The Collaborative International Dictionary
myo-
myo- \my"o-\ A combining form of Gr. ?, ?, a muscle; as, myograph, myochrome.
Douglas Harper's Etymology Dictionary
myo-
before vowels my-, word-forming element meaning "muscle," from comb. form of Greek mys "muscle," literally "mouse" (see muscle (n.)).
Wiktionary
myo-
pre. Forming compounds words pertaining to muscle.