSPORT

Slam Dunk: At 5 meters, it’s the biggest drop of the course. Competitors must avoid a powerful recirculating hole at the bottom, or they could get stuck.

Faux rocks: Anchored to the bedrock with steel, these artificial rocks enhance the rapids and speed up the water.

Callihan Ledge: Shallow water leads up to this natural ridge. Since the river curves away, competitors can’t see the finish line.

Humongous: This 6 percent drop in the river sports the most formidable rapids of the course. They end with a sticky hole for exhausted paddlers.

Route: 415 meters (1,362 feet) Width: 18-40 meters (60-130 feet) Water flow: 10,000 gallons per second

SOURCES: USDA FOREST SERVICE; MCLAUGHLIN WATER ENGINEERS; JOHN ANDERSON, ARCHITECT. RESEARCH: BRAD STONE.