A prefix meaning muscle. Most often found in the more complete form myo-. The 'o' is dropped in words beginning with a vowel.
Myasthenia - muscular weakness or debility.
Myatonia - poor muscle tone.
Myalgia - pain in a muscle.
The prefix myo- can also refer to mice, as in
myomancy (divination from the movements of mice),
myophobia (fear of mice), myoxine (mouselike), but I have not yet found a mouse-based word that shortens myo- to my-. (Please /msg me if you find one).
The prefix myo- comes to us from the Greek mys (and later mūs), meaning muscle, and also mouse. (In Greek and Roman times, certain muscles were thought to look and move like mice under the skin). In Latin mūs (mouse, muscles) became musculus, literally 'little mouse', from which we get our modern word muscles.
It is possible that the prefixes myelo- and myel- are related; while they come from the Greek muelos, meaning bone marrow, muelos may come originally from mūs ('bone muscle', as it were).