Scientists saw these soon built-up more muscles than their untreated companions.
The genetic treatment involves blocking a pair of proteins associated with limited muscle mass.
A SpaceX capsule safely parachuted all 40 mice back in good condition at the start of the year.
The researchers completed the groundbreaking experiment as the coronavirus crisis began to be felt around the world.
Dr Emily Germain-Lee of Connecticut Children’s Medical Center, who was also involved in the study, said: “The only silver lining of COVID-19 is that we had time to write it up very intensively and submit the results for publication.”