The intermediate muscle fibers are polymers of muscle cells that, thanks to their characteristic adaptability to the effort, can be specialized acquiring more aerobic (oxidative) or anaerobic (anaerobic glycolysis and creatine kinase) metabolic characteristics.
Specializing intermediate muscle fibers means orienting the training stimulus based on the results to be obtained; taking into consideration training with overloads, specialization can evolve:
- in the oxidative direction increasing the duration and reducing the intensity
- in glycolytic-anaerobic direction increasing the intensity and reducing the duration.
Genetics of motor unit and performance variability of intermediate fibers
In the sports field it is common to hear from the trainers an "old saying": " With training, a sprinter CAN become a cross-country skier ... but it is not at all certain that a cross-country skier can become a sprinter! "
This concept is NOT absolute, but it is a statement certainly supported by many factors among which the most important is GENETICS. Each of us has a well-defined "muscle project" and responds effectively (to a stimulus rather than to the other) based on the prevalence of the different motor units: motor neuron + muscle fibers (see the dedicated article Motor Muscle Units - white fibers and red fibers ).
We already know the biochemical characteristics of DIFFERENT muscle fibers ... but what do motor neurons have to do with them? There are different types and in practice they differ in the transverse section of the axon which affects the SPEED of conduction of the impulse. Practically, the motor unit with red fibers is innervated by a narrow section (slow) motor neuron, while the corresponding one with white fibers has a large section (fast) motor neuron.
To see what has been written so far, the reader might understand that the genetic predisposition to the dominance of one motor unit over the others (fast with white fibers or slow with red fibers) constitutes the only variable that determines the success or failure of a sportsman in the various disciplines; in reality (and fortunately), this concept is only partially acceptable.
Leaving aside for a moment the importance of the SPECIFICITY of training, we analyze in more detail another variable able to determine the muscular predisposition and the potential improvement of a sportsman towards his favorite physical activity: the intermediate fibers. From the metabolic point of view, the intermediates are real "jokers" able to direct the energy production towards the aerobiosis or anaerobiosis; it follows that a high percentage of these fibers determines both a great athletic potential and an extreme athletic flexibility .
Ultimately: " it is not at all certain that a cross-country skier can become a sprinter! ", But if his red fibers are largely made up of specialized intermediate fibers, modifying the training, there is a good chance that he can get good results even in disciplines of strength and speed. Let me be clear, sometimes the muscular predisposition is quite evident even "observing" the morphology and the anthropometric phenotype of the subject; a cross-country skier of 60kg can hardly become a 100 elite metrista ... but this does not exclude that many endurance athletes can find satisfaction even in disciplines of intermediate duration (such as the middle distance runner).
Intermediate fibers - how to specialize the metabolism
The first classification (antediluvian!) Used to catalog the muscle fibers is the "chromatic" one: red fibers and white fibers; subsequently, given the discovery of the intermediate fibers, the numerical solution was proposed: type I (red), type IIA (white - intermediate) and type IIB (white). Further expanding the biochemical and structural knowledge of muscle cells, the fibers were further cataloged using other differentiation criteria:
- Contraction speed: Slow and Fast (slow [S] and fast [F])
- Energy metabolism: Oxidative and Glycolytic (oxidative [O] and glycolytic [G]
By intersecting these two characteristics it is possible to distinguish THREE types of cells:
- SO - slow oxidative red fibers
- FOG - intermediate glycolytic / oxidative white fibers
- FG - fast white glycolytic fibers
The peculiarity of the FOGs lies in the adaptation potential; by themselves they contain good amounts of glycolytic enzymes, glycogen, oxidative enzymes, mitochondria and capillaries. Furthermore, they are innervated by medium-low velocity conduction neurons (medium-small axon), produce an intermediate tension but have medium-high contraction velocity and resistance.
To specialize the intermediate fibers it is necessary to carry out a specific training that guides the metabolism towards the desired one. Through the right stimulus, the intermediate fibers can acquire:
- a greater anaerobic enzymatic pool, with greater reserves of glycogen and creatine phosphate (energy substrates characteristic of lactate and aldacid metabolisms)
- or a set of aerobic-oxidative catalysts associated with several mitochondria, myoglobin and vascularization capillaries.
In short, the intermediate fibers are modified along with the training and can act synergistically to those SO in endurance athletes, synergistically to those FG in sprinters or synergistically to both in mixed sports.
Example of specialization of intermediate fibers in a runner
Subject: runner 100 metrista
Objective: increase of pure strength
Tools: overloads
A midfielder who aims at maximizing running speed must necessarily increase the pure muscular strength of the lower limbs (nerve conduction, fiber recruitment, intramuscular and intermuscular coordination, hypertrophy). The preferred methodology involves performing heavy gymnastics (exercises with overloads) to be transformed later into the specific athletic gesture. In the gym, the centometrist will perform exercises such as the "squat" in more or less extensive series but NEVER above 12-15 repetitions; recovery MUST be total or subtotal. In this way, in addition to enhancing the effectiveness and efficiency of FG fibers, it is possible to specialize FOG fibers in anaerobic metabolism (lactacid with many repetitions and / or alactacid with few repetitions and ample recoveries). Recall that in the development of pure strength the intermediate fibers will participate significantly in approaching the FG fiber metabolism, but will NEVER equal them in "effectiveness" due to the difference in conductivity of the dedicated motor neuron (slower in the intermediate).
Metabolic conversion of the runner:
Subject: 100metric rider who engages in the prolonged middle distance
Objective: increased endurance and aerobic power
Tools: race
Our centometrist decides to try his hand at the middle distance run, specifically the 10, 000 meters. Although the world record is close to 26 minutes, for a "common mortal" this discipline exceeds 30 minutes in duration and, while boasting a certain LACTACID component, it also needs a good anaerobic threshold. The effort is predominantly aerobic but located above the anaerobic threshold; in order to convert the intermediate fibers to the oxidative metabolism, the runner will have to abandon the exercises of maximal strength and mass to leave space for exercises on specific runs. In particular, the future 10.000metricist will have to carry out repetitions of medium length (above the anaerobic threshold) to develop the oxidative mechanisms to the maximum without losing the ability to produce lactic acid and resist its accumulation. In this case it is better to omit the repeated briefs which, on the contrary, would have been better suited to the opposite conversion, or of a marathon runner at the middle distance.
