Bulked-up, mutant “mighty mice” held onto their muscle during a monthlong stay at the International Space Station; returning to Earth with ripped bodybuilder physiques, scientists reported on Monday. The findings hold promise for preventing muscle and bone loss in astronauts on prolonged space trips like Mars missions, as well as people on Earth who are confined to bed or need wheelchairs. A research team led by Dr Se-Jin Lee of the Jackson Laboratory in Connecticut sent 40 young female black mice to the space station in December, launching aboard a SpaceX rocket. In a paper published in the Proceedings of the National Academy of Sciences, Lee said the 24 regular untreated mice lost considerable muscle and bone mass in weightlessness as expected – up to 18 per cent. But the eight genetically engineered “mighty mice” launched with double the muscle maintained their bulk. Their muscles appeared to be comparable to similar “mighty mice” that stayed behind at NASA’s Kennedy Space Center. Besides, eight normal mice that received “mighty mouse” treatment in space returned to Earth with dramatically bigger muscles. The treatment involves blocking a pair of proteins that typically limit muscle mass. A SpaceX capsule brought all 40 mice back in good condition, parachuting into the Pacific off the California coast in January. Some of the ordinary mice were injected with the “mighty mice” drug after returning and quickly built up more muscle than their untreated companions, Lee said.