A new study shows that despite exercise regimens designed to mitigate the atrophying effects of microgravity, astronauts on the International Space Station (ISS) are experiencing dramatic muscle loss on their missions.

Scott Trappe, a professor of exercise science and director of Ball State University's Human Performance Laboratory, and his colleagues report in the current issue of the Journal of Applied Physiology that the volume of calf muscles of nine ISS crew members decreased by an average of 13 percent during six months in space. (The researchers selected the calf for their NASA-funded study because it shows atrophying effects more dramatically than other muscles.) Trappe says that the magnitude of loss in muscle mass is akin to the difference between a 25-year-old and an 80-year-old.

The team used magnetic resonance imaging (MRI) and muscle biopsy pre- and post-flight to ascertain muscle characteristics at the macroscopic and microscopic levels, as well as physical testing to gauge muscle performance. Similar studies have shown that living on the space station also exacts a toll in terms of both the density and strength of bones.

The ISS has various conditioning devices, including an exercise cycle and a treadmill, to help offset the deleterious effects of extended stays in microgravity. Astronauts aboard the station also perform a series of exercises such as squats and lifts to stave off atrophy. The programs do seem to be yielding some benefits—the amount of muscle loss on the ISS is slightly less than that found in a 2000 study of astronauts on the Russian Mir space station and is significantly less than that experienced in shorter-term (two- to four-month) bed-rest studies without any exercise at all. But most of the onboard exercise programs are of moderate intensity.

The study authors say that higher-resistance training is needed to better protect the health of ISS denizens and that of astronauts on longer spaceflights in the future. "Quality (i.e., intensity) is more effective than quantity for protecting skeletal muscle size and performance during long-duration unloading," the researchers conclude.

Photomicrograph of normal muscle fibers (above) and atrophied muscle fibers (below) courtesy of Johnson Space Center/NASA