But NASA has an answer, a pair of them -- the Earth Shoes.
Astronauts aboard the ISS are set to begin donning a pair of Earth Shoes during their daily two-hour exercise routines using the specially designed Advanced Resistive Exercise Device.
The Earth Shoes will collect data as the astronauts work out on the machine, data which scientists back on Earth will parse in order to better understand the interplay between exercise and zero-gravity's unwanted consequences.
"There is still progress to be made in understanding the effects of exercise on bone and muscle health," said Andrea Hanson, an aerospace engineer with Wyle Science, Technology & Engineering, a company contracting with NASA on the Earth Shoe project.
The Earth Shoe, as NASA calls it, is actually the ForceShoe; and it is developed by a company called Xsens. Each shoe is outfitted with three sensors, which measure up-and-down, side-to-side and front-to back movements.
"We are eager to understand how joint forces may be different between exercise performed on the ground and in space," Hanson explained.
Several pairs of ForceShoes traveled to the space station this week aboard Russian Soyuz capsule -- which also delivered new crew members.