Astronauts who spend six months on the International Space Station (ISS) may experience back pain and muscle loss, despite temporarily gaining up to two inches in height, a study has found. Since the late 1980s, astronauts have reported back pain, with over half of US astronauts experiencing it, and 28% indicating it is moderate to severe pain. A further risk is herniated discs, with astronauts experiencing a 4.3 times higher risk in the first year after their mission. According to Dr. Douglas Chang, first author of the new study, the pain is caused by degeneration and atrophying of the supporting muscles in the lower spine due to the flattening of the spinal curvature in microgravity.