Seventeen races over seven days: three, sometimes four swims a day. Six events covering all four stokes, at three distances.

The task Maryland swimmer Michael Phelps set for himself at the Olympic swimming trials seemed killing. Surely he'd be worn out, ground down, beaten up by the workload in the giant parking lot pool by the sea.

But quietly, over the first five days of competition, the opposite has occurred. Rather than tire, Phelps, 19, seems to have grown stronger, according to his coach.

Instead of getting worn out, Phelps says he feels better each day.

Instead of a gas tank headed for empty, Phelps says he has plenty of octane left -- some of which he used to win four more semifinal races Sunday.

Phelps's coach, Bob Bowman, is cautious about revealing the blood test figures that support the trend: "We're not telling the Aussies," he said of the Australians, likely the main U.S. swimming rivals at next month's Olympics in Athens.

But the curve is unmistakable.

On Saturday Bowman said he noticed that post-race tests showed Phelps was recovering from exertion faster than at the start of the trials three days earlier.

The trend continued after Sunday's morning events, he said, defying conventional wisdom. Most swimmers would be more tired after such an arduous program, he said, joking: "Maybe it's the food he's eating."

But he had a theory.

"I theorize that he would, actually, as the meet goes on do better and better," the coach said. "I don't know why that is. But I think that in the course of an eight-day meet, by the time you get halfway through there's actually a training effect from the first couple of days."

"His body adapts to what happened on the first day, so after a period of time . . . he actually gets an adaptation which makes him better."

"That's my theory. I don't even know if it's true. Nobody knows. Because nobody's ever done this before. But he's brighter [after each race]. I think he's in really good condition. Somebody said you kind of race yourself into shape.

"Your body makes certain adjustments to training stimuli over a period of time. By the time he got to [Saturday] he'd had three days in this mode, which is more than enough time for his body to have had the training stimulus."

Bowman said post-race testing for lactic acid in Phelps's system is one marker of the trend. Lactic acid, which can inhibit performance, is produced in the muscles during a race.

A measure of a swimmer's condition is how fast he "clears" the substance during the post-race warm-down swim. Blood is taken every 10 minutes from the swimmer's earlobe and checked for lactate, the acid's ion.

Phelps's clearance rates were extremely good. Now they're getting better.

From peak lactate levels after a race, Phelps now clears about 20 percent in the first 10 minutes and 50 percent in the second 10 minutes, said Genadijus Sokolovas, director of physiology for USA Swimming. That is better than his rate at the start of the trials, he said.

Bowman said he hadn't told Phelps about the figures. "He doesn't know any of the numbers . . . I don't want him thinking about, 'Oh I didn't recover fast, or this or that.' So I'm the only one that knows that."

He declined to provide all the new figures, saying he didn't want to tip off the Australians, or other countries' swimmers. "But they're good," he said.

He said rival coaches and athletes might glean from the data "more about what [Phelps's] physiology is and we don't want them to know.

"Very clandestine," he said.

He said Phelps had gone through the same kind of improvement at meets last summer, and more recently at a big meet in Santa Clara, Calif., in May, where he swam multiple events, including three in one evening session.

Phelps said Saturday he still had plenty of "fuel" in the tank, and after Sunday's morning session, in which he swam two 200-meter races within an hour he said he felt fine.

"I'm starting to feel good," he said. "The races . . . felt comfortable, really smooth."

As of Sunday night, Phelps had been in 12 contests in five days, with four more Monday -- three in the evening session -- and one Tuesday, still to go.

He set a world record in the 400-meter individual medley last Wednesday.

Eddie Reese, the men's swimming coach at the University of Texas and the head coach of the men's Olympic team, said of Bowman's theory: "That could definitely be true. I mean, we've all got our theories on it."

John Walker, assistant director of national team technical support for USA Swimming, the sport's U.S. governing body, said Sunday it may be that, with the trials, Phelps's training is just starting to peak. "That would be scary for the rest of the world," he said.

Jon Urbanchek, the just-retired swimming coach at the University of Michigan, said Phelps still is a very youthful swimmer with a certain innocence and a kind of blue-collar work ethic, that of "a real tough guy, the steel mill, not a country club type of environment."

He said it's as if Phelps is saying to Bowman: "Bring it on. Come on, Bob. Give me more. Give me more."

Last week, Mark Spitz, whose record of seven gold medals at the 1972 Olympics Phelps is seeking to match, said Phelps's heavy Olympic swimming load may benefit him in another way.

"Those who just want to swim one event sometimes sit around and second-guess whether or not they've trained properly," Spitz said.

"When you're watching your competitor start to come in and break records, that momentum is hard to stop."

Michael Phelps competes in 200-meter backstroke preliminaries. "I think he's in really good condition. Somebody said you kind of race yourself into shape," Coach Bob Bowman said.