The Working Muscles

As a reputable personal trainer, who is equipped with the knowledge necessary to be considered a fitness “expert”, you need to know the basis of human movement. That is, how muscles work.

It is likely that you know that there are three types of muscle: cardiac (the heart), smooth (found in the organs and blood vessels); and skeletal muscles. But what you might not know is that 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. It is on these muscles we usually target our training recommendations. 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 about the skeletal muscles, the basis of type of every exercise (and movement, for that matter) performed through the use of skeletal muscle contractions.


 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: fast-twitch and slow-twitch. 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.) slow-twitch, oxidative (energy provided through the use of oxygen);

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

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

As you can see, when discussing voluntary muscle action, we concern ourselves and our training recommendations with three major types of skeletal muscle fibers. And each fiber type is quite different in function than the others. Therefore, it is recommended that you know the properties of each fiber type in order to accurately address results-specific training recommendations.


Slow-twitch muscle fibers are those which are most activated during slow, long-term movements like marathon running and long-distance cycling. Connecting nerves are thinner than those which 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. And 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. Slow-twitch muscle fibers have a larger amount of blood flow through them than the fast-twitch fibers. This should signify to you that they 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 fast-twitch fibers) makes them more resistant to fatigue and together with the fact that these fibers contract slowly, make them more endurance oriented. Therefore, slow-twitch muscle fibers must be trained by way of low resistance and long duration (e.g. light weights and high repetitions of 15-40 reps per set of a resistance exercise).


Intermediate 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 often 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). These fibers are 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 10-15 reps per set of a resistance exercise).


Fast-twitch fibers are those which are highly active in 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 to you, 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, fast-twitch muscle fibers are those which enlarge the most and are most capable of displaying greater degrees of strength, power and speed. Training of these fibers is accomplished by use of heavy resistance and low repetitions (e.g. 3-8 reps per set of an exercise).


Muscle fiber type is a product of heredity. That is, fiber composition (fast vs slow-twitch) is predetermined at birth. And, although fiber types remain unchanged, it appears that fast-twitch glycolytic muscle fibers can become more endurance oriented through endurance training, and display characteristics similar to those of the slow-twitch fibers.2


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 which 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


You should know by now that 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, red or slow-twitch muscle fibers recover much quicker than fast-twitch 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.


By knowing the various properties and functions of muscle tissue, you should become more capable of designing workout programs accordingly. That is, you will know the following:

1.) The goal oriented type of muscle fibers that need to be affected.

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.

So how does this information apply to a personal trainer? Well, without knowing how the body works, how are you ever going to accurately instruct others to improve bodily functions they may need or want?

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.


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.