Every muscle in the body contains both fast twitch and slow twitch fibres. The ratio between fast and slow depends entirely upon the individual. For instance, I watched a fascinating documentary with British olympic gold medalist hurdler Colin Jackson the other week, where he endeavoured to find out whether his talent was God given (nature) or more down to dedication and will to succeed (nurture).

As part of the documentary, Colin visited a lab to have a chunk of his thigh muscle removed, and analyzed. The aim was to see if his muscles were significantly different from the average persons’.

What they found was, not only did Colin have more fast twitch fibres than the average person, but, he also had an even higher abundance of even faster fast twitch fibres known as “super fast twitch fibres.”

They found Colins’ leg muscle contained something like 20-30% of these super fast twitch fibres, whereas the average person would have just a couple of percent.

I would guess unless you had this test performed on every major muscle group and had the hardcore data in front of you, you’d never know for sure what percentage you was. My advice would be, to build maximum size, power, and endurance, you’ve got to train for both strength (low reps, very heavy) and size (higher reps, less heavy).

So not only does this confirm that there is a third type of muscle fibre, but, it’s generally accepted there is an even greater spectrum of fibres than this - all of which, no doubt, have their ’sweet spot’ which specific training methods can maximise the potential of.

and what about type a and b, which I have also heard mentioned.

It sems there’s great diversity of opinion regarding this subject and the answers vary considerably from place to place.