Space shuttle Columbia's astronauts are breezing through their orbital workouts, but that should change as their medical-research mission wears on, a NASA doctor says.

Today is only the fourth day of the flight, which is supposed to last 16 or 17 days."Feel great," Dr. Charles Brady Jr. said after a vigorous workout on a stationary cycle Sunday that measured both his lung function and muscle power.

"It's actually more comfortable," added astronaut Richard Linnehan, "because you can kind of float and relax while you're exercising."

NASA scientist Victor Schneider said it should become more difficult for the astronauts to exercise in orbit as their muscles weaken from prolonged exposure to weightlessness.

As for space motion sickness, which strikes two-thirds of all astronauts, Columbia's crew has little to report.

Astronaut Susan Helms, who's in charge of the laboratory experiments, said Sunday the entire crew adapted "very, very quickly" to weightlessness.

"We've had just an outstanding flight from the standpoint of adaption, so if the scientists were hoping to capture a lot of data on that on our crew, they're probably not going to get a whole lot," she said.

The fact that the four medical subjects - male first-time space fliers in their 30s and 40s - seem to be in such great shape is immaterial from an experimental standpoint, Schneider said.

"If it's a little (sick) or none or some, all that is very helpful information for the future and for the research itself," he said.

"Full-blown, terrible space motion sickness" would have been disastrous for researchers, Schneider said, because it would have been impossible to gauge the extent of the astronauts' illness before and after some of the more nauseating tests.

In one motion sickness-inducing experiment, the astronauts were instructed to move both their heads and bodies while looking at moving objects, as opposed to moving just their heads and eyes. The more motion the more likelihood of motion sickness, so if the crew members did not get sick during this test "then that's a good sign," Schneider said.