Athlete or marathon runner?

Types of muscle fibers

One of the main parameters that distinguish the innate marathon runners from sprinters is the ratio of fast and slow types of muscle fibers. Among other things, it is this ratio that largely determines whether it is easy for a particular person to burn fat and gain muscle mass.

On the other hand, ignorance of the fundamentals of the physiology of the work of muscles leads to the selection of an erroneous training strategy. In order to successfully build muscle or have a relief and tight body with a minimum amount of effort, you only need to understand how the body works.

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Fast and slow muscle fibers

Human musculature consists of connective tissue, capillaries, sarcoplasm and, directly, muscle fibers. These fibers, in turn, are divided into fast and slow, depending on the speed of their involvement in the work. Also, the color and energy source they use are different.

Slow( red) fibers responsible for static or continuous monotonous loads use fat as the main energy source. The fast( white) fibers needed for short and high-intensity loads are the reserves of glycogen( carbohydrates) and creatine.

Differences in muscle fibers

A good example of the difference in physiology of types of muscle fibers is chicken meat. The breast and wings have a characteristic white color and minimal amount of fat, while the thigh and thighs are characterized by dark red color of meat and higher fat content.

Since most of the time the chicken spends standing, the musculature of its legs experiences a constant static load - the main work is done by slow fibers( 1).In contrast, the muscles of the wings are used for vigorous strokes - the load goes to the fast muscle fibers.

Slow / Red Fibers

Although the slow fibers themselves are thin and weak, they can support the load for a long time. Their red color is due to the presence of oxygen molecules necessary for the oxidation of fats( triglycerides), serving as the main source of energy for slow fibers.

That's why aerobic training and prolonged cardio are ideal for losing weight - they involve slow muscle fibers and literally melt fat stores. However, it is important to train in the fat-burning zone of the pulse for optimal nutrition of body tissues with oxygen.

Fast / White

fibers For high-intensity and short-term loads, human muscles require fast energy. Fat does not fit, as its transportation and oxidation takes at least a few minutes. Energy should be in an easily accessible form as close as possible to the muscle fibers.

For explosive efforts, fast muscle fibers are used that work primarily on glycogen( muscle carbohydrate stores), ATP and creatine phosphate( 2).Recall that the growth of muscles as a result of strength training is largely due to the increase in energy reserves.

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What fibers do you have more?

Human musculature consists of a plexus of muscle fibers of various types. In the stabilizing muscles of the body and spine, the internal muscles of the abdomen and in the muscles of the legs, slow-type fibers usually predominate, and in the other skeletal muscles - fast-type fibers( 3).

However, the body is able to adapt under the influence of regular physical exertion and to change this ratio. In runners at marathon distances, more than 80% of all muscle fibers are slow, in contrast to sprinters, in which fast fibers predominate( about 65-70%).

How to train fast fibers?

For strength muscle training, strength exercises are best for training fast muscle fibers. The higher the working weight and the lower the number of repetitions( and, correspondingly, the shorter the time spent under load), the more active the quick muscle fibers are involved.

Since the main source of energy for this type of fiber is glycogen stores, it is extremely important to maintain a sufficient amount of carbohydrates in the diet - that is why, for muscle growth, carbohydrates, and not just a sports protein, as many mistakenly believe, are needed above all.


Human muscle fibers are divided into fast and slow. Strength training involves fast fibers, requiring carbohydrates and glycogen. In contrast, long-term, low-intensity aerobic loads are needed to involve slow fibers and burn fat.

Scientific sources:

  1. Muscles - Fast and slow twitch, source
  2. Skeletal striated muscle, source
  3. Speed ​​and power training, source
  4. Fast Twitch, Slow Twitch. ...Which One Are You?source