Since astronauts in orbit around the earth don't feel the effects of gravity, they don't have to use their muscles as much during normal, everyday activities. For example, every time you take a step on earth you have to lift your entire body weight up with your legs. In orbit, however, where astronauts experience "weightlessness," it takes almost no effort to propel yourself forward. So astronauts have to intentionally exercise a lot or else they will get out of shape.
Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of the development for the former version of the site.