What are hybrid muscle fibers?

What is the best technology for testing muscle fibers composition?

• Is there a technology that permits cheap, accessible and precise testing of slow twitch and fast twitch muscle fibers? This could be used for fitness advice (sprints vs marathons)

• Answer:

  I can't think of anything more accurate than biopsy... EMG is still not accurate enough as far as I know. If you want a rough estimate, I would be asking person to do some weight exercise (e.g. squats with barbell) to a max rep and then ask him to do let's say 80% to the max number of reps. If he can only do 3-5 reps with that weight - than he has is predominantly fast fibers, if closer to 8-10 reps or more, then predominantly slow.  For exact proportion you would still need biopsy, I afraid.