Physical Fitness: What Is It? The Theory

physical fitness

The main task of modern, scientifically-based physical fitness is morphological changes in the body tissues, allowing you to adapt to physical activity. The most obvious example: muscle growth in response to strength training. But there are also less conspicuous changes.

For a physical training program not to be empirical, but based on biological data, it is necessary:

  • To know what structures inside the human body exist and why (biochemistry, physiology)
  • To know which ones and how we can change (i.e., training methods)
  • We must have a means of diagnosing/measuring these structures in the human body (i.e., control)

Well, knowing the physical form of top athletes in a particular sport and the shape of the athlete we are interested in, we can draw up a training program and bring the second to the first. That is: we analyze the athlete, compare with the sample (samples), take the most lagging element, and draw up a training program to pull it to an acceptable level. Then, the second element, etc., until we get the condition.

What Is Physical Fitness

This is a direction of mass, sports, and health physical culture, which aims to improve the general condition of the human body, its training and ability to resist the negative effects of the environment by performing simple and complex exercises with music or in a certain measure, helps to correct body shape and weight and allows you to consolidate the results achieved.

The Theory of Physical Fitness

Myofibrils and Physical Fitness


A muscle consists of muscle fibers, the number of which is set genetically and is not amenable to training. The functional units of the fibers are myofibrils, which are contractible (under the action of ATP) muscle element. Strength training is aimed precisely at increasing the number of myofibrils in muscle fibers.

The maximum contraction rate of an individual myofibril depends on the activity of the ATPase enzyme, which is set genetically and does not lend itself to training. The force of contraction of myofibrils is the same for all people, but their number can be changed: the more contractile elements in the muscle fiber (myofibrils), the greater the strength it can manifest. That is, the force essentially depends on the cross-sectional area of ​​the muscle/muscle fiber. In this case, the specific force (force/per cross-sectional area) is the same for children and the elderly, for athletes and beginners.

Strength and speed are two sides of the same coin (power), related by Hill’s law.

Muscles can show maximum strength in the inferior mode (eccentric), slightly less in static (isometric), even less in overcoming (concentric). The faster the muscles contract in a concentric mode, the less force they can exert. That is, in the concentric mode, the maximum power is constant (muscles consume energy in the form of ATP molecules at a constant speed), but the force and speed of contraction can vary in an inversely proportional manner (N = F * v).

Different muscle fibers have different ATPase activity and, therefore, different contraction rates. They usually talk about slow (SMF) and fast muscle fibers (FMF), sometimes highlighting more intermediate (IMF) fibers, but in fact, it’s worth understanding that in the muscles there is a whole continuum of fibers of different contraction rates: from the fastest to the slowest. Therefore, you can optionally select a few more intermediate types.

what is physically fit

In the abstract muscle, the SMF and the FMF are approximately equal, but the ratio changes depending on the specialization. Typically, SMFs predominate in muscles requiring precise or continuous action. For example, we have more SMF in the muscle of the forearms, moving fingers, in the muscles responsible for posture, in the muscles of the legs. The greater quantity of BMW is in large and rarely used muscles, for example, the muscles of the arms and chest.

In addition to the specialization of muscles, the ratio of SMF/ FMF in a particular muscle also depends on genetics. A different ratio is suitable for different types of physical activity: for example, for good sprinters, the front of the thigh consists of IMF, and the back of FMF, for good jumpers, the front of the thigh consists mainly of FMF, etc.

The difference in the speed of contraction between the fastest and slowest fibers is 20-40%, according to one data, almost doubled according to others. True, real sports actions are rarely performed at a speed of more than 50% of the maximum, so not everything is so scary.

Muscle fibers are combined into groups innervated by one motor neuron and axon. Such groups are called motor units (MU). All muscle fibers in MU have the same contraction rate, so we can talk about slow MU (SMU) and fast MU (FMU). The composition of the SMU includes hundreds of muscle fibers, the composition of the FMU includes thousands. Therefore, the SMUs are more accurate, and if we need to connect an FMU (that is, show great effort), the accuracy drops.

Growth factors of myofibrils
physical fitness define

Modern science distinguishes the following growth factors of myofibrils in muscle fibers:

  1. The stock of amino acids in the cell
  2. An increased concentration of anabolic hormones in the blood (muscle)
  3. An increased concentration of “free” creatine in MF
  4. An increased concentration of hydrogen ions in MF
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The process of accelerating the construction of new myofibrils in muscle fibers (MF) is possible only with their simultaneous inclusion. Nevertheless, people have many questions on this topic. For some reason, these arguments are not entirely convincing for them. Many still continue to firmly believe that the main reason for starting the construction of new protein molecules is the microtrauma of myofibrils received during training. Microtrauma has no effect on the growth of muscle fiber. It is high time for us to abandon this error. Modern research regularly confirms this. For example, the website of the National Center for Biotechnological Information shows 2011 research data that showed that the presence of muscle damage during training did not affect growth. The studied groups with minimal damage (which was measured by detecting the level of creatine kinase, as well as subjective sensations – the presence of post-training pain) showed similar growth rates of strength and muscle mass, as well as a similar increase in the level of growth factor IGF-1Ea and mRNA. Readers who don’t support this thesis also refer to the work of Vadim Protasenko, “How Muscle Grow.” In it, the author, referring to numerous studies, concludes that the multinucleation of muscle fibers itself indicates that the volume of muscle fiber that can serve one cell nucleus is limited. And he also expresses the idea that it is the fission of satellite cells and the increase in the number of nuclei in the muscle fiber and not the acceleration of “protein synthesis by existing nuclei” that causes muscle hypertrophy. He also concludes that drugs containing testosterone should increase the number of nuclei in muscle fibers. #weightgain #gainweight #weightgaining #gainingweight #weightgainfood #weightgainmeals #gainweightfast #gainedweight #muscle #muscles #musclegain #musclegains #musclemeals #musclefoods #musclefood #musclebuilding #musclemass #gainmass

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Amino acids. The supply of amino acids in the cell is the building material for the creation of myofibrils. We note that amino acids must have time to accumulate in the cell. Therefore, for example, drinking BCAA before, during, or after training does not make much sense, because the level of these three amino acids will increase only in the blood. For the growth of myofibrils, it is more important how much protein you consumed the day before.

In addition, the creation of myofibrils is not a quick task (it is 90% built for 10-14 days, tendon ends are built 90 days), so it is equally important how you eat a week or two after training.

Finally, the construction of the protein requires not three, but all 20 amino acids. If even one is not enough, the synthesis will stop. Conclusion – drink protein regularly.

The concentration of anabolic hormones. Hormones are secreted as a result of psychophysical stress and provide an adaptive response of the body in adverse conditions.


What does it mean? Muscles do not grow from the “pumping” of iron, but from the regular achievement of muscle failure and attempts to go beyond it.

The first sets are done in order to simply tire the muscle, but the last one (especially the last repetitions in the last set) – when there is no more energy, and you need to work – it just leads to intense stress.

In response, the body responds by releasing anabolic hormones (testosterone, growth hormone) that enter the nuclei of working cells and start the synthesis of new proteins there, so that the next time this stress does not happen.

If hormones trigger protein synthesis informationally, then creatine provides this process energetically. The more free creatine released in MF, the more intense the plastic processes will be.

The concentration of hydrogen ions. Hydrogen ions (a byproduct of anaerobic glycolysis) increase the pore size of cell membranes, which facilitates the access of hormones to hereditary DNA information.

Therefore, although hormones are carried by the blood throughout the body, they give the greatest effect in working muscles. For example, if you squat on one leg for a long time, the second will also increase in volume, but not as much as the first.

Mitochondria and Physical Fitness

The second important organelle in muscle cells is mitochondria, the cell’s energy station.

Mitochondria are a very interesting thing. The fact is that oxygen is poison. Have you seen how iron rusts (oxidizes)? That’s about the same way oxygen acts on living tissue, so all organisms that use oxygen have protective mechanisms against its destructive effects. However, even they do not save 100%. And 2.4 billion years ago, when the oxygen revolution was just beginning, living organisms did not have such protection, so the increased oxygen concentration led to the mass extinction of living things.

Who survived, first learned to defend against oxygen, and then use it for their own benefit (for energy).

The first to learn how to use oxygen are proteobacteria – the ancestors of mitochondria. And it was in symbiosis with proteobacteria that allowed “breathing poison” that eukaryotes (i.e., all living organisms except bacteria and archaea) and multicellular organisms arose. Semi-autonomous mitochondria still have many features of their independent ancestors. For example, mitochondria have their own DNA.

Mitochondria “multiply” in constantly working muscles/tissues. That is, if some muscle is constantly working, mitochondria appear there, which allows the use of oxygen for more efficient energy production.


It sounds simple, but it is deceiving. For example, many people have legs that are much hardier than arms because they are constantly at work. The intensity of such work is small, so not all muscle fibers work, but conditionally, 15%.

It is the myofibrils of these 15% percent of MF that braid the mitochondria, making them tireless (literally, under ideal conditions, an oxygen muscle can work forever; myocardium and respiratory muscles are good examples). Such fibers are called oxidative (OMF).

However, if such a person tries to jog, then for a long time, he will not be enough. Why? Because running is more power work, involving, conditionally, 40% of MF. Of these, 15% with mitochondria (oxidizing) and 25% without (glycolytic, i.e., anaerobically-glycogen-based glycogen).

A byproduct of anaerobic glycolysis is lactic acid, which consists of a negatively charged lactate molecule and a positively charged hydrogen ion.

Lactate is a large molecule. Therefore, in the absence of appropriate enzymes, it cannot damage the cell. And a hydrogen ion is the smallest atom that easily penetrates tissues and causes chemical damage. It is the hydrogen ion that is responsible for burning muscles and a feeling of fatigue.

How to better physical fitness
what is physical fitness

What needs to be done to increase jogging stamina? Just run regularly. Then this 25 % of MFs will gradually become entangled with mitochondria and will also become indefatigable. But having received endurance in jogging, we will not become more resilient in the sprint.

Because in the sprint, 40% of MF is no longer working, but conditionally, 80%. If we often (so that the muscles work more,) and not for long (so that the muscles are not strongly acidified and the mitochondria are not killed by hydrogen ions) run sprints. Then we can turn almost all the muscle fibers of the legs into OMF.

Training arms. For mitochondria to grow in high-threshold MU of arms, you need to constantly make them work. Making them work is not difficult – by making a bench press, for example. But how to make them work constantly? From 1-2 two presses, well, even bodybuilders’ 5 x 8-12, mitochondria will not grow. Hundreds of repetitions per day are needed.

The way out of the situation suggests the Hill’s law – “force-speed”. We do not have to lift 80% of 1RM hundreds of times a day. We can take 30-40% RM but increase the speed. As a result, the exercise looks like this:

  • sharply press the barbell,
  • lower it,
  • relax,
  • rest (make it possible to decrease the concentration of hydrogen ions),
  • press again.

Bench press 10 times, rest for one or two minutes, and another 9 sets.

physical fitness components

So for a workout, you can make all MU of muscles of the arms “breath” 100 times, including high-threshold ones. The most important thing is not to become acidic. After training, you should feel slight fatigue.

Such training does no harm, so you can conduct it at least several times a day. The more often it is performed, the more muscles “breath,” the more mitochondria are formed in them.

Mitochondria grow quite quickly (like bacteria), but they also die quickly. So, you can increase the number of mitochondria in muscle fiber by 50% after a week of training. Another 50% – in a month and a half. When the mitochondria braid all myofibrils, a person reaches the peak of a sports form in endurance.

Want to be even more hardy? Grow strength (i.e., increase the number of myofibrils), then train your stamina (i.e., braid myofibrils with mitochondria).

In connection with the foregoing, in reasonable training programs, cycling of training for strength and endurance is usually used. This is either strength-endurance-strength-endurance or strength-strength-strength, and a month and a half or two before the competition, strength training stops (or rather, goes into maintenance mode) and all the time allocated for physical fitness training is endurance workouts.

Physical Fitness Components

what is a fitness

So, from the point of view of physical fitness in muscles, we only have myofibrils and mitochondria (plus reserves of energy substances). There is no strength, speed, stamina, and other physical qualities there.

What is strength?

This is the ability of muscles to overcome external resistance. By and large, it depends on two things:

  • the number of muscle fibers (set genetically)
  • the number of myofibrils in MF (amenable to training)

That’s all. True, if we talk about the muscle as a whole or the strength of the multi-joint movement, then the ability to recruit high-threshold MU (neuromuscular control), performance technique, and some other things still play a role here, but these are delights.

What is speed?

Speed ​​, as such, does not exist. This is a pedagogical generalization of various phenomena. So, the contraction rate of muscle fiber depends on the activity of the ATPase enzyme, which is genetically set on the one hand, and on the other, depends on temperature, degree of acidification, the concentration of adrenaline and norepinephrine in the blood. Also, the reduction rate of MF depends on external resistance.

The speed of multi-articular movement depends on technique and coordination.


There is still such a thing as the speed of repeated reductions (pace). The pace depends on the rate of muscle relaxation (the power of calcium pumps). Relaxation is an active process. The body spends energy on it. Therefore, the more energy we have (higher stamina), the higher the pace is.

What is power?

It is determined by the rate of energy consumption (ATP) by the muscles. There are two extreme types of power manifestation:

  • power (high power, low speed)
  • high-speed (low power, high speed).

Plus intermediate.

What is endurance?

This is the ability to perform an exercise without losing power. It depends on various mechanisms of energy supply in the muscles.

  • alactate power is determined by the reserves of ATP and CrPh in muscle fiber.
  • anaerobic glycolytic power depends on the mass and the ratio of GMF / OMF, as well as the buffer properties of GMF, OMF, and blood. For example, the more OMF are in a muscle, the better it resists acidification, the more alkaline the blood, the better the body resists acidification.
  • aerobic glycolytic power depends on the mass of mitochondria.

What is agility?

Agility is not a physical quality, but the result of proper technical training.

Additionally, see why progressive overload is the key to muscle growth and awesome physical fitness.

What is more, find out whether it is ok to eat cottage cheese while cutting.