The reason is that muscles take orders from more than the brain, Baldwin says. Some nerves from motor units go to the spinal cord and up to the brain, but others loop back and connect to other motor units, to the skin and to other body tissues.
Through these loop-back circuits, muscles and skin can communicate among themselves, allowing muscles to react faster than the brain can. The classic example is touching a hot surface. Your skin sounds the alarm, and the muscles pull your hand away before the news even reaches your brain.
Similar, routine reactions are happening in your body every waking instant. The fact that you can, without thinking, stand on two legs or hold your head erect is because of direct communication among muscles assigned to oppose gravity.
In fact, some tasks are easier to do if you keep your brain out of it. Cocktail servers are taught to balance their drinks on a tray supported by the palm of one hand and to avoid watching it. Even the most top-heavy trays rarely spill, because the arm and hand muscles balance the tray continually and automatically. But start watching the tray and the brain will almost always over-correct and spill.
This back-door communication is the key to why our muscles change, Baldwin says. Through this system, signals flow to muscles and prompt quick adaptations.
Muscles can change because they are constantly being built and rebuilt, Baldwin says. In only seven to 14 days, half of the protein in your muscle cells has been broken down, discarded and replaced.
"It's as if you had a contractor constantly tearing down the rooms in your house and rebuilding them with new materials," Baldwin says. "After a month, you've got a brand-new house. Next month you'll have another brand-new one."
What every bodybuilder knows is that increased stress on muscles--lifting more weight than you did last week--somehow signals cells to build bigger, and therefore stronger, muscles. (Bigger muscles are always stronger, Baldwin says.)
But what more recent studies discovered is that a further change takes place. The character of the stress was changing the character of the muscle fibers.
Marathon runners were developing a type of slow-moving but high-stamina fiber, named Type I or "slow twitch." Sprinters and power lifters were developing a type of high-speed, high-output fiber--a group called Type II or "fast twitch."
The typical person has half one type, half the other throughout the body. But when muscle cells are somehow informed of the type of unusual activity the muscle is performing--slow-moving or fast, long-lasting or intense--some muscle fibers change character to the appropriate type, Baldwin says.
The change begins quickly, says Baldwin's research associate, Vince Caiozzo, assistant professor of orthopedics at UC Irvine's College of Medicine.
"We have rats that pump iron. It sounds funny, but they aren't standing up curling weights. They take their feet and push against an object. The whole thing is computerized, so it can simulate every aspect of you going to the gym.
"If we have them do about two minutes of weight training a day, within two days we begin to see changes in the (muscle fiber types) they are producing."
Five of Baldwin's rats went up on a space shuttle last November to determine just what was happening to muscles in space. On their return, Baldwin discovered that being in zero gravity for 14 days had converted a large portion of their muscle fibers from Type I, which is particularly suited to opposing gravity, to Type II.
Muscles, freed from the effects of gravity, were merely adapting to their new environment, normally a good thing. "When they're up in space, they no longer need their legs to balance and control gravity," Baldwin says. "They can float around from one location to another with just fingertip force."
But astronauts return to Earth suddenly, where zero-gravity adaptation instantly becomes a liability. They found that not only were the muscles that helped them stand weakened, but the nervous system that operated their muscles had become lazy in weightlessness. So had the astronauts' breathing and blood circulation systems. In some cases, the astronauts could not stand without fainting.
This is merely an inconvenience when returning to Earth. With the flight over, the astronauts can spend several days or even weeks to work back to normal.
But it could be a major problem on a long mission to another planet.
"Let's take Mars; that's where they plan to go next," Caiozzo says. "It's projected to take 1 1/2 years just to get there. The problem is, when they land on Mars (and once again encounter gravity), are they going to be able to function?"