Anatomy And Physiology

Anatomy Physiology

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Ever heard of people commenting on how an athlete has God-given talent? Although reluctant, I have to admit that in a way, they are technically correct. Genetics has determined for us our potential for height, intelligence, body frame, etc. Having so much influence in our lives, genetics play a role in muscle as well.

There are 2 kinds of muscle fibers in a muscle: Slow-twitch (Type I) and fast-twitch (Type II). As the name suggests, slow-twitch muscles are slow in nature but have great endurance. Fast-twitch, on the other hand, performs in short bursts of speed. In an average person, there is a equal distribution of fast-twitch and slow-twitch muscle fibers. Some unlucky ones have distribution of muscle fibers which works against them. (Picture a sprinter with marathon runners attributes)

According to a study, these muscle fibers can be manipulated to change their characteristics into their counterparts. Which means, although genetics play a role in pre-determining our muscle fiber-makeup, we can change the characteristics of our muscle fibers to suit our needs. Before discussing in detail about how to use this to our advantage and build muscle quicker, you have to understand more about the different muscle fiber types.

Slow-twitch muscle fibers

Marathon runners and joggers have a large amount of slow-twitch muscle fibers as compared to fast-twitch ones. This is because of the fact that these endurance athletes perform a low power output activity lasting for a long period of time.

Slow-twitch muscle fibers use oxygen to fuel the activity done. The muscle contractions of slow-twitch fibers are slow but have a huge capacity for endurance. Excessive walking, jogging or any activity which lasts a long period of time has an effect to change the characteristics of fast-twitch muscle fibers to slow-twitch ones.

Fast-twitch muscle fibers

Sprinters have a large amount of fast-twitch muscle fibers and little slow-twitch ones. Fast-twitch fibers are anaerobic in nature and uses glycogen directly as fuel. The contractions of type II muscle fibers are faster than type I by as much as 10x. Fast-twitch muscle fibers can be sub-categorized into type IIA and type IIB. High intensity movements like sprinting and plyometrics can change the characteristics of slow-twitch fibers to fast-twitch fibers.

Type IIA muscle fibers have properties of type I and type IIB muscle fibers. It has the ability to use both aerobic and anaerobic metabolism equally. It is called intermediate fast-twitch muscle fibers for being able to perform bursts of speed in a extended period of time compared to type IIB muscle fibers.

Type IIB muscle fibers are the opposite of type I muscle fibers. They are only capable of using anaerobic metabolism and have extreme speed in contractions. Though they have great potential in speed, they cannot last for long periods of time and fatigue easily.

How to manipulate the muscle fibers for muscle growth

Take a look at a sprinter and marathon runner; which has more muscle mass and less body-fat? Most people would think the answer is the marathon runner, well, they're wrong. Sprinters have absolutely more muscle mass and less body-fat than long-distance runners.

By examining the difference in their muscle fiber make-up, we can see that the sprinter has more type IIA and type IIB muscle fibers and the marathon runner has more type I muscle fibers. This is proof that fast-twitch muscle fibers have more potential for growth than slow-twitch muscle fibers.

Some of you might think that this information would lead to me advising you to use mainly low repetitions and higher sets in your workouts. Although this would be good advice to most people in their current situations, there are other ways to maximize the benefits.

Rep ranges and weight

Repetition range and weight is the most direct method to manipulate the change in muscle fiber type. In the past, bodybuilders in the golden age used every rep range; as high as 20; as low as 3. They targeted every muscle fiber type and reap the benefits of both.

Since fast-twitch muscle fibers have more potential for growth, it is advisable that you base your training program on rep ranges of less than 10. Although some might argue that when lifting a heavy weight, you appear to lift it slow. It is the intention to move the weight as fast as possible which will send signals to your body to change. Just try it, move a heavy weight slowly in the concentric phase. Done? You can't do it without attempting to move it as fast you can did you?


Not many people use tempo anymore in their lifts. A pity, since it is one of the best ways to recruit both type I and type II muscle fibers.

Tempo refers to the time taken to perform the exercise, usually split into the lowering (eccentric) and lifting (concentric) movements. A tempo may read like this in a program: 4010. This means lowering the weight for 4 seconds, pause 0 seconds, then lifts the weight up in 1 second and finally pauses 0 seconds to lower the weight.

The lowering part (eccentric) of the movement will recruit the type I muscle fibers and the lifting part (concentric) of the movement will recruit the type II muscle fibers. In addition, the time under tension of your muscles and the strengthening of the tendons will increase your muscle growth as well. (Note: there are some movements which do not have tempo but still as effective e.g.: power cleans)


Everybody is different; the amount of muscle fiber distribution is very different and unique to others. As such, you must train differently and specific to your needs. A person who is very fast-twitch dominant, will gain best when he does low reps and high sets with heavier weights. A person who is very slow-twitch dominant will gain best when he does high reps and low sets with sub-maximal weights.

How to know if you're slow or fast-twitch dominant? Jump. Execute a vertical jump and if you squat just down before jumping, you are fast-twitch dominant. If you dip and squat down more than parallel before jumping, you're slow-twitch dominant.

Of course, if you train for a sport, you'll have to asses yourself the situation. Does your sport require you to do short bursts of speed followed by activity with little to no rest? Or does your sport require you to run all the time? By understanding the requirements for your sport, you can cater to them by designing training programs which modifies your muscle fiber make-up.

Muscle fiber distribution is unique to every individual, asses yourself and your needs so you can design a training program specific to you. Remember to read up on designing a training program before you actually do it!


F Ingjer
Effects of endurance training on muscle fibre ATP-ase activity, capillary supply and mitochondrial content in man.

More about this author: Victor Roffel

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