None of the dilemmas threaten the life of the station or its crew. Russian Yuri Usachev and astronauts James Voss and Susan Helms replaced the first station crew March 18.

The stubborn problems they are fighting include:

• Breaking or cracked slats on the treadmill.
• A malfunctioning probe measuring the electric charge around the station.
• A balky communications antenna.

The treadmill has provided perhaps the most perplexing glitch so far -- a tendency of its slats to break during exercise sessions.

The device is central to the exercise regimen used by station residents. Astronauts and cosmonauts use a stationary bike and a space weight bench for other exercise, but Flight Director Rick LaBrode said human muscles and bones need the repeated shock of treadmill running to deter the effects of weightlessness.

Runners use a harness and elastic bands to hold them to the treadmill's surface.

Astronaut Bill Shepherd, who returned to Earth last week after four and a half months in space, credited exercise with making the transition to gravity easier on his body.

While treadmill breakdowns could be explained on Earth, it is causing some confusion in space.

LaBrode said engineers will have to look at the breaks firsthand to find the problem.

"Until we get it home we can't get a handle on what happened," he said.

Ground controllers also are deciding how to handle an electric probe that hasn't worked since December. It is mounted atop Alpha's solar arrays but has not relayed any information since Alpha's first crew moved a Soyuz capsule from one port to another three months ago.

The unit had problems before and scientists have been running through options. They may build a replacement if the experiment does not show signs of life, LaBrode said.

Alpha's crew has activated four science experiments, but a research rack must wait until an antenna on the station begins working correctly. Controllers hope the antenna, which handles vast amounts of information, including video, will find its satellite target and transmit by next week.

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