muscle fibers - Differences between red and white fibers

Types of muscle fibers – what are they?

Science recognizes two types of muscle fibers: red (slow) and white (fast). The ratio of muscle fiber types is one of the parameters that differentiate a top sprinting athlete from a marathoner.

Knowing what types of muscle fibers are worked with the exercises is beneficial to improving performance and an athlete’s capacity. Thus it is helpful for the selection of muscle recovery methods.

Ignorance of the physiology and muscle genetics of the body causes a person to choose the wrong training strategy. To gain lean muscle mass and sculpt an athletic body, It is necessary to understand how the muscular system works and what types of muscle fibers predominate.

Differences between red and white muscle fibers

The human musculature comprises connective tissues, capillaries, sarcoplasm, and the two types of muscle fibers. The latter, in turn, are divided into White (fast) and red (slow) fibers. The speed of contraction at which they work is the main difference. The color differs due to the source from which they take energy.

The slow muscle fibers (red) are responsible for supporting homogeneous, static, and endless loads; this type uses fat as its primary energy source. The fast muscle fibers (white) have the function of making short but high-intensity movements; They use muscle glycogen and creatine for energy.

How do you know if a muscle fiber is slow or fast?

An example of distinguishing between the two types of muscle fibers is to make an analogy with birds. For example, the chicken breast and wings; are characterized by their white color and a minimal presence of fat. The thighs and legs have a characteristic darker shade of red muscle fibers.

Most of the time, the birds are on the ground, where the muscles of their legs withstand a constant static load. Slow muscle fibers mainly bear this load. (1) On the other hand, the wing muscles are used for short and intense movements. It is this type of movement that fast fibers do.

In the stabilizing muscles of the torso, the spinal area, the internal abdominal muscles, and the leg muscles, there is a more significant presence of slow muscle fibers. Fast muscle fibers prevail in the rest of the regions (3).

Slow / Red fibers

Slow muscle fibers are thin and weak. These are capable of withstanding loads for long periods. Its reddish color is due to the presence of oxygen molecules. Oxygen is essential for the oxidation of fats. Its correct transport in the body requires iron.

Aerobic exercises like running or walking act on slow muscle fibers. When doing this exercise, something fundamental is to keep the pulse in the fat-burning zone (60-70% FCM). In this way, oxygen is supplied to all muscle tissues.

The best exercises to target red muscle fibers are static exercises. The abdominal planks and fixed squats are the most classic exercises.

Fast fibers / White

To work fast fibers, high intensity movements are needed. To move, muscles need quick access to some source of glucose. Fat is useless for the work of this type of muscle fiber. Its transport and oxidation take at least a couple of hours. Ideally, the energy should come from a place that is accessible and very close to the muscles: the muscle glycogen stores.

White muscle fibers perform explosive movements; they work based on glycogen and phosphocreatine adenosine triphosphate (creatine phosphate or PCR) (2). The increase in muscle volume is mainly due to the growth of these types of muscle fibers.

The proportion of muscle fibers depends on the body’s somatotype.

The human musculature is made up of a network of different types of muscle fibers, and this depends on the body’s somatotype.

The good news – the body can adapt under the influence of regular workouts and slightly modify the ratio. Of course, it depends on the type of sport that is practiced. In the case of a marathoner, the percentage of slow muscle fibers can reach 80% of the total. On the contrary, in high-intensity athletes, fast fibers predominate.

How to exercise fast fibers?

Strength training is best for exercising fast muscle fibers. The higher the weight to be lifted and the lower the number of repetitions, the better the tight muscle fibers will be worked.

The primary energy source for this type of muscle fiber is glycogen. Therefore, it is essential to consume sufficient carbohydrates, especially before and after an exercise routine. It is common to see athletes recharging energy with fruit after training. It is not only proteins that are important in sports nutrition. Oatmeal with fruits is an example of an ideal breakfast before a workout.


  • There are two types of muscle fibers: white (fast) and red (slow) muscle fibers.
  • Each type of fiber uses different sources of energy to generate muscle movement.
  • The ratio of muscle fiber types can be modified with exercise.
  • Strength training acts on the white threads; before doing this type of exercise, consuming the appropriate dose of carbohydrate foods is recommended.
  • Slow muscle fibers are activated by performing long-duration, low-intensity aerobic exercises.
  • The ratio of carbohydrates, proteins, and fats must be balanced to provide energy to the two types of muscle fibers. 

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