Though they don't bear the burden of gravity, astronauts who run aboard the International Space Station can commiserate with participants in the New York City Marathon, though weightlessness presents them with different types of challenges.
PHOTOS: Where NASA Simulates Space to Train Astronauts
NASA astronaut Michael Hopkins, a 46-year-old Air Force colonel who served more than five months as a space station flight engineer from September 2013 to March 2014, said running on the station's treadmill often wasn't a cardio workout, like it is on Earth, but an exercise in bearing a load.
The treadmill includes a bulky harness and bungee cords to hold astronauts in place while they run.
"For me the biggest challenge is wearing a harness that you're loading up. If you were to only load that up to say one-quarter of your body weight or something it's not going to be as bad, but I would try to get it up to as close to my body weight as possible. You're carrying that on your shoulders and your hips," Hopkins told Discovery News.
PHOTOS: Spacemen at Work: Astronauts Snap Amazing EVA Photos
"So imagine in my case, 185 pounds, and in some sense it's like carrying a 150-pound pack on your back and trying to run down here on Earth. The straps start to dig into your shoulders and your hips as well. You have that pressure, that load that's bearing down on you. You can get pains in your shoulders and things like that while you're running, and so there are times when you just have to stop and kind of just squat down and get the load off your shoulders, just kind of float near the treadmill just to get a little bit of relief. For me, that was probably one of the more challenging parts of running," he said.
Another peculiarity of microgravity exercise is that perspiration won't roll down your face and drip to the ground.
"It definitely behaves differently. You get this thin layer of sweat that just covers your arms and your neck and your head. You can just start to feel it pool up around your eye sockets, for example. It tends to stick with you, so I always kept a towel close by that I could grab and wipe that off," Hopkins said.
PHOTOS: Inside the First 100 Days of a Year in Space
Astronauts typically spend two- to 2.5 hours per day exercising aboard the station, part of a protocol to mitigate muscle wasting, bone loss and other potentially dangerous effects of long-duration spaceflight.
"The longest I ran while I was up there was 12 miles and it took me basically 90 minutes, one orbit of the Earth," Hopkins said. " I like to tell people I ran around the Earth."
In 2007, NASA astronaut Sunita Williams spent more than four hours on the station's treadmill as a remote participant in the Boston Marathon, the first person to run a race in space.
Williams completed the 26.2-mile (42 kilometer) race in four hours and 24 minutes.
The New York City Marathon, which is expected to draw more than 50,000 participants, is on Sunday.