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Maximize Your Performance By Training For The Right Muscle Fiber Type

Posted Mar 09 2009 3:02pm


There was a very interesting article posted over on the Performance Menu forums a few days ago: Muscles, Genes, and Athletic Performance. I’m also going to use these two articles in this post: Skeletal Muscle Fiber Type Part I and Part II.

The crux of the article is about genetic engineering and “genetic doping” at the elite athletic level. But there’s some interesting stuff in there for the rest of us, from a performance perspective. Note that much of the article and my discussion invokes sprinting, but sprinters aren’t the only ones that can make use of this information. Sprinting just happens to be the best example of raw speed.

Anatomy Of A Muscle

The image above does a good job of showing the overall structure of a muscle. Basically, a muscle is a bundle of cells called fibers, or more technically myocytes. A fiber is a bundle of myofibrils that run the full length of the cell. Each myofibril is composed of contractile units called sarcomeres, which are in turn composed of filaments known as actin and myosin that slide together to produce contraction. Here is a visual of the actin/myosin interaction snagged from Page 4 of the PDF above.


A component of the myosin molecule is known as the “heavy chain” and determines the overall functional characteristics of the muscle fiber. So in order of smallest to largest: actin & myosin -> sarcomere -> myofibril -> myocyte (fiber) -> “the muscle”.

Muscle Fiber Types

Now, let’s have a brief overview of the types of muscle fibers that are found in all mammals. At a base level, there are two fiber types: slow-twitch (Type I) and fast-twitch (Type II). Fast-twitch is further divided into Types IIa, IIx, and IIb, in increasing order of speed of contraction. Humans possess Type IIa and Type IIx, whereas the fastest muscle type, Type IIb is found in animals like small rodents.

Here is a table from Brian Mackenzie that describes the base attributes of the 3 fiber types. I copied this exactly, but it’s likely the final column should be Type IIx rather than Type IIb since as far as I’ve found, Type IIb is not found in humans.

Muscle Fiber Attributes

Fibre TypeType I fibresType II A fibresType II B fibres
Contraction timeSlowFastVery Fast
Size of motor neuronSmallLargeVery Large
Resistance to fatigueHighIntermediateLow
Activity Used forAerobicLong term anaerobicShort term anaerobic
Force productionLowHighVery High
Mitochondrial densityHighHighLow
Capillary densityHighIntermediateLow
Oxidative capacityHighHighLow
Glycolytic capacityLowHighHigh
Major storage fuelTriglyceridesCP, GlycogenCP, Glycogen

As you can see, as you go up the scale, you increase speed of force production, but you decrease the ability of the muscle to sustain maximal force production. A simple example is sprinting versus jogging. If Usain Bolt tried to run a mile at the speed he runs his 100-200m sprints, he’d make it…approximately 200m before slowing to a crawl. The muscle fibers that produce 100m speed don’t have the endurance to produce that speed over 1600m, which is why the record for the 1 mile run is around 3:45, rather than the 2:35 it would be if Bolt could sustain his 100m world record pace for another 1500m.

The Type IIa muscle fiber is essentially a hybrid of the Type I and Type IIx fibers. It possesses some characteristics of the Type I fiber, namely a higher oxidative capacity than Type IIx fibers, along with some characteristics of the Type IIx fiber, such as being about five times faster to fully contract than a slow-twitch fiber (for comparison, the Type IIx fiber contracts about 10x faster than Type I).

Strength Characteristics Of The Muscle Types

So we know that Type II fibers are faster than Type I and that Type IIx is faster than Type IIa. But what about in terms of strength?

Gram for gram, the two types are not different in the amount of force they produce, only their rate of force production. So, having a lot of fast twitch fibers only makes a positive difference when the time available for force production is very limited (milliseconds), like the 100ms or so the foot is in contact with the ground during a sprint or long jump. It makes no difference to the powerlifter who may use 3-4 seconds to execute a slow, smooth lift.

So fiber type doesn’t determine raw strength potential. It can only determine power potential, i.e., force over time. An example is picking 250lbs off the floor. Picking up 250lbs is picking up 250lbs whether it takes you 1 second or 1 minute. But doing it in 1 second is significantly more explosive and powerful than doing it in 1 minute.

Muscle Type Conversion And The “Overshoot” Principle

Okay, so basically we all have muscles made up of the three main types of muscle fibers (counting IIa and IIx as different fiber types) and the fibers in turn determine how good we are at specific sports. A person with lots of slow-twitch fibers will never make a good sprinter. Someone born with lots of fast-twitch muscle will probably gravitate to sprinting rather than distance running. Let’s get down to what we really care about…is it possible to “configure” your muscles to excel in your chosen sport?

The average active person has about 50% slow-twitch and 50% fast-twitch fibers throughout their body, with about 5% being the super fast Type IIx. An elite sprinter on the other hand has around 20% slow-twitch, 45% Type IIa, and 35% Type IIx. For a world-class marathoner, you’re looking more like 80:20 slow-twitch to fast-twitch, with virtually no Type IIx.

Converting One To The Other

Is it possible to change your muscle fiber composition? Technically, yes, a muscle fiber can change its type. This happens commonly between Type IIa and Type IIx (in both directions) with training. But can Type I fibers become Type II fibers or vice versa? Evidence in this area is weak, but it does appear that Type I fibers can convert to Type IIa fibers with the traditional tools used by sprinters to increase speed. If Type IIa fibers can convert to Type I, the conversion is very slow and there’s no evidence to prove it happens as there are no before-and-after biopsies done on marathoners.

Bulking Up

Unfortunately, you cannot grow new muscle fibers. If a muscle has ten fibers today, it’ll have ten fibers tomorrow and ten the day after that (it could actually have 9 or 8 as people lose muscle fibers through aging, but it’ll never have 11 or 12). But you can grow new myofibrils within the fibers, so all isn’t lost. Basically, you’re increasing the size of the fibers, not increasing the number of fibers.

So if you workout in a way that grows your Type IIa and IIx fibers, you can change the overall ratio of Type I-to-Type II muscle mass, though the ratio of actual fibers remains the same. The slow-twitch fibers don’t tend to bulk and heavy strength training converts Type IIx fibers to Type IIa (relax, we’ll come back to this). Effectively, by strength training, you are pushing your muscles towards that Type IIa fiber that has some component of power and some component of endurance.


Here’s a very interesting phenomenon discussed in the paper: overshoot.

As expected, the relative amount of the fast myosin IIx isoform in their vastus lateralis muscle was reduced from an average of 9 percent to about 2 percent in the resistance-training period. We then expected that the relative amount of the IIx isoform would simply return to the pretraining level of 9 percent during the period of inactivity. Much to our surprise, the relative amount of myosin IIx reached an average value of 18 percent three months into the detraining.

Physiologically, there’s no real explanation for why this happens. The question is, do we care why? Understanding that a tapering period before a major competition will bring about a significant increase in our fastest muscle fibers is enough. Sprinters use this to their advantage all the time before their most important meets…it’s called “peaking”.

Then again, the deloading period in the study was 3 months, which is unreasonable for an athlete. How much of “peaking” is a result of overshoot vs. simple recovery? Unfortunately there’s no way to know just yet. Again though, the phenomenon is interesting from a science-geek perspective, but knowing that tapering works is enough until we’re able to better take advantage of the science behind it.

Training For Sport

So which is better? That’s an impossible question to answer. For the average person, the standard mix of about 50% slow twitch and 50% fast twitch is just fine. But for the athlete interested in improving performance in their chosen sport, it is a very important question. Other than running/rowing/cycling for long periods of time, I can’t think of any sport that wouldn’t benefit from the raw power of the fastest-twitch muscles.

Obviously, the short sprints (200m and below) in track and field rely heavily on the fastest fibers we have, but every field event is also reliant on a very quick, explosive movement to move either an external object (shot put, discus, javelin) or to move the body itself (pole vault, high/long/triple jump). Sports like basketball, baseball, football, and soccer are characterized by bursts of speed or jumping intermixed with periods of light to no effort.

Given that, unless you’re a distance athlete, you probably have enough slow-twitch muscle fibers and would benefit from devoting some time to increasing the size of your fast-twitch fibers. But frankly, who really cares about muscle fiber composition? The important thing is performance.

Training For Power

But since we’re here, let’s look at the kinds of activities that increase power, focusing prominently on growing our fast-twitch muscles, the ones that increase our speed and ability to jump. Basically the goal is to increase the size of our engine (get stronger) while also keeping or increasing the firing rate of that engine.

  • Heavy Strength Training - High weight (85% of 1-rep max or more), low reps in exercises like the deadlift and back squat for serious posterior chain targeting, as well as bench press and overhead press (don’t neglect the upper body here). The goal is to try to move the weight as quickly as possible (with good form, of course), whether the weight actually moves quickly or not. This focuses on increasing the overall size of the Type IIa muscles (bigger engines). Some Type IIx will convert to Type IIa as will some Type I.
  • Plyometrics - Depth jumps, bounding, explosive skips…all work to give a very quick stretch-reflex that is difficult to mimic in any other way. It teaches the muscles to fire hard and fast and I can’t think of any other movement that is as quick as things like depth jumps and bounds. Ease into plyometrics slowly as they are intense and very hard on muscles and connective tissues. Here are a few precautions and some ideas for plyometric workouts.
  • Sprinting - If you want to jump higher, you’d incorporate plenty of jumping in your training protocol. So if you want to be able to outsprint the next guy, whether you’re a wide receiver that needs to blow past a cornerback or a 1-mile runner wanting a stronger kick at the end of the final lap, sprinting more will speed you up. Uphill & downhill sprints are great, but the incline should not increase or decrease your speed by more than 10%.
  • Olympic Lifts - While not as quick as the short time your foot is planted when running full speed, the Olympic lifts are great for teaching you to activate explosively. Hang power cleans are the easiest to perform without instruction, but a few sessions with an Olympic lifting coach might be good.

What other ways have you used to successfully increase your speed?

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