Myofibrilin was a growth hormone prescribed for extended zero gravity missions. The invention of artificial gravity caused myofibrilin to fall out of use by 2155, except for during certain military exercises, and for workers in remote places where artificial gravity was impractical, such as mining colonies.
In 2155, Doctor Phlox discovered traces of myofibrilin in the blood of Susan Khouri, a nurse who had been murdered by Terra Prime, the xenophobic group to which she once belonged. With this information, the crew of the Enterprise NX-01 was able to deduce that Terra Prime was based out of the Orpheus Mining colony on the moon. (ENT: "Demons")