By Nenad Rodic,
posted 04/05

Muscles can be considered the engine of exercise, as they make all the movement possible. Human body is equipped with three different types of muscle fibers: smooth (most internal organs are built from these fibers); cardiac (specific to the heart only); and skeletal (they connect and move various bones in the body). It is this third group that is the focus of our interest and discussion.


Muscles receive messages form the central nervous system (CNS) via the nerves stretching from the brain to the muscle fibers. These messages/impulses are of electrical nature. The impulse travels through a neuron at a speed of approximately 120 m/s. Each muscle consists of a number of muscle fibers (see picture). Each muscle fiber is a single muscle cell as thin as human hair and as long as several centimeters. Groups of fibers are surrounded by a connective tissue/membrane. Each fiber contains a myofibril, a contractile element containing actin and myosin. During a contraction, myosin filaments connect and pull the actin filaments inward, causing the fiber to contract. Only a limited number of muscle fibers can contract simultaneously depending on the message received form the CNS. Muscle fibers are further organized into motor units. Each motor unit receives impulses from a motor neuron (nerve cell) and all fibers contract simultaneously (all-or-none law). How many muscle fibers (motor units) will contract, depends greatly on the load. If you are trying to perform a maximum lift almost all fibers will contract*.

*Motor units cannot contract halfway or half-strong. Depending on the force required to complete the movement, the CNS will send impulses to more motor units. It is impossible to engage all motor units at once. This is a biological mechanism that protects us form from over-exertion and injuries. At any point during muscle activity some motor units are resting. When a motor unit gets fatigued, the rested ones take over the work.


There are two types of muscle fibers:

1) Slow-twitch (ST) AKA slow oxidative (SO) or red or Type I fibers
2) Fast-twitch (FT) AKA white or Type II fibers

The key difference is that the FT fibers contract at a rate of 30-50 contractions per second, while the ST fibers contract at about 10-15 times per second. FT fibers also shorten at six fiber lengths per second while ST can shorten at only two fiber lengths per second. ST fibers possess a much greater endurance since they contaiin more of myoglobin (substance that gives them red color and transports oxygen within a muscle cell) and contain more mitochondria (a cell organelle in which the aerobic metabolism occurs). The anaerobic capacity of ST’s is very limited though, due to a lack of enzymes that catalyze the release of energy when oxygen is not available. Even when they are called upon to deliver the energy anaerobically they cannot do so as rapidly as the FT fibers.


Most humans have an equal amount of FT and ST fibers. However, there are individuals who have more FT fibers and accordingly, they have a predisposition for power sports and sprinting. People with higher percentage of ST fibers on the other hand, have a greater affinity towards distance events.


One of the most commonly encountered misconceptions is that the ST fibers engage only when we perform slow movement, and the FT ones when action is performed quickly. The ST fibers are always the first to engage regardless of the speed. FT fibers will engage if more force is required to complete the action. A specific motor unit always contains one type of muscle fibers. An ST fiber motor unit usually has 10-180 fibers while an FT motor unit can contain up to 800 fibers. This is the main reason why FT motor units can provide more force when engaged compared to ST motor units.

CONCLUSION: It is a common belief that percentage of FT and ST muscle fibers cannot be changed with training, whereas contractibility of ST fibers and endurance of FT fibers can be increased. FT fibers are larger in size, having also a higher capacity for anaerobic work. ST fibers are smaller in size and have higher capacity for aerobic work. Light to moderate exercise will recruit ST fibers only, so if your goal is to work your entire muscle, you need to increase the load.
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By Nenad Rodic, founder of