White fibers

Red Fibers vs White Fibers

In physiology the distinction between white and red fibers derives from the correlation between the color of the muscle cell and its own speed of contraction.

The "white muscles" (or better clear) are mainly glycolytic (energy metabolism of anaerobic glycolysis), therefore faster but less resistant of the red ones; vice versa, the red muscles are more "efficient" (less strength and greater economy in effort) but, from an energy point of view, less "effective" in contraction.
Thanks to all the physiological parameters collected it is possible to make a more specific description

  • both fast fibers (white glycolytic - type IIB - αw - Fast Glycolytic [FG])
  • both of the slow ones (oxidative red - type I - βr - Slow Oxidative [SO]).

In fact, between these two categories there is a third intermediate, given by

  • light fibers (type IIA - αr - Fast Oxidative Glycolytic [FOG])

which has the ability to specialize as a "glycolytic" or as an "oxidative". Practically, depending on the training stimulus, intermediate IIA fibers can evolve to glycolytic white or oxidative red (but also somewhere in between).

Remember that ALL muscles contain a percentage of white and red fibers, therefore, their function is NEVER totally of strength or endurance; moreover, by comparing the various districts and their respective tissues, it is also possible to observe a certain HETEROGENITY "in the specific composition of the fibers. NB. This characteristic is determined by the function of the muscle in question (for example, the ileo-psoas does not perform a work comparable to that of the pectoral) and is influenced by subjective predisposition as well as physical exercise (see specialization of intermediate IIA fibers).

Characteristics of white fibers

White fibers are functional units of skeletal muscle that transform the chemical energy of the bonds present in the adenosine triphosphate (ATP) into kinetic / mechanical energy.
White fibers include both glycolytic (IIB) and intermediate (IIA) fibers, but the latter (which need a broader and more detailed description) will be dealt with carefully in a dedicated article.
The white fibers are paler than the red ones, because:

  1. They DO NOT contain significant amounts of mitochondria and myoglobin
  2. They have a minor density and capillary branching.

It is therefore established that, compared to the red ones, the white fibers have a greater speed of contraction and mainly exploit the ANAEROBIC GLYCOLYSIS (from the reserve glycogen). In reality, the white fibers are able to effectively catabolize also the Creatin Phosphate ( CP - metabolism almost absent in the red fibers), taking full advantage of the ANAEROBIC ALACTACID pathway, albeit for a few seconds at the beginning of the effort. It follows that, in addition to the aforementioned structural differences, the white fibers contain a pool SPECIFIC enzymatic, therefore totally different from that of red fibers; translating physiology into athletic terms:

  • the white fibers are more suitable for fast and intense efforts [anaerobic type, both lactacid (anaerobic glycolysis) and alactacid (creatine phosphate hydrolysis)] rather than long and moderate.

The muscles (or rather, the motor units) that contain more white fibers than the red ones are those that mainly perform rapid contractions, to the detriment of resistance but to the advantage of the tension produced (pure strength); This category includes the large muscles of the torso (a large part of the pectoral major) of the back (great dorsal), of the arms (triceps) and of the legs (such as the vastus medialis, the rectus femoris and the soleus).

It should be clear to the reader that the distribution of white fibers, rather than red or intermediate ones, It is NOT well defined; in addition to subjectivity, training and the type of effort to which the muscle is delegated, there are significant heterogeneities also within the same district (various types of motor units). The high, intermediate or low bundles of a muscle do not NECESSARILY contain the same amount of white fibers, quite the contrary!

  • The pectorals and quadriceps are a clear example of how the concentration of white or red fibers can change from one bundle to another in the same district.

Training: optimization of white fibers

The training of white fibers must focus on the development of contractile strength and speed. The specific type of training varies according to the athletic discipline but, by virtue of the correlation between strength and speed, the most used general preparation technique is that of "overloads".
The training of the white fibers is anaerobic, lactacid or alactacid type. If you want to stimulate the alactacid component (CP) it is essential to perform very short series of "weight lifting" (for strength) or repeated sprints (for SPECIFIC speed). recovery must be abundant or at least sufficient, and the number of series weighted according to the level of preparation and objectives.
On the contrary, if the intent is to stimulate the lactacid metabolism of the white fibers (short-lasting resistant force or resistance to speed), the number of series and the execution time of the same increase in proportion to the resistance time required (30 seconds , 1 minute, 3 minutes etc.) and the recoveries must be calculated based on the number of programmed series.
A practical example of the development of maximal and explosive strength (alactacid metabolism - CP) in the white fibers is that of weightlifter or even the thrower (weight, disc or hammer) which develops through the execution of multi-joint exercises (deadlifts, pushes in the flat bench, etc.) repeated in various series of 2-3 repetitions and interspersed with complete recoveries ranging from 2 to 5 minutes.
Wanting to propose an example of training for the "short resistant strength" of the legs (and not for the long resistant strength which also involves a significant commitment of the aerobic metabolism - red fibers), the most classic exercise, as well as one of the most effective, is the squats (with or without a jump) in series of at least 15 repetitions.
Finally, remember that white fibers, although mainly anaerobic, are also involved in medium and long-term executions; they (with the intermediate ones IIA) are responsible for the production of lactic acid in the efforts ABOVE the anaerobic threshold, therefore, it is also possible hasten the intervention of white fibers during specific-aerobic workouts. This can be achieved by exploiting the intensity peaks through training in short repetitions and through the execution of rhythm variations (for example in middle distance running or other disciplines of the same duration ).


  • Movement Neurophysiology. Anatomy, biomechanics, kinesiology, clinic - M. Marchetti, P. Pillastrini - Piccin - pages 29-30.
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