Weightless yoga might be the answer to bad backs suffered by astronauts on long missions, a study suggests.

Scientists have found that a lack of gravity weakens the muscles supporting the spine.

The resulting low back pain and lack of mobility is a common problem among International Space Station crew members who spend long periods of time in orbit.

Adding yoga to astronauts' exercise routines could be a solution, according to experts writing in the journal Spine.

It had been thought the back pain accompanying space flight was linked to changes affecting vertebrae discs.

The new research, based on magnetic resonance imaging (MRI) scans of returning astronauts, shows the real culprit to be shrinkage of the paraspinal muscles.

Even after several weeks back on Earth following long-duration space flight, the muscles are not fully recovered, the study found.

Current astronaut exercise programmes focus on maintaining bone density, large muscle strength, and endurance.

Dr Douglas Chang, from the University of California at San Diego, and his team of experts called for "core strengthening" lumbar extension and isometric exercises to protect astronauts' backs.

They added: "Another promising exercise countermeasure for low back pain is yoga, which might be particularly effective in addressing spaceflight-associated lumbar stiffness and hypomobility (poor mobility)."

Further studies were needed to see how such exercise might be performed in microgravity with available equipment, they said.