Energy Systems and Muscle Fiber Types

To understand the rationale behind various forms of exercise, it is first necessary to understand how the human body generates energy and produces muscular force. If a trainee has no comprehension of the “why” behind the workouts, then he or she is just blindly trusting me while going through the movements. I assure you, when someone chooses to commit to an exercise program under my guidance, I don’t demand their blind trust. If life has taught me anything thus far, it is to thoroughly question absolutely everything. Naturally, I’m an advocate of transparency from any form of authority. Formulating questions and seeking answers is the foundation of knowledge. More knowledge will make you more interested in — and better at performing — any activity you choose to pursue. Certainly, more interest in exercise is something many people need in our increasingly sedentary culture.

We’ll begin at the cellular level…


Chemical energy is used within cells by a type of molecule called Adenosine Triphosphate. ATP fuels muscle contractions, metabolic processes and is utilized in assembly of amino acids used for protein synthesis.

Muscles don’t provide much ATP storage, so when it becomes depleted — which happens within seconds of activity — ATP is re-synthesized through a series of chemical reactions. This occurs through the body’s three main energy systems.

Energy Systems

ATP-CP (Phosphocreatine) System — durations of effort approximately up to 10 seconds. Phosphocreatine converts adenosine monophosphate into ATP. Maximum 1-Rep weightlifting and 100-meter sprints utilize this system primarily.

Anaerobic (Glycolytic) System — durations of effort approximately up to 2 minutes. Glucose is transformed into ATP and pyruvate, creating lactic acid. This system takes over for the ATP-PC if higher intensity activity carries on past ten seconds, as is the case for moderate-to-high rep resistance training or a 400-meter sprint.

Aerobic (Oxidative) System — longer durations of effort. Mainly glucose and fatty acids are broken down and synthesized into ATP. Running, cycling, and most long term activities rely primarily on the oxidative system.

The three energy systems work in tandem to supply the muscles with fuel, which is why it is essential to develop all three to the best of your capability. Sports such as soccer and basketball require the development of each system due to the anaerobic nature of sprinting and jumping combined with aerobic activity of lower intensity movement between bursts of action.

Development of both endurance and power brings me to my next topic, muscle fibers…

Skeletal Muscle

Of the three types of human muscle — skeletal, smooth, and cardiac — skeletal muscle is responsible for all voluntary movements, under the control of the somatic nervous system. Skeletal muscle fibers are composed of cells of myofibers, which are bundled together into myofibrils made up of strands of protein called actin and myosin (it is the myosin which binds to ATP to provide energy). These are groups into segments of sarcomere, which shorten during contraction to create movement. When a muscle contracts, it pulls on connective tissue coverings and tendons, which tighten and pass the tension on to the bones they are attached to. This is the mechanism that moves the body.

Every person has a blend of skeletal muscle fiber types in each individual muscle that are primarily derived from genetics, training methods, and daily activity.

Muscle Fiber Types

Type I are slow twitch muscle fibers that are primarily utilized during aerobic activities (ex. distance running). These fibers contract slowly and do not generate much power, but they are capable of working for very long periods of time without fatigue. They contain large amounts of myoglobin, mitochondria and blood capillaries, which makes them have a reddish coloring.

Type IIa are fast twitch muscle fibers that are primarily utilized during long term anaerobic activities (ex. rope climbing). They are faster to contract than Type I fibers and therefore generate much more force, but they are also faster to fatigue. Like Type I fibers, these also appear red due to their large number of capillaries. Type IIa also are capable of creating ATP by oxidative processes.

Type IIb are also fast twitch muscle fibers. They are primarily utilized during short term anaerobic activities (ex. Olympic weightlifting). They are the fastest contracting, most powerful muscle fibers, but fatigue very quickly due to the nature of the all-out effort they are recruited to provide. Unlike Type I or Type IIa fibers, Type IIb have low content of myoglobin, mitochondria and few capillaries, which results in whiter coloring.

Skeletal muscle is a mixture of these fiber types, but the mixture varies throughout the body. For example, muscles of the neck usually have a high proportion of slow oxidative fibers because they support the weight of the head throughout the day, whereas muscles of the legs have a more even blend because they are used for standing and walking, but also for more athletic movements like running and jumping.

It is important to note the recruitment order of muscle fibers — when a task only requires a weak contraction, such as when someone picks up a bag of Burger King hamburgers and french fries to stuff his/her face with, only slow twitch fibers are activated. If the task requires more explosive movements, such as when the same person decides to burst into an all-out sprint to reach a pile of Burger King hamburgers and french fries to stuff into his/her face, progressively larger fast twitch fibers are recruited to power the body to a higher production of force.

Let’s break the all of the above information into a real life (sort of) example…

Walk from your house to Walmart. Type I fibers will be doing the work and the oxidative system will be fueling activity.

Buy a fifty pound bag of dog-food and carry it from the store out into the parking lot. Type IIa fibers and the glycolytic energy system will be seeing the majority of action.

In the parking lot, put the bag down and rest briefly. Then, pick the bag of dog-food up and throw it as high as you can into the air. Type IIb fibers will be heavily recruited, using ATP-PC energy.

Understanding how each type of energy system and muscle fiber functions will help you target them in training to help best achieve your goals. I’ll address these topics in much greater detail in future articles.


Verkhoshansky, Yuri. “Supertraining, 6th edition.” 2009.

Zatsiorsky, Vladimir. “Intensity of Strength Training Fact and Theory: Russian and Eastern European Approach.” Biomechanics Lab The Pennsylvania State University, University Park, Pennsylvania and Central Institute of Physical Culture – Moscow, Russia