Muscle Fibre Types and Getting Big

There are several different types of muscle fibres in humans. According to Wilmore, Costill and Kenney (2008), we have the following fibre types (and as research techniques develop, we may distinguish fibre types even further): "I; Ic (I/IIa); IIc (IIa/I); IIa; IIax; IIxa; and IIx." Clear as mud? Excellent.

For the sake of this post, we'll narrow that down to Type I, IIa and IIb (with IIb being more or less equivalent to IIx). Note that fibre types aren't so much a distinct classification as they are a sliding scale of qualities. Here's more or less what each of the above three types look like:

	Type I	Type IIa	Type IIb
Twitch	Slow	Fast	Fast
Colour	Red	Pink	White
Energy	Aerobic	Aerobic, anaerobic	Anaerobic
Power	Low	High	High
Endurance	High	Moderate	Low
Hypertrophy Potential	Low	High	Debatable

The twitch is a full stimulus --> contraction --> relaxation period. Thus slow-twitch muscle fibres go through that sequence slowly, and fast-twitch fibres go through the sequence quickly. Type II fibres can typically go through 5 twitches in the time it takes a Type I fibre to go through 1 twitch. Type I and II fibres of the same length and width in the same location will produce about the same amount of force, but because Type II fibres can perform many more twitches in the same duration, they can produce much more power.

The colour of the muscle is directly related to the energy source. As aerobic energy sources use blood oxygen, aerobically dense muscle fibres are red. As anaerobic energy sources don't use blood oxygen, anaerobic muscle fibres lack redness. As aerobic energy pathways allow long-duration tasks, while anaerobic pathways do not, the colour of the muscle fibre is also related to its endurance.

The hypertrophy potential of the muscle fibres is an interesting issue. Hypertrophy seems to occur when two training conditions are met:

1) Adequate load is placed on the muscle fibres.

2) Adequate volume is placed on the muscle fibres.

I'm not sure how much biological potential slow twitch fibres have for hypertrophy, but from a loading perspective, we run into an immediate problem: they can't take much load, and with the load that they can take, it's really hard to wear them out.

As for Type IIb fibres, I'm led to believe that biologically they do have high potential for hypertrophy. The problem, however, is that their is very low, and exposing them to high training volumes may promote them converting to Type IIa fibres. Following this is how impractical it is to expose Type IIb fibres to high volume. If we assume the classic hypertrophy vs max strength rep ranges (6-12 = hypertrophy; 1-5 = strength; in reality, just like fibre types, this is actually a sliding scale), the closer we get to max strength and thus focusing on white fibres, the less reps we can do in a set. Let's take the extreme and compare 12 rep sets vs 1 rep sets. In a standard hypertrophy workout, you might do 3-5 sets of 12, for 36-50 total working reps, and that might take you 10-30 min, depending on intensity and rest periods. In contrast, with enough weight on the bar to make heavy singles worth doing (probably 90% 1RM), you're looking at 3-5 min between sets, maybe longer as you get fatigued, which means it would conservatively take you 36 min plus actual lifting time just to make up the volume of the first hypertrophy set.

For these reasons, focusing on Type IIa (pink) fibres is far more practical when it comes to hypertrophy. I don't know if they are biologically better at hypertrophy than white fibres (ie, given the same volume, I'm not sure that pink fibres would still grow faster than white fibres), however their combination of being both fast-twitch and decent at endurance makes them generally the easiest fibre type to hypertrophy. Moving closer to max strength (white) loads makes it hard to accumulate sufficient volume for hypertrophy. Moving closer to endurance (red) loads makes it hard to apply sufficient intensity for hypertrophy.