Pumping Iron in Microgravity
Video feeds from the International Space Station (ISS) invariably show crew-members exercising in the background. Exercise is serious business on the ISS because microgravity causes skeletal muscles to lose power and stamina. Workouts help astronauts fight back.
Yet despite rigorous workouts, astronauts return to Earth shockingly weaker than when they left. Only 11 days in microgravity may atrophy (shrink) muscle fibers as much as 30 percent and cause soreness as damaged muscles tear while readjusting to Earth's gravity.
Two electron micrographs of human oleus muscle cells before and after 17 days in microgravity. (Image Credit: Danny Riley and James Bain, Medical College of Wisconsin)
Fortunately, muscles recover rapidly after weeks in microgravity. But what might happen during a years-long mission like a trip to Mars? How long would muscles atrophy during spaceflight, and what levels of muscle strength would the astronauts have when equilibrium is reached? Could more vigorous aerobic workouts prevent wasting, or would other types of exercise be more effective? Such questions intrigue Robert H. Fitts, professor of biology at Marquette University in Milwaukee, Wisconsin. He has been examining astronauts' muscle tissue before and after ISS missions to project how longer spaceflights would answer these and other questions.
Fitts, an expert on exercise physiology and a serious runner, says that his ISS research will help develop workouts for astronauts to minimize or even prevent atrophy. Any success would also deliver benefits on Earth, where similar exercises could help the elderly stay strong and speed the rehabilitation of certain patients after lengthy illnesses.
Muscle atrophy involves many subtle chemical as well as physical interactions, but the basic principle is simple. Muscles, explains Fitts, are adaptable tissues. Increase the load on them by lifting weights or other types of exertion, and they grow larger and stronger. Reduce the load by lying in bed or living in microgravity, and they grow smaller and weaker.
"When you load a muscle," Fitts continues, "its fibers begin a series of intracellular signaling steps. Genes within the cell nucleus make RNA [ribonucleic acid], which synthesizes proteins that make up muscle fiber. Pumping iron activates the expression of these proteins, which accumulate and enlarge the muscle fibers."
Microgravity has the opposite effect. It reduces the load that gravity naturally places on muscles, interrupting protein synthesis so that fibers begin to atrophy. This loss of muscle mass contributes to reduced skeletal muscle strength when astronauts return to Earth.
Not all muscles atrophy at the same rate in microgravity. Back and leg muscles that work against Earth's gravity to maintain an erect posture waste fastest in microgravity. Yet even among these muscles, there are differences. In microgravity, astronauts naturally assume a modified fetal position, with legs bent at the knees and feet extended downward. This posture shortens the calf muscles in the back of the lower leg, removing tension and speeding atrophy. It also lengthens the shin muscles in the front of the leg, creating enough tension to impede atrophy.
Fast and slow
Microgravity also has a profound effect on fast- and slow-twitch muscle fibers. As the name suggests, slow-twitch fibers contract gradually and generate little power but have high aerobic capacity and resist fatigue. Fast-twitch fibers contract more quickly and generate more power but tire quickly.
"Slow-twitch muscle fibers dominate in running marathons and fast-twitch muscle fibers in the 100-yard dash," says Fitts. Analysis of rats exposed to microgravity initially led researchers to believe that spaceflight degraded slow-twitch fibers more rapidly than fast-twitch fibers, but more recent human studies indicate that both types of muscle fibers undergo significant atrophy.
Surprisingly, spaceflight alters the balance of fast- and slow-twitch fibers. "Not only is there a change in the amount of protein synthesized but also the type synthesized," says Fitts. "During extended flights, about 15 to 20 percent of slow-twitch muscle fibers become fast-twitch fibers." Muscle conversion is likely caused by changes in the type of muscle proteins synthesized by the body, he explains. Changes in fiber type may also be responsible for muscle tears when astronauts return home. "Our theory is that microgravity may suppress expression of proteins that anchor contractile filaments to the muscle fiber surface," he says.
Any microgravity exercise routine must maintain not only muscle mass but also the right mix of proteins to balance fast-twitch muscle power with slow-twitch muscle endurance while firmly anchoring contractile filaments. This sounds like a tall order, but Fitts believes that preserving muscle mass will automatically balance protein synthesis as well.
Muscle atrophy during spaceflight has always been tough to avoid. Historically, U.S. and Russian astronauts have relied on aerobic exercises, primarily pedaling a cycle ergometer (an exercise bike) and running while tethered to a treadmill. Unfortunately, aerobic exercises are designed to condition the cardiovascular system rather than apply loads systematically to a wide range of muscles, Fitts explains. Cycling, for example, applies a good load to the upper leg but not the lower leg or back, he says. "It does not preserve muscle."
"At this point," Fitts says, "we know we're losing muscle mass and not getting the proper muscle activation with aerobics." Although the ideal microgravity exercise program remains undefined, Fitts believes it will include more strength training.
Strength training, says Fitts, involves two different types of resistance exercises: high-intensity isotonics, which shortens and lengthens muscles (for example, lifting and lowering a dumbbell), and isometrics, which fully contracts muscles without movement (for example, pushing against a doorway). Both types of exercise could potentially reduce muscle atrophy in microgravity. Fitts' experiments with rats, however, suggest that isometrics may protect slow fibers better than isotonics because slow fibers develop very little force during relatively fast isotonic motions.
Astronaut Robert Overmyer works out on a treadmill on the Space Shuttle
It is easy to develop a strength training program that combines isometrics and isotonics on Earth. In microgravity, where a dumbbell "weighs" no more than a feather, it is difficult -- but NASA may have a solution. Studies are under way to evaluate the efficacy in microgravity of the interim resistive exercise device, which was installed aboard the ISS in April 2001. The device generates up to 300 pounds of resistance for various exercises. (NASA licensed the technology to Schwinn, which now sells it as the RiPP Pro unit.)
Fitts says several research groups are working to develop similar units that could provide adequate loads to protect skeletal muscle while living in microgravity. The challenge, he says, is designing compact, reliable devices that can generate consistent, measurable loads for the various skeletal muscle groups located throughout the body.
After determining the best kind of exercise for astronauts, the next question is, how much exercise is enough? "If you're pumping iron on Earth, two or three times weekly is enough to build muscle," says Fitts. "In [microgravity], you'll have to do it maybe one or two times per day" -- and that's only to maintain muscle strength, not increase it.
The operative word here is "maybe," because the effects of prolonged microgravity on muscle fibers remain largely uncharacterized. Without fully understanding how each type of muscle fiber changes over time, any proposed countermeasures are educated guesses at best. Yet Fitts' guesses are more educated than most. He led a team that characterized human limb muscle fibers for the 17-day STS-78 flight in 1996. Now he has begun to run similar tests on astronauts before launch and immediately after they return from extended ISS missions.
Fitts is studying the two major muscles of the calf: the outermost gastrocnemius, which contains both fast- and slow-twitch muscle fibers, and the soleus underneath, which contains mostly slow-twitch fibers. Instead of measuring gross muscle performance, he is assessing individual muscle fibers from small tissue samples taken from the calf muscles of astronauts before and after spaceflight.
Fitts chemically activates the muscle fibers to cause them to contract so he can measure force, velocity, and power. When the force they generate reaches its peak, he stretches the fibers 10 percent -- equivalent to the stretching that occurs during walking or climbing stairs -- and measures their peak force again. Muscle fibers from people who exercise on Earth typically show little change in strength after five stretches; Fitts expects postflight muscle fibers to show significant damage after only one or two stretches. Fitts then determines where tears occur in the fibers by examining them with an electron microscope. He also uses antibodies that react with specific proteins in the fibers to identify protein changes. Together, the information helps him precisely characterize variations in muscle fiber size, peak force, power, speed, and tear resistance.
On Mars and Earth
Fitts will use the data obtained from his experiments on single muscle fibers to try to predict changes that may occur in muscles during such prolonged space missions as a voyage to Mars. His measurements will also provide important baseline data for evaluating future microgravity exercise programs.
Some of Fitts' discoveries may have an immediate impact on Earth. People steadily lose muscle mass and aerobic power after age 55. Until recently, physiologists have typically recommended aerobic exercise to counter this effect. "The problem with walking or bicycling is that many elderly are too weak to get out of a chair," says Fitts. "Until very recently, most fitness programs for the elderly didn't emphasize muscle mass. Therapists are just starting to get the word that the elderly need resistance exercises, too."
The challenge, says Fitts, is choosing the right exercises. That's where space research comes in. "Spaceflight acts like accelerated aging," he explains. "It causes rapid loss of muscle mass. If we can learn how to prevent muscles from wasting during spaceflight, that information can help people on Earth stay strong as they age." The same insights could help patients with serious burns, who are often confined to bed for months and treated with steroids that break down muscle. "Optimal exercise could reduce rehabilitation time in many clinical settings," Fitts explains.
By honing today's best approaches in the demanding microgravity environment, NASA researchers may learn how to keep muscles strong and healthy in microgravity as well as on Earth.
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