While all offered benefits, so, too, did they present drawbacks.

Sprinting in the water creates excessive resistance, often necessitating a forward lean.

Cycling can cause overuse of the quads. The elliptical trainer can't accurately replicate the running motion. But in the past two years elite athletes have discovered a new invention to help them recover from injuries - the Alter-G treadmill.

The Alter-G, which was derived from NASA research, is an anti-gravity treadmill that utilizes air pressure to reduce impact. Wearing neoprene shorts similar to wetsuit material, athletes are zipped into an enclosure to create an airtight environment from the waist down. Envision the way skirts keep water out of a kayak. Using a touch screen, athletes can adjust the air pressure, reducing the impact by up to 80 percent of their body weight.

Dathan Ritzenhein, who finished second at the men's marathon Olympic Trials in November and won the USA Cross Country Championships in February, suffered a slight leg injury in January. The last three weeks before the cross-country championships, Ritzenhein ran 220 miles on the Alter-G, 110 miles on land. "It lets you basically train right through injuries so much better than any other form of cross-training," Ritzenhein said.

The Olympic Training Center in Chula Vista, Calif., has leased an Alter-G treadmill for four months with favorable reviews from athletes and trainer Vinnie Comiskey.

"I loved it," said pole vaulter Stacy Dragila, the 2000 Olympic gold medalist who has undergone two Achilles' surgeries the past two years. "It gave me confidence to put my foot down properly and not feel like I was babying it or altering my running."

"If I had one," said steeplechaser Kassi Andersen, who has been recovering from foot injuries, "I'd use it all the time." Andersen described the sensation of running with the air pressure as "kind of like you're floating. People say it's kind of like walking on the moon. It's not like any of us have, but you're just lighter."

Many runners, in fact, are using the treadmill to augment their training. By doing a small percentage of their mileage on the Alter-G, the runners alleviate some of the wear and tear on their bodies. Shot-putters, discus throwers, kayakers and rowers, who are heavier than distance runners and therefore put more stress on their bodies when running, have supplemented their conditioning on the Alter-G at the Olympic Training Center.

Said Comiskey: "We've had athletes with Achilles' strains to stress fractures who initially adjust the amount of pounding to their lower extremities and can decrease their pain and get their workouts in."

The treadmill is not cheap. It costs $75,000. The training center Chula Vista is negotiating with Alter-G officials to extend its lease.

Some runners say that one slight drawback to the treadmill is that they have to alter their arm positioning slightly. "It's not perfect," said Andersen. "But it's the closest you can come to actually running (while cross-training)."

Recently, Alter-G received approval from the Food and Drug Administration to market the treadmill for medical and therapeutic use. Dragila said the treadmill can help patients recovering from knee and hip replacements by enabling them to gain range of motion more quickly.

Said Comiskey: "We had a group of triathletes here and their eyes opened like a kid in a candy store (when they saw the treadmill). They couldn't wait to run on it. It's a very useful machine."