A non-invasive MRI scan that measures carnosine in the thigh muscle can accurately predict how many fast-twitch muscle fibers a person has, making it a useful tool to classify muscle type without biopsies.

Scientists used a non-invasive scan to measure a chemical in muscles (carnosine) and used that to tell if people had more fast-twitch muscle fibers — and it worked well enough to group people into types for the study.