So far, Baldwin says, exercise programs in space using treadmills, cycles and bungee cords have not solved the problem. The solution, he says, is more likely to be a combination of physical stress and drugs that either prevent the change in muscle fibers or deceive the muscle cells into thinking that they have been exercising.
Tests on Baldwin's rats have shown that a chemical called beta guanidinopropionic acid acts to deplete muscles of a naturally occurring substance called phosphocreatin. Normally, this substance is used up when muscles work. Baldwin believes that artificially depleting it makes cells react as if the muscles had been through a workout. So far, rats have reacted to the chemical by converting some Type II muscle fibers to Type I fibers.
Removing the thyroid hormone from circulation seems to block the conversion of muscle fiber that occurs in zero gravity, Baldwin says. This shows the process can be regulated chemically, he says.
Changing or maintaining muscle size is a different matter, Baldwin says. Researchers believe that the simple brute force generated by muscles serves to signal how much muscle mass is needed. The muscle builds up or shrinks accordingly.
"We have not been able to unlock a chemical that would act in place of force, but we think we have some clues," Baldwin says. A substance called insulin-like growth factor is being studied. It seems to work directly on muscle cells to increase their size, Baldwin says. Unlike steroids, the increase in size occurs without exercise.
(It appears that steroids only allow a person to exercise more strenuously; it is the increase in exercise that increases muscle size, Baldwin says. Steroids administered to a couch potato would probably have no muscle-building effect, but this has not been scientifically tested.)
Another researcher--Frank Booth, professor of physiology at the University of Texas Medical School, Houston--believes that the answer may be genes rather than drugs. Injecting certain genes into muscles, like a vaccination, could promote production of desired substances within the muscle that would maintain muscle fiber without exercise, he says. Something that could be used by people is only five to 10 years away, he estimates.
"What I'm describing will be possible on a very limited scale for a very few people," Booth says. "I don't think it would be used by the normal population. It would probably be used very selectively. I think it would be costly and not applicable in all cases."
But, he adds, subsequent advances "may take it to the general public."
Another researcher, V. Reggie Edgerton, professor of physiological science at UCLA, says he is concentrating on the nervous system. It is known that the efficiency of a body's nervous system plays a role in how much force its muscles can apply.
Within certain bounds, a person with smaller muscles but a better nervous system can be effectively stronger than the person with the opposite attributes. Edgerton and others say the typical person cannot voluntarily exert all the potential force of his or her muscles. But some scientists disagree, and the point remains controversial.
Edgerton says the nervous system may by itself play a role in determining how muscles will change, but adds: "We're pretty far away from knowing all the answers.
"Humans have been wanting to believe there's an easy way for a long time. Do you remember that book about how to stay fit with only 30 minutes of exercise a week? It stayed on the bestseller list for months. It just told everybody what they wanted to believe."
Only 10 years ago, muscle biologists thought every property of muscles was determined by the muscle's activity. Now it's now plain that's not true, Edgerton says.
"I think we could easily get to the point where we could minimize some of the negative effects of aging" by controlling the chemistry of muscle cells.
Division of Labor
Taxing muscles more than usual signals muscle fiber to grow.
* "Slow twitch" fibers: Used for routine movement and endurance; built up with slow exertion.
* "Fast twitch": Used for explosive, high-power movement; built up with quick, extreme exertion. * Average person: About 50% slow-twitch muscles, 50% fast-twitch. * Marathon runner: As much as 80% slow-twitch muscle for extreme endurance. * Linebacker: As much as 70% fast-twitch for explosive speed and power.