Understanding Muscle Fibers and Function: The Foundation of Being a Personal Trainer


To become a personal trainer who is equipped with the knowledge necessary to be considered a fitness “expert”, you need to know the basics of human movement. That is, how knowing the different muscle fibers and how they work.

There are three types of muscle: cardiac (the heart), smooth (found in the organs and blood vessels); and skeletal muscles. Each type of muscle has respective and quite different functions. Within the realm of practical application rather than theory, you must concern yourself primarily with the cardiac and the skeletal muscles to which our professional programming is targeted.

Each type of muscle needs to be trained accordingly in order to achieve the best goal-specific results. This article describes information necessary to understand the skeletal muscles and the types of appropriate exercises (and movement, for that matter) performed to produce specific skeletal muscle contractions.

Muscle Fiber Classifications

Muscle fibers are classified by the characteristics of the motor unit serving that specific muscle fiber. The fibers are characterized on the basis of speed, force, and the length of time a muscle fiber can contract without losing tension (endurance). There are two classifications of muscle fibers with respect to speed: Type I and Type II.

With respect to force, fibers fall into three classifications: low, moderate, and fast. The two classifications in which muscle fibers fall into with regard to endurance are fatigue-resistant and fatigable.1 Although we usually classify muscle fibers, in general, by means of the two classifications of force production (fast and slow), you should be familiar with the fact that there are three major classifications, characterized by the primary energy sources facilitating muscle contraction:

1.) Type I: slow-twitch, oxidative (energy provided through the use of oxygen);

2.) Type IIa: moderate or intermediate fibers that are fast-twitch, high-oxidative, and glycolytic (energy provided through oxygen and the breakdown of sugars); and

3.) Type IIb: fast-twitch, glycolytic (energy provided through the breakdown of sugars).


Low Intensity-Long Duration Muscle Fibers

Type I muscle fibers are those that are most activated during slow, long-term movements like marathon running and long-distance cycling, or even simply walking. The connecting nerves are thinner than those that are prominent in the fast-twitch fibers, allowing for continuous muscular contractions of low-force output. These fibers appear red in color when viewed under a microscope. Although you probably will not be analyzing your client’s muscle tissue under a microscope, it is important that you know the relationship of various muscle fibers to their respective levels of blood flow and subsequent color representations.

Type I muscle fibers have a greater blood flow than Type II fibers and are thus highly oxidative (perform better with the use of oxygen), making them more aerobic in nature. This increased ability to utilize oxygen (as compared to Type II fibers) makes them more resistant to fatigue; coupled with their ability to contract slowly, this makes them more endurance-oriented.

Hence, these muscle fibers are optimally trained via low resistance and long duration (e.g. light weights and high repetitions of 16-40 reps per set of a resistance exercise).

Moderate Intensity-Moderate Duration Fibers

Intermediate, Type IIa fibers are those which contain both, slow and fast-twitch muscle fiber characteristics, and are the main source of power output in activities like the 800-meter run and lengthy competitive rowing. Varying sizes of nerves innervate these muscle fibers.

Intermediate fibers are also referred to as red, fast-twitch fibers because they receive a relatively high degree of blood supply (less than slow-twitch and more than fast-twitch fibers). They are somewhat resistant to fatigue (not as much as slow twitch), and can be trained to contract with high degrees of force for moderately long periods. These fibers are also referred to as highly oxidative and glycolytic, because they are capable of being fueled by the breakdown of sugars (through glycolysis) and through the use of oxygen (oxidation).

Intermediate muscle fibers are best trained through the use of moderate resistance and over periods moderate in duration (e.g. moderate weight for moderate repetitions 11-15 reps per set of a resistance exercise).

High-Intensity Low-Duration Muscle Fibers

Type IIb fibers are those which are recruited for explosive or fast movements like Olympic weight lifting and sprinting. They contract much faster than slow-twitch fibers due in part, to thicker nerves innervating these fibers. When viewed under a microscope, fast-twitch fibers appear white in color, indicating the limited blood supply serving them.

These fibers utilize energy supplied via the anaerobic energy system (without the use of oxygen), and through the process referred to as glycolysis. Although these fibers fatigue rather quickly, white, fast-twitch muscle fibers are those that hypertrophy the most and are most capable of displaying greater degrees of strength, power, and speed. Training these fibers is accomplished by incorporating heavy resistance and low repetitions (e.g. 4-6 reps per set of an exercise).

A Muscle Fiber is a Muscle Fiber is a Muscle Fiber

Muscle fiber type is a product of heredity. That is, fiber composition (Type 1 vs Type II) is genetically predetermined. And, although the proportion of fiber types remains relatively unchanged, it appears that Type IIa muscle fibers can become more endurance-oriented through endurance training, and display characteristics similar to those of the Type I fibers.2

Fiber Recruitment

When a movement is begun, it is the slow-twitch muscle fibers that first become activated. But the more the movement is resisted (e.g. via heavy weights) assistance is provided for by the fast-twitch fibers. As a training effect of lifting heavy weights, an earlier recruitment of fast-twitch fibers is experienced.

If slow movements are continued with little resistance, it is the slow-twitch fibers that are called upon to facilitate the muscular contractions. In sports activities that require a single, all-out effort, the synchronous recruitment of muscle fibers may be more important than the element of fatigue resistance.3 Research has shown that the synchronous recruitment of fast-twitch fibers exhibits the greatest high peak torque.4

Recovery Time

The greater the blood supply to a muscle, the greater the ability of that muscle to recover (provided good eating habits are practiced). That is, Type I muscle fibers recover much quicker than Type II fibers because of this greater blood flow — “delivery system” — serving them. Blood flow aids in the recovery of a muscle first by removing waste products resulting from muscular contractions, and second, by delivering the necessary nutrients (e.g. protein and carbohydrates) to the worked muscles for recovery.

The recoverability of a muscle group helps you determine how frequently a particular muscle group can be trained. Obviously, the shorter the recovery time required of a muscle, the more frequently the muscle group can be trained. And fiber type is only one of several factors affecting the length of time needed for complete recovery.

Practical Applications

By understanding the various properties and functions of muscle tissue, certified personal trainers are capable of designing optimal workout programs accordingly. That is, you will know and be able the following:

1.) Selecting the correct muscle-fiber training protocol for the goal.

2.) Whether the exercise prescription will require gross or fine coordinated movements.

3.) If the muscles worked will have a high or low degree of blood flow serving them.

4.) Whether the recovery time must be short or long.

5.) Whether the resistance must be high or low.

You need to address exercise prescriptions on an individual basis. That is, the reasons for exercising must be met through individually designed programs that will yield specific results. This is known as training specificity.


1. Hatfield, F. Complete Guide to Fitness Training, International Sports Sciences Association, 1991.
2. Kaijer, L. and Jansson, E. Effect of extreme endurance training on muscle fiber characteristics. Proc. Int. Union Physiol. Sci. 11, 723 (abstract), 1977.
3. Saltin, B., Henriksson, J., Nygaard, E., and Andersen, P. Fiber types and metabolic potentials of skeletal muscles in sedentary man and endurance runners. Ann. N.Y. Aca. Sci. 301: 3-29, 1977.
4. Thorstensen, A., Larsson, L., Tesch, P., and Karlsson, J. Muscle strength and fiber composition in athletes and sedentary men. Med. Sci. Sport. 9: 26-30, 1977.


NFPT Publisher Michele Rogers, MA, NFPT-CPT, manages and coordinates educational blogs and social media content for NFPT. She’s been a personal trainer for 20 years with a lifetime passion for all things health and fitness. Her mission is to raise kinesthetic awareness and nurture a mind-body connection. After battling chronic lower back pain and becoming a parent, Michele aims her training approach to emphasize corrective exercise and pain resolution. She holds a master’s degree in applied health psychology from Northern Arizona University. Follow Michele on Instagram.
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