RED FIBERS - PHYSIOLOGY

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Red Fibers

Red fibers VS white fibers

The differentiation between white and red fibers is the result of the association between the color of the muscle and the respective speed contraction; the "red muscles" are mainly slow but resistant, while the "light muscles" are more "effective" (greater strength and speed of contraction) but less "efficient" from an energy point of view (less autonomy during the effort).
Later more accurate classifications have been proposed that consider specific parameters such as the MEASUREMENT of the speed of contraction and the METABOLIC prevalence of muscle fiber cells.
Today, all known parameters are UNIFIED in the specific and detailed description of:

slow fibers (red - type I - βr - Slow Oxidative [SO])
intermediate fibers (light - type IIA - αr - Fast Oxidative Glycolytic [FOG])
fast fibers (white - type IIB - αw - Fast Glycolytic [FG]).

In adult skeletal muscles there is a third type of fibers, called IIx, with intermediate characteristics between IIa and IIb.
Obviously each muscle contains a certain percentage of ALL the fibers and its composition is never 100% of one type or the other; furthermore, remember that:

Between them, the various skeletal muscles have a composition of different fibers.
Muscle predisposition is ALSO genetically determined.
Muscle fibers can be partially specialized with training.

Characteristics of red fibers

Red fibers are functional units of skeletal muscle; they, like white fibers and those defined as "intermediate", are responsible for the transformation of chemical energy (adenosine tri phosphate - ATP) into mechanical or kinetic energy.
The red fibers have a color very similar to that of blood by virtue of some biochemical and structural characteristics; particularly:

Dense capillary branches.
High concentration of myoglobin, a storage protein (similar to the hemoglobin contained in red blood cells) which acts as a RESERVE of muscle oxygen.
High concentration of mitochondria.

Compared to IIA and IIB, red fibers have a rather reduced speed of contraction; in ALL humans (and all mammals), the highest muscle concentration of red fibers is:

In the muscles responsible for maintaining posture (e.g. spine supporters)
In the muscles responsible for the "execution of" slow and repeated "movements (such as some muscles of the thigh and leg useful for walking, eg psoas-iliac and soleus).

Furthermore, the red fibers contain a large amount of mitochondria that work effectively in the oxidative (aerobic) energy production, supported by the great blood circulation of the dense capillary bed.
NB. Often in body-building the muscles table is varied - by increasing: 1. the repetitions 2. the series and 3. the volume of training - with the aim of partially favoring the increase in muscle mass, ALSO managing the proliferation of mitochondria and of the capillaries. In reality, while constituting a valid alternative in training cycling, it should be specified that, through this variant, the increase in mitochondria and capillaries is quite limited and does NOT significantly affect the increase in volume and overall muscle mass.

Ultimately, the red fibers are suitable for mild, slow and repeated efforts; they brilliantly resist fatigue even if they do not contain large volumes of glycogen (higher in fibers IIa and IIB).

To summarize the concepts expressed above, we refer to the critical reading of the following tables

Slow or red fibers or II

Fast or white fibers or IIb

Intermediate fibers or IIa

Atp production

Oxidative phosphorylation
(aerobic)

Glycolysis

(anaerobic lactacid)

Phosphocreatine
(anaerobic alactacid)

Oxidative phosphorylation

(aerobic)

Glycolysis

(anaerobic lactacid)

Oxidative enzymes Abundant Scarce

Intermediate characteristics

Glycolytic enzymes Scarce Abundant

Color (myoglobin)

Intense red

Clear

Mitochondria

Numerous

Scarce

Energy substrates Mainly lipids Mainly carbohydrates

Fiber diameter

Small with many

capillaries

Great with a few

capillaries

Features

motor neuron

Small axon and body

cellphone, slow speed

of conduction e

discharge rate

Large axon and body

mobile phone, high speed of

conduction and frequency of

download

Speed of

fatigue

Slow

Quick

Characteristic

They maintain activity

toned for long

periods

They keep an activity

explosive and powerful for

a few moments

Percentage of slow and fast fibers present in human skeletal muscles (*)

MUSCLE % Red fibers % Intermediate fibers % White fibers

Short adductor

Great adductor

Gluteus maximus

Ileo psoas

Pettineo

Psoas

Gracile

Semimembranous

Tensor of the fascia lata

Broad Intermediate Quadric. Femor.

Vasto Medial Quadric. Femor.

Soleus

Great dorsal

Brachial biceps

Deltoid

Rhomboid

Trapezium

Long adductor

Twins

Medium / small buttock

External / internal shutter

Piriform

Biceps femoris

Sartorio

Semitendinosus

Popliteal

Wide lateral

Quadric rectus femoris. Femor.

Tibialis anterior

Rectus abdomen

Brachioradialis

Grand Pectoral

Brachial triceps

Supraspinatus

Training: optimization of red fibers and specialization of intermediate fibers

Personally, I have always been of the idea that every athlete should make "predisposition" his strong point. Although apparently paradoxical, at times, favoring the development of a "natural" tendency can determine an absolutely incomparable increase in performance. Obviously, it is not possible to oppose the will of the student or the client ... if a potential marathon runner wants to become a weightlifter ... little remains to be done!

However, a method frequently underestimated by most personal trainers - and which (unexpectedly) is quite successful - is to PROMOTE athletic and motor development while respecting the physiological tendency of the athlete.
Practical example:

Objective: development of the general resistant force
Subject: singlet (middle distance runner) characterized by a genetic prevalence of red fibers
Method: CIRCUIT TRAINIG (see the article resistant force)

According to this principle, the choice of the number of repetitions and the "intensity of exercise" could be oriented more on the aerobic component (sets of 7 "for each station) rather than aerobic / mixed anaerobic (sets of 3" for each station). In this way, the naturally present red fibers have the possibility of expressing their development to the maximum, both in structural terms (capillaries, mitochondria) and in biochemical and enzymatic terms (myoglobin, enzymes of the oxidative chain, etc.); at the same time, the intermediate fibers (always present even if in variable quantities) evolve on the basis of the preponderant stimulus (in this case AEROBIC).
The limit of this technique is obvious; using ONLY a training of this kind there is the possibility of significantly LIMITING the development of the athlete and insufficiently stimulating all the white-anaerobic muscle fibers ... but on the other hand, persisting in training a genetically poor "lactic acid power" it could mean:

Obtain poor results on anaerobiosis
Limit the development of the genetically strongest component.

The speech changes significantly in the eventuality in which the percentage of red and white fibers depends almost exclusively on the specialization of the intermediate fibers (IIA); if the quantity of the latter prevails over the others, the athlete will boast a greater ability to adapt to the stimulus, consequently, the training can be managed with greater freedom and also with more room for improvement.
Unfortunately, in addition to muscle biopsy, there are no PRECISE techniques that can evaluate the prevalence of one or the other fiber; on the other hand, aptitude tests are able to provide us with "good" information of a "metabolic" type, but in this case, understanding whether the red fibers are genetically determined or whether they are already specialized IIA fibers is very difficult.

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