Fibras musculares, ¿cómo se las estimulan?

Muscle fibers, how are they stimulated?

To analyze the mechanism that induces muscle fibers and, consequently, hypertrophy, we need to understand certain principles.

A study from the 1980s on muscle fibers and hypertrophy, which remains conclusive to this day; it suggests that muscle hypertrophy induced by weight (or resistance) training may be regulated by different mechanisms depending on exercise volume and intensity.

It is also noted that this depends on the distribution of work across the muscle fibers: fast-twitch fibers, slow-twitch fibers, and different incidences in certain areas of the fibers (1).

Muscle fibers: Types

In general, it is widely accepted that there are two different types of muscle fibers: slow-twitch muscle fibers (Type I) and fast-twitch muscle fibers (Type II). From there, fast-twitch muscle fibers can be further subclassified into Type IIa and Type IIb.

Type I muscle fiber

  • Type I muscle fibers have the slowest contraction speed, the smallest cross-sectional area, the highest oxidative (aerobic) capacity, and the lowest glycolytic (anaerobic) capacity.
  • They contract slowly and are able to maintain a steady contraction rate for long periods without fatigue.
  • These type I fibers are predominantly used in endurance activities, such as long-distance running, swimming, and cycling.

Type II muscle fiber

  • Type IIb muscle fibers have the fastest contraction speed, the largest cross-sectional area, the lowest oxidative capacity, and the highest glycolytic capacity.
  • They are ideal for short, rapid bursts and are used in activities such as sprinting, powerlifting, and bodybuilding (in certain exercises).
  • Type IIa fibers are intermediate, and their properties fall between those of type I and type IIb fibers.

Muscle Fibers: Differences Between Type I and Type II

Type I muscle fibers differ from type IIb fibers for many reasons; ideally, you can think of them as opposites. Type I fibers are for long-duration activities, while type IIb fibers are for short, rapid bursts.

  • Type I fibers are highly oxidative and are less likely to hypertrophy as much; they are known as red fibers because of their abundant blood supply.
  • Type IIb fibers are highly glycolytic and tend to hypertrophy more than type I fibers; they have a poor blood supply, which gives them a pale appearance and is why they are called white fibers.

Muscle Fibers: Recruitment

How does your body recruit muscle fibers?; well, even the small muscle groups in your body have more than 100,000 muscle fibers.

A motor neuron is what stimulates your muscles to contract: it carries impulses (messages) from your brain and spinal cord to your muscles. A motor neuron controls between 2 and 2,000 muscle fibers.

The motor neuron and the fibers it stimulates are called a motor unit, and each motor unit mainly contains muscles of its own type. In addition, the motor unit fires at a frequency that favors the fibers it stimulates.

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In short,

  • A slow-twitch motor neuron will make the muscles contract slowly,
  • A fast-twitch motor unit will activate quickly.

The faster it fires, the more energy it produces; therefore, if the activity is light, type I muscle fibers will be stimulated primarily, but when it becomes too intense, type IIa muscle fibers will be recruited. Finally, for the highest-intensity movements, type IIb fibers will be recruited.

This is why type I fibers are called low-threshold fibers, while fast type IIb fibers are called high-threshold fibers. To take a look at these fibers, you can view them under a microscope.

They have a low threshold because they are the first muscle fibers to be recruited, and a high threshold because they are recruited only under the most intense circumstances… your body always activates its muscle fibers this way…

To better understand how things work, we have a video made by our professional trainer Rober Castellano at a seminar held in Cádiz

Muscle Fibers: how do they grow?

The most relevant question here is, what determines whether a muscle fiber increases in length or diameter?… let’s analyze things…

After resistance training, muscles as a whole increase in volume and, therefore, also in mass. In humans, this occurs largely because of an increase in the volume of individual muscle fibers, rather than an increase in the number of fibers.

Individual fibers can increase in volume by increasing their length or diameter.

  • Increases in length occur through the addition of new sarcomeres in series, which are probably added to the end of existing fibers.
  • Increases in diameter occur through the addition of myofibrils in parallel.

Changes in the shape and structure of the muscle adapt to these increases in size, but… what stimulates a muscle fiber to increase in diameter or length?…

Hypertrophy from mechanical tension

In addition to the mechanisms of muscle damage and metabolic stress, hypertrophy also results from mechanical tension, which must be generated by the muscle fibers themselves.

Below is a section of the sarcomere that will help you understand the elements related to mechanical tension.

When the mechanical tension experienced by a muscle fiber is produced more by passive elements (structural parts of the fiber, including the giant molecule called titin), the fiber appears to increase in volume mainly by increasing in length, with sarcomeres being added in series.

This effect may be stimulated by titin detecting the stretch imposed on it, since the fiber deforms longitudinally.

Conversely, when the mechanical tension experienced by the muscle fiber is produced more by active elements (actin-myosin cross-bridges), the fiber appears to increase in volume mainly by increasing in diameter, adding myofibrils in parallel.

This effect could be stimulated by the outward bulging of muscle fibers that occurs when actin-myosin cross-bridges form, deforming the muscle fiber in a transverse direction.

The following graph shows the relationship between the active and passive tension of a section of muscle, since agonist/antagonist muscles are the ones working at a given moment, both to contract and to lengthen.

The contribution of passive and active force to overall mechanical tension is determined by muscle length, the mode of contraction, and the rate of lengthening.

Muscle length

Resistance training using exercises that involve larger ranges of motion (ROM) increases the proportion of mechanical tension arising from passive elements because the structural elements stretch after the fibers reach a certain length.

Contrary to popular belief, full-ROM and partial-ROM concentric-only resistance training produce similar hypertrophy when the same amount of work is performed in both training programs (2).

However, the type of hypertrophy is slightly different after each type of strength training:

  • Full-ROM training primarily causes hypertrophy by increasing fascicle length.
  • Partial-ROM training predominantly causes increases in the cross-sectional area of the muscle fibers.

The mode of contraction

Strength training with lengthening contractions (eccentric contractions) increases the proportion of mechanical tension arising from passive elements because titin is activated once the fiber begins to lengthen and automatically begins contributing to force production.

Titin contains two elements in series with each other (Ig domains and a PEVK segment), separated by a small N2A segment.

  • When passively lengthened, the highly elastic Ig domains increase in length, providing only a small amount of resistance to stretching.
  • When actively lengthened, N2A binds to the thin myofilaments, limiting the amount of change in titin length that can be achieved by lengthening the Ig domains.

Consequently, the much stiffer PEVK segment must lengthen instead (and may also wrap around) the thin myofilament, providing substantial passive resistance to stretching, but only during active lengthening contractions (eccentric contractions).

Although it was once believed that eccentric-only strength training could produce greater muscle growth than concentric-only or conventional strength training, it is now widely accepted that when programs are matched for volume or work, the amount of hypertrophy that occurs after eccentric-only and concentric-only strength training is very similar.

Importantly, recent research shows that although eccentric-only and concentric-only strength training produce similar increases in muscle volume (3):

  • Eccentric-only training primarily increases fascicle length.
  • Concentric-only training primarily increases the muscle’s cross-sectional area.

The velocity of lengthening

Strength training with eccentric contractions may involve different lengthening velocities, and because the force–velocity relationship is much flatter during the eccentric phase than during the concentric phase, these lengthening velocities do not substantially influence the amount of force exerted.

However, the velocity of lengthening affects the proportion of force produced by the passive and active elements, respectively.

Faster velocities reduce the proportion of mechanical tension arising from passive elements because the detachment rate of actin–myosin cross-bridges is faster, which reduces the amount of active force produced.

On the other hand, the proportion of mechanical tension arising from the passive elements increases due to the viscoelastic properties of titin and the other structural elements of the muscle fiber that resist lengthening.

Consequently, according to the scientific conclusion (4):

  • Fast, eccentric-only strength training produces greater increases in fascicle length than slow, eccentric-only strength training.
  • Slow, eccentric-only strength training causes greater increases in the muscle's cross-sectional area.

Conclusions

Muscle fibers increase in volume by increasing either their length or their diameter:

  • Increases in length occur through the addition of sarcomeres in series.
  • Increases in diameter occur through the addition of myofibrils in parallel.

During strength training, when the mechanical tension experienced by a fiber is produced more by the passive elements, the fiber appears to increase in volume primarily by increasing in length.

In contrast, when the mechanical tension experienced by the fiber is produced mainly by the active elements, the fiber appears to increase in volume primarily by increasing in diameter.

The contribution of passive and active force to overall mechanical tension produces increases in fiber length or diameter, which is determined by muscle length, the mode of contraction, and the rate of lengthening.

References

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