What types of muscle fibers do humans have and what are they responsible for?

Muscle memory is formed under the influence of regular physical activity. The better it is developed, the longer a person retains a good shape and the faster he recovers from illnesses.

Muscle fibers inside the human body are usually classified according to 4 main features. Fibers are divided into red and white, slow and fast, low-threshold and high-threshold, as well as oxidative, intermediate and glycolytic. Below we consider the specifics of each classification separately.

1. Red and white

  • The shade of the muscle fiber is noticeable on its cross section. It is determined by the concentration in the sarcoplasm of myoglobin, a specific muscle pigment. At a low concentration, this substance gives the tissues a light pink hue, and at a high concentration, a rich reddish-brown.
  • The content of myoglobin, in turn, depends on the number of mitochondria. The main purpose of this substance is to transfer oxygen from the surface of the muscle fiber to the mitochondria. As a result of training, the concentration of myoglobin in tissues can increase – accordingly, the fibers tend to darken or lighten over time.

White fibers use glycogen as energy, while red fibers use triglycerides. White fibers predominate in the body of sprinters and jumpers, while red fibers predominate in the body of long-distance runners.

The color of almost every human muscle is heterogeneous, since it contains fibers of both shades, as well as tissues with mild pigmentation.

2. Slow and fast

Fast fibers are those with high ATP activity. ATP in this case stands for adenosine triphosphatase. This enzyme determines the rate of muscle contraction. One ATP molecule contains enough energy to turn the myosin bridges once when it is released. This rotation is known as a stroke and results in muscle contraction.

Bridges move in the following sequence:

  1. Uncoupled from the actin filament.
  2. Return to original position.
  3. They bind to a new site of actin.
  4. They make a row.
  5. Then another ATP molecule starts the next stroke.

All muscles make single strokes at the same speed. The faster adenosine triphosphatase is broken down inside the muscle fiber, the more strokes the bridges manage to complete per unit of time and the faster the muscle contracts.

ATPase activity is an inherited parameter. It cannot be influenced by training.

3. Oxidative, intermediate and glycolytic

The oxidative potential of a muscle is determined by the number of mitochondria within the fiber. Mitochondria is an organelle of a cell (that is, one of those parts of it that specializes in performing certain functions). Mitochondria are responsible for the following chain of processes:

  1. The breakdown of fat or glucose into water and carbon dioxide.
  2. Resynthesis of ATP.
  3. Resynthesis of creatine phosphate with the help of ATP.
  4. Resynthesis of myofibrillar ATP molecules using creatine phosphate.
  5. Muscle contraction.

Glucose can be broken down in muscle tissue, including outside the mitochondria. Then pyruvate is obtained from it, and ATP resynthesis is also started. A person will feel fatigue in the muscle, as lactic acid is formed there.

According to this criterion, muscle fibers are classified into three types:

  1. Oxidative – Mitochondrial mass is initially large. No matter how long a person trains and no matter how he increases the load, this mass will not increase significantly.
  2. Intermediate – Compared to the previous type of fibers, the mitochondrial mass is significantly less. As the muscle works, lactic acid accumulates in it. Fatigue builds up slowly compared to glycolytic fibers.
  3. Glycolytic — The content of mitochondria is minimal. The fibers are characterized by anaerobic glycolysis, leading to an accelerated accumulation of lactic acid and a rapid onset of fatigue.

During aerobic glycolysis, glucose inside the mitochondria is broken down to ATP, water and carbon dioxide with the participation of oxygen. In the anaerobic version of the process, oxygen is not involved, and glucose is broken down to ATP and lactic acid.

If a person does not train regularly, his oxidative muscle fibers are of the slow type, and the other two types are of the fast type. If desired, the endurance of the body can be increased.

To do this, you need to choose workouts, during which part of the glycolytic and intermediate fibers will turn into oxidative ones – which means you will be less tired.

4. Low-threshold and high-threshold

We are talking about the threshold of excitability of motor units. Each such unit consists of:

  • motor neuron;
  • axon;
  • collection of muscle fibers.

The number of motor units in the body does not change over time. Muscle contraction occurs under the influence of nerve impulses of an electrical nature emanating from the brain.

This happens when the magnitude of the impulse is at or above the threshold value for a given motor unit. When the impulse is below the threshold, the motor unit remains passive.

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