By Andrew Golin,


Movement is one of the most distinctive characteristics of human life. Body motion is facilitated by specialized cells called muscle fibers and is controlled by our nervous system (1).

Three broad classes of muscle fibers exist: skeletal, cardiac and smooth. Skeletal muscle fibers are multi-nucleated long fibers that have a cross striated outer appearance under a microscope (1).  Skeletal muscles are voluntarily commanded, that is, humans are able to consciously control skeletal fibers. This class of muscle fibers are attached to our bones by tendons, and commonly known examples of skeletal muscle fibers are the biceps and triceps. Cardiac muscle fibers are also cross-striated, but our autonomic nervous system, which controls our involuntary nervous system, regulates the movement of these fibers (1). Skeletal and cardiac muscles are striated due to the overlapping and crossing of myofilaments. Myofilaments are chains of actin and myosin proteins, which are the predominant tissue in all muscles.   Unlike both skeletal and cardiac muscle fibers, smooth fibers are not striated (1). Smooth muscle fibers’ activity is regulated by our autonomic nervous system. The body’s organs possess the largest portions of smooth muscle fibers (1).

Muscle fibers can be further distinguished into two subcategories: slow and fast twitch fibers. Slow twitch fibers, also known as type I fibers, contain more mitochondrion and myoglobin molecules than fast twitch fibers (2). Mitochondria are organelles where biochemical processes that generate fuel for the cell through cellular respiration occur. Myoglobin proteins are functionally similar to hemoglobin molecules. Myoglobin proteins carry and store oxygen molecules in muscle cells. Since mitochondria generate fuel from cellular respiration, oxygen molecules, being the primary reactant, type I fibers are energetically supplied by aerobic processes (2).

Fast twitch fibers, or type II fibers, have fewer mitochondrion and myoglobin proteins than slow twitch fibers (2). Despite the decreased amount of mitochondria, type II fibers are still able to synthesize large amounts of energy through anaerobic processes. Anaerobic processes do not require oxygen and utilize glucose, a simple unit of sugar, as their primary energy supply. Though type I and type II fibers have different sources of energy, the consequences of both energy-synthesizing processes are similar: to produce adenosine tri-phosphate (ATP), a molecule that contains large amounts of energy (2).

The body utilizes ATP as the primary source of energy currency. But before ATP can be converted into energy, the brain must send electrical impulses to muscles in order to initiate contractions (1). These electrical impulses travel rapidly across coatings or “sheaths” on the outside of the nerve cells for increased speed. Multiple sclerosis is an autoimmune disease where the body attacks its own myelin sheaths. If the damage is minor, nerve impulses will continue to travel with minimal interruptions. If the damage is sufficient to cause the myelin to be replaced with scar tissue, nerve impulses may not travel through at all (4). Multiple Sclerosis Society of Canada’s list of symptoms include extreme fatigue, lack of coordination, weakness, tingling, impaired sensation, vision problems, bladder problems, cognitive impairment and mood changes (4).   Fast twitch fibers generate quicker contractions compared to slow twitch fibers, due to the greater thickness of their myelin sheaths (3). The thicker the myelin sheath, the faster nerve impulses may travel from the brain to the muscle (3). Therefore, slow twitch fibers have thinner sheaths than fast twitch fibers (3). Once the signal reaches the muscle fibers, ATP is used in exchange for contractions.

Type I fibers do not fatigue as quickly as type II fibers (2). This is due to the different chemical by-products that arise from either aerobic or anaerobic processes. The by-products of type I fibers are carbon dioxide and water, which do not cause muscles to fatigue quickly. The primary by-product of fast twitch anaerobic processes is lactic acid. Lactic acid increases the acidity of muscles and causes the fibers to fatigue quickly. Staying hydrated during physical activities, breathing deeply during rest periods, and eating foods rich in magnesium will help decrease lactic acid build up during training sessions.

Aerobic exercises are physical activities performed at low to moderate intensity. Common examples are jogging, swimming, cycling and walking. Anaerobic exercises are physical activities performed at high to maximum intensity. Sprinting, Olympic weight lifting and jumping are anaerobic activities. Aerobic exercises can be performed for long periods of time, where anaerobic activities are often performed in high intensity intervals. While both forms of exercise utilize all muscle fiber types, aerobic activities utilize more slow twitch fibers where anaerobic exercises employ more fast twitch muscle fibers.

By understanding which fibers are used in either aerobic or anaerobic activities, people may configure their training sessions to focus on specific muscle fibers. Individuals involved in anaerobic activities should configure their workouts towards fast twitch development. Fast twitch development requires low volume, high intensity, and low frequency repetition schemes (3). Individuals involved in aerobic activities should alter their training sessions towards high volume, low intensity, and high frequency repetition ranges (4).

By applying the knowledge above, training sessions can be configured to increase optimal specificity, and therefore optimal efficacy towards one’s goals.




1. Gardner, Ernest Dean, Donald James Gray, and Ronan O’Rahilly. “Muscular System.”Anatomy: A Regional Study of Human Structure. Philadelphia: Saunders, 1975. 28-30. Print.

2. Dreams. Muscle Fiber Types_Energy Production and Cardiovascular(n.d.): n. pag. Web. 3 Oct. 2015.

3. “Muscle-Specific Hypertrophy: Chest, Triceps and Shoulders By Menno Henselmans.”

SimplyShreddedcom. N.p., n.d. Web. 03 Oct. 2015.

4. “Multiple Sclerosis Society of Canada.” What Is MS? MS Society of Canada. N.p., n.d. Web. 03 Oct. 2015.