Resistance training (weight lifting) is one of the best ways to maintain bone strength. In space, the astronauts do not feel the same gravitational loading on their bones that we do on the surface. This leads to deterioration of bone tissue. Unfortunately weight training isn’t too easy in a free fall environment, like a space station. On-board the International Space Station (ISS), the crew now have a device called aRED (advanced Resistive Exercise Device) that simulates weight lifting using air pressure.
Here’s a video of Expedtion 18 crew member, Mike Fincke, using the aRED. Studying astronauts using devices like aRED will be incredibly important if we are ever to achieve the goal of sending people on long duration flights to destinations like Mars.