How Do Bones React in Space?4 min read

You’d think that floating around in space would be good for bones and joints because you aren’t putting any pressure on them, but in reality prolonged weightlessness can take a toll on the body’s musculoskeletal system.

Mike Renlund
A Treadmill for Running

Astronauts actually lose bone mass in space, in fact. The lack of gravity causes a decrease in bone density which weakens the bones. In order to maintain healthy bones, we need weight-bearing activities like walking or jogging. Even our daily running of errands around town is weight bearing if you think about it. We need earth.

“On Earth, gravity applies a constant mechanical load to the skeletal system that causes healthy bones to maintain a certain density so that they are able to support the body,” says the Canadian Space Agency.

That’s why exercising is so important not only on Earth but in space. Just like with our muscles and joints, if we don’t use our bones, they weaken. Our bones, muscles and joints (plus tendons, ligaments and cartilage) make up the musculoskeletal system and allow us to do physical activities. Apparently we can’t escape exercise no matter where we go!

An astronaut typically loses as much as 40 percent of muscle and 12 percent of bone mass after five months of orbiting earth, according to an article on BBC.

“The muscle loss is the equivalent of a 20-year-old turning into a 60-year-old over a period of three months,” says UK’s Space Agency’s Jeremy Curtis in the BBC article.

Microgravity in space also contributes to knee problems often associated with rheumatoid arthritis, which causes pain and inflammation in the joints, and osteoarthritis because the cells undergo genetic changes even after only three weeks in space.

In space, it’s tough to duplicate the kind of exercise we typically get on Earth, but astronauts do work out while on their missions. The International Space Station has a special treadmill, stationary bike, and a weightlifting device. In order to make it weight bearing, the astronaut is strapped to the machine. Bone loss can be slowed in the process but not completely mitigated. When they aren’t exercising, the astronauts are weightless again.

Other research shows that some people in space lose bone mass as much as 1-2 percent a month. That’s a crazy acceleration rate when you consider that’s the same amount of bone loss that occurs over an entire year for older populations on Earth. When skeletal systems don’t need to support their weight as is the case in space, less new bone is made yet more old bone is being absorbed. This is what causes bone mass loss.

“The magnitude of this effect has led NASA to consider bone loss an inherent risk of extended space flights,” says Dr. Jay Shapiro in NASA Science

Bone loss and osteoporosis is a big problem on Earth, too. Considered a major public health threat, almost 44 million people in the U.S. over 50 suffer from low bone mass or osteoporosis — representing 55 percent of people over 50 years old. 

The NASA Science article says that hopefully figuring out the bone loss problem in space will unveil “important clues about what causes osteoporosis (and other bone disorders) right here on Earth.”

An article on says that bone density starts to change after several days living in microgravity.  Over time, a large amount of calcium that’s normally stored in the bones gets released into the bloodstream and in turn decreases bone density.

“This drop in density, known as disuse osteoporosis, leaves bone weak and less able to support the body’s weight and movement upon return to Earth, putting the astronaut at a higher risk of fracture,” according to the NASA article.

Because space flight takes a toll on the human body, radiologic technologies, such as MRI, are often used to help determine if bones are fractured and the result of osteoporosis.

As you can see the negative effects of zero-G are serious. After returning to Earth, astronauts participate in a rehabilitation program to rebuild muscles and bones. They can rebuild muscle, but previous bone mass may never return. The positive aspect is that continued scientific studies will further new treatments for diseases such as osteoporosis and cancer for the earthbound population. For more effects on long-term space flight, read our blog post about astronaut twins Mark and Scott Kelly.


Avery Phillips is a freelance human based out of the beautiful Treasure Valley. She loves all things nature, especially humans, and was driven to pursue an Anthropology degree due to her childhood love of Indiana Jones and Laura Croft. Now she lovingly writes about all things great and small.

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