Training For Rep Ranges and Muscle Fibers Types

Rep ranges are a funny thing. Lots of people talk..

Training For Rep Ranges and Muscle Fibers Types

Rep ranges are a funny thing.

Lots of people talk about doing this many reps for this many sets but really have no idea why they are doing that particular number or the effect it will have on their bodies.

Generally, lower repetitions like 3-8 are most productive for creating absolute strength.

Repetitions in the medium range like 10-20 produce anaerobic strength endurance.

The higher rep ranges between 20-40 produce aerobic strength endurance.

The reason for such varied ranges even within a category of rep range is because people will take varying amounts of time to complete the rep.

We know that it is not the actual number of reps but the time that the muscle is under tension that really causes the adaptive response.

So if you do 4 reps that take 6 seconds each and I do 6 reps that take 4 seconds each. 

We have both worked in the 3-8 range and caused that adaptation response that would be caused by putting a muscle under 24 seconds of tension.

Now on to fiber types.

There are actually 3 major types of muscle fibers in the body that we are concerned with.

Type 1, type 2a and type 2b.

Some people have natural tendencies to do well at certain activities more so than others and this is because of the ratio of one muscle fiber type to another, among other things.

Type 1 

Type 1 muscle fibers (or slow twitch muscle fibers,sometime called red) are your endurance muscle fibers because they are very resistant to fatigue and injuries.

The sad part is that their ability to produce power is very low also.

Type 2a

Type 2a muscle fibers (fast twitch muscle fibers – intermediate – sometimes called white) are much larger and stronger than type 1 muscle fibers.

These fibers have a high capacity for glycolytic activity and can produce high force output for longer periods of time.

Type 2b

Type 2b muscle fibers are the extreme end of the power scale.

These are the survival fibers.

The whole purpose for 2b muscle fibers is to allow enough power and strength to survive emergency situations.

Sixteen percent of an inactive persons body is 2b fibers.

Now even though these fibers are powerful, they have no resistance to fatigue or injury.

Fact of the matter is that if and when you need to use these guys. Most untrained people will damage that tissue beyond repair.

Now I’ve said all that to say this.

Research has show that it is the white muscle fibers. The type 2a and 2b that give the greatest returns in size and strength when trained.

These white muscle fibers are for high force output. That means lots of calories burned along with a tissue that requires a lot of calories to stay alive.

So what about the red muscle fibers, the type 1?

Well your body becomes efficient at what it is needed for.

So if all you do is only cardio and other high rep activities. 

Your body is going to favour the development of type 1 muscle fibers and in the end rob you of your ability to achieve your maximum strength, size and power.

What if strength, size and power are not your goals?

Well not having at least some of those attributes will result in a suppressed metabolic rate. That means you will not burn as many calories in a day.

This means that if you are on a weight loss program. You need to work the type 2a muscle fibers in the 10-12 rep range in order to boost your metabolism and stay strong and healthy.

Now the body can only handle increasing levels of stimulus in a particular area for a given amount of time.

This is why in good programs you will find several totally different workouts. Each stressing a different muscle fiber type for a given period of time.

An individualized program has the majority of the workout time spent in the phase that best effects the goal that is trying to be achieved.

Muscle Fibers & Nerves

You see, the type of fiber expressed as far as type I vs Type II is controlled by the nervous system. Nerves that control and connect to a group of motor units run from the brain to the motor unit and are hardwired in the brain.

Fast twitch motor units are controlled by fast twitch nerves. Slow twitch motor units are controlled by slow twitch nerves.

In the laboratory you can take a nerve from a motor unit that supplies a slow twitch muscle fiber and replace it with one that supplies a fast twitch fiber. The slow twitch fiber will behave just like a fast twitch fiber!

The reverse is also true.

You can take a slow twitch nerve and connect it to a fast twitch motor unit and the fast twitch will behave like slow twitch.

Unforunately, it’s impossible to change a slow twitch nerve into a fast twitch nerve and vice versa.

However, you can make the Myosin Heavy chain expressed in a fast twitch fiber either more or less fast twitch or a slow twitch fiber more or less slow twitch but more on that later.

Muscle Recruitment

So, aside from muscle fiber involvement why is the nervous system so important?

The majority of the time, the real limit to your performance is the number of motor units your nervous system can recruit in the short amount of time you have in a sporting movement.

The amount of horsepower (size of the muscle cells) under control of those motor units, not the type of muscle fiber (slow twitch or fast) that comprises those motor units.

Remember, the nervous system determines the degree of motor unit involvement.

It should also be noted that with regards to peak “force” production, the only real difference amongst the fibers is their size.

Type II’s are bigger yet an equal volume of type I’s can produce roughly the same peak force.

Therefore, for displays of maximum force (strength), fiber type is of little consequence.

Now this next part is important. Recall that the average person can only recruit around 50% of their muscle motor units anyway.

It normally takes anywhere from .4 – .6 seconds for the nervous system to call on all the available muscle motor units to contract.

This is the same length of time it takes to demonstrate max strength or apply maximum force. However, it takes only .2 seconds to perform something like a vertical jump.

Main Determining Factor

So the main determining factor is how many of ALL the available muscle motor units one can get turned on in .2 seconds and not necessarily how much fast twitch fiber one has.

Therefore, if one lacks fast twitch fiber but also has a very efficient nervous system capable of recruiting nearly all the FT fiber they do have. 

They will tend to have superior performance in comparison to someone with a less efficient nervous system and lots of fast twitch fiber.

Normally the body inhibits the contraction of all available muscle fibers as a protective mechanism. An example of this phenomenon in reverse can be seen when looking at weight-lifters.

Often people can considerably increase their strength without any increase in muscle size.

Why is this so?

It’s simply because the body becomes more efficient at muscle recruitment and firing synchronisation.

By engaging in the correct training programs over a period of time with an emphasis on speed, explosiveness, and power you can better teach your body and nervous system to recruit it’s FT fibers.

Slow To Fast Conversions

Another reason that fiber typing may be largely disregarded is that studies in both man and animal have consistently shown a fast to slow conversion in response to training of any kind. That is, IIB fibers convert into the slower contracting and less powerful IIA.

In fact, guess what group of people has the highest percentage of the fastest contracting IIB fibers? COUCH POTATOES!

With just about any type of training, the higher threshold fibers (IIB) change into slower contracting IIA fibers.

When training is ceased these fibers once again revert back to IIB. The likely reason why this occurs is because of metabolic efficiency.

The body will deal with stress in the most efficient manner possible. A slow transformation is metabolically more efficient while it still allows the body to adapt to stimuli.

In fact the amount of either type II type only becomes even remotely important when a resistance is less than 30% of max.

As noted, the main difference between IIA and IIB is their speed of contraction. They contract at about the same force but the IIB/IIx contract quicker and are better at creating force at high speeds.

Therefore, with typical training schemes the relationship between IIA and IIB is also inconsequential.


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Daniel Messer, RNutr, CPT

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