Disclaimer: nerd alert, requires a lot of attention, my native language is Russian - so please accept my apologies in advance had I misused the English language or allowed some typos.
Our body is a very complex mechanism that features vast functionality and a variety of individual skills. Whereas the muscles, bones, and connective tissue play a crucial role in it, nothing will be possible without the brain and the nervous tissue.
Everything we do starts inside the brain in the form of chemical-electrical activity, which is transmitted to the body parts we are about to engage. It is worth noticing that the brain is in charge of many other processes such as digestion, respiration, metabolism, sleep, etc., but we will talk only about its involvement in muscle-related activities.
To communicate with the body, the brain uses a highly ramified network of the tissue we know as nerves. For example, if we want to walk, run, or jump - first, the brain needs to generate electrical signals and send them to the corresponding muscles. Only after that, we can expect any activity to happen. The brain also receives signals from the body parts to monitor their current state and control movement outcomes. This incoming information is called biofeedback. There are two primary means by which the brain sends the signals to run various processes within the body:
The chemical-electrical impulses, propagated via the nerves and produced by the neurons, the interconnected cells inside the brain, and the spinal cord.
The hormones, chemical compounds, transported via the bloodstream, and manufactured by the major glands, parts of the endocrine system. These chemical messengers are delivered into the targeted cells and affect their intrinsic activity in the way the brain deems necessary.
We have two main nervous systems: central and peripheral. The central nervous system (CNS) consists of the brain and the spinal cord. The latter has myriads of nerve breaches spread into the body, including the muscles, joints, and inner organs. These branches comprise the peripheral nervous system (PNS). A couple of more nervous systems exist, but they are subdivisions of the PNS. For the sake of simplicity, we will omit them.
The significance of brain involvement in everyday fitness is much overlooked. In pursuit of better well-being, we often focus on muscle training: muscle strength, muscle power, muscle flexibility, muscle coordination, and so on. You might be surprised to learn how little the muscles themselves are responsible for all of it. Let’s have a detailed look at the fitness aspects we talked about in my previous post. But first, let me address the burning question of how we gain the muscles.
To gain muscle mass, we need to lift weights. Sometimes it works, sometimes it does not. Especially after the age of 25, we see a significant decrease in our ability to put on badly wanted lean mass. What is the matter? The conventional theory is the following.
When we lift weights, we slightly overexert the muscle tissue, creating micro-tears inside. In other words, we cause damage to the muscle cells’ inner structure. Luckily, it is repairable. The damaged cells produce biochemical compounds leaking from them. These compounds interact with the other chemicals in the vicinity sending the biofeedback to the brain about the damage which triggers this chain of reactions:
The hypothalamus, a part of the brain, responds by producing hormones, which go to the pituitary gland, also located in the brain.
The pituitary gland produces other hormones that go to the testes. The latter secrete around 95% of testosterone.
When the testes receive the pituitary hormones, they increase testosterone production.
The extra testosterone goes into the damaged muscle cells via the bloodstream.
Every cell in the body, including the muscle cells, has a nucleus. The latter stores our DNA. The DNA is a very long molecule, shaped like a double-helix and comprised of millions of chemical elements, called nucleotides. These nucleotides form sequences of different lengths, known as genes. The DNA has 20 000 - 25 000 genes; most of them have protein-coding functionality. The genes tell the body how to build different proteins from amino-acids derived from food (the proteins we consume are disassembled into amino-acids and reassembled in a different sequence according to the genome code to form the body’s native proteins).
Upon arrival into the damaged muscle cell, testosterone goes into its nucleus.
Inside the nucleus, a molecule of testosterone binds with a specific gene that provides coding instructions on how to assemble a protein for this particular cell thereby initiating protein-production using amino-acids taken from the bloodstream.
After the assembly has finished, the new protein molecule is ready to be structurally incorporated into the cell to repair its damage.
Muscle growth occurs.
It is a long process with quite a few steps. Here is a condensed version of it:
lifting weight -> muscle cells damage -> extra testosterone production -> the gene activation -> new proteins assembly from available amino-acids -> damage repaired with the new proteins -> muscle growth happens
We can see that the key component of muscle growth is testosterone. Unfortunately, with age, the testes' ability to produce testosterone declines significantly. With not enough testosterone, there will be no gene activation, consequently no new protein assembly, therefore, no muscle growth. Even if the bloodstream swarms with amino-acids, the laying bricks of proteins, there will be no “workers” to lay them.
At a young age, the level of testosterone is much higher to facilitate tissue growth while the organism is still taking its final form. That is why in your teens, almost any sports activity produces a noticeable result for the body's musculature. In early adulthood testosterone level starts declining, making it harder to assemble new muscle cells and repair their internal damage caused by physical activity. There are other factors limiting muscle growth, but testosterone reduction is the most significant of all. In a way, it is a natural body's mechanism to prevent the tissue from constant development.
In no way, I want to discourage you from lifting weights. There is a potential for some muscle growth at any age, especially if one has no previous experience of weightlifting. Genetics is enigmatic and might still facilitate some gain. However, this gain will be a far cry from a professional bodybuilder's musculature. It is crucial to set our expectations right.
After the muscle formation has finalized, it is crucial to continue training to maintain and improve what we gained starting with strength.
In my last article, we have already defined what strength is - the ability to produce the force required to overcome an object’s weight. That force is produced by the muscles, more precisely by contraction of their inner structures.
The inner structures are protein filaments, called myofilaments.
The myofilaments are bundled into myofibrils.
The myofibrils are grouped to form a muscle cell or muscle fibre.
The muscle cells are arranged into a muscle fascicle.
The muscle fascicles are packed into a single muscle.
You can think of a muscle as tiny wires (myofilaments) within slightly bigger telephone wires (myofibril) inside big telephone wires (muscle cells) bundled up to make huge telephone wires (muscle fascicles) which wrapped around by electrical tape into a vast single cable - the muscle itself. (Pic. 1)
The myofibrils are divided into compartments, called sarcomeres, where each of them has myofilaments, protein structures: actin, myosin, and titin. The actin and myosin are partially overlapping with each other. (Pic. 2)
When the electrical impulses, generated by the CNS, reach the muscle, the actins and myosins in each sarcomere start pulling against each other using their cross-bridges, thereby producing muscle contraction. When the electrical activity ceases, the actin and myosin filaments stop interacting and the muscle completely relaxes. Without the participation of the CNS, no muscle activity is possible, no pulls are produced.
Stepping aside from the topic, I need to mention that during lifting weights we cause damage to the myofilaments (actin and myosin), which the body repairs resulting in their extra thickness or quantity - the muscle gain.
The fact that the muscles need commands from the CNS to produce any activity makes them the receiving side. Now let's have a look at the related organization of the sender, the CNS.
As we already know, the CNS is constituted from the brain and the spinal column; both are composed of nervous cells (neurons) and their interconnections. A neuron sends and receives information (electrical activity) to and from other neurons. It does so using an axon (only one) and myriad dendrites. The dendrites are used for inbound information, and the axon is for outbound information. Whereas a neuron might have hundreds of thousands of dendrites, it has only one axon (pic. 3).
Among these neurons, there is one type of great interest for us - the motor neurons. They are located in the motor cortex (a part of the brain), brainstem (another part of the brain), and the spinal cord. A motor neuron can have hundreds or even thousands of connections with the muscle cells. The number of muscle connections varies from neuron to neuron. Some might have only a few, and others might have quite a few.
As mentioned before, a neuron has only one axon. A motor-neuron axon connects to a muscle and can ramify on the other end connecting its neuron with many other muscle cells at once. Bifurcation of a single axon is interesting because it means that once the neuron starts generating outcoming electrical signals via its only axon, those signals will be delivered to all muscle cells associated with this motor neuron, thus simultaneously activating all muscle cells linked to this particular motor-neuron. The compound of a motor neuron and its muscle cells is called a motor unit. (Pic. 4)
Now, after we got some idea of how the muscles are structured and connected to the CNS, we can have a look into the strength concept.
When the motor neurons are firing, sending electrical currents down along their axons to their muscle cells; the latter are responding with contraction producing a muscle pull. The strength of the pull does not depend on the power of the electrical stimulus. If the stimulus is above a certain threshold ( -70 mV known as resting potential), the muscle cells fire generating maximum pull. This principle is the all-or-none law. However, there are factors the strength of the pull will depend upon:
On the side of the CNS:
The number of motor neurons in the CNS. The higher the number, the more neurons can be potentially involved to generate a single contraction. Nevertheless, this factor is genetically predetermined and varies only slightly amongst individuals.
The number of motor neurons active at the time of muscle exertion. The more neurons are active, the more muscle cells can be recruited.
Quantity of connections with the muscle cells each acting motor neuron has. The all-or-none law states that once the motor neuron fires, it excites all associated muscle cells to their max. Therefore, the more muscle cells connected to a single-acting motor neuron, the stronger contraction is produced.
The firing frequency of the motor neurons. If the firing frequency is high, then the muscle cells produce individual twitches with less delay in between. As a result, those twitches superimpose generating a more forceful and constant contraction; a phenomenon called tetanic contraction.
Unison of the motor neurons activity. If the motor neurons fire in unison, their associated muscles cells contract all together at the same time producing a more potent general pull.
On the side of the muscle:
The number of muscle cells inside the muscle. A higher number gives us a better possibility of engaging more muscle cells in a single contraction.
The size of a muscle cell. If the inner structure of a single muscle cell has more myofilaments (actin and myosin), then its contraction will be more powerful.
If all of those seven factors are excellent, then the muscles deliver top-notch strength performance. Keep in mind, whereas we can improve some of those factors by training, we cannot improve them all. The amount of motor neurons is given to us at birth and cannot be increased, furthermore, past the age of 60, the CNS starts losing them diminishing our strength potential. Also, as I already mentioned before, our testosterone level and its production slow down with time. It presents a challenge for muscle gain. However, we absolutely can reduce age-related regression by staying physically active and taking up new activities. With appropriate and regular training, we certainly can:
Expand the number of connections with the muscle cells each motor neuron has.
Make more motor neurons fire during a single muscle contraction.
Intensify the frequency with which motor neurons fire.
Improve the synchronicity of the motor neurons’ stimuli production.
Slightly build up the inner structure of the muscle cells.
Even within diminishing physical potential owing to time-passing, we can significantly build up our strength at any age by reorganizing the work of the CNS to make the current resources function more effectively and efficiently.
Stamina, or endurance, is the ability to generate purposeful muscle contraction for an extended period. It depends on the conditions and cooperation of the following body systems: the heart, lungs, blood vessels, blood chemistry, and the muscles.
The heart. Prolonged physical activity leads to increased demands for nutrients and oxygen in the active muscles. To meet the demands, the CNS increases the heart rate. Such a workload, if recurrent and consistent, leads to strengthening the heart muscles, resulting in a higher volume of blood excreted with each stroke. A typical stroke volume of an untrained person is 70-75 mL and can be improved up to 120 mL with training. Given the fact that the human body contains approximately 5 L of the blood, with a higher stroke volume, it takes fewer beats per minute to circulate all 5 L around the body. Thereby, it results in a lower resting heart rate and makes it easier to provide a greater amount of blood to the working muscles during exercising.
The lungs. Their functions are: to move the air in and out, get oxygen from the air into the blood, and remove carbon dioxide. When the muscles work, they use oxygen and produce carbon dioxide as byproducts. Exercising can increase lung efficiency by improving the muscles, that physically expand the lungs (the diaphragm and Intercostal muscles). If those muscles are well-conditioned, then they open the lungs to their full volume allowing the maximum amount of air in. The higher the inhale volume, the more air floats in, the more oxygen is extracted, the more carbon dioxide removed with the following exhale. It makes any physical activity more efficient. If those muscles are left untrained, they might not open the lungs up to their full extent leading to insufficient oxygen supply to the working muscles and poor CO2 removal rate. The other factor which can improve the lungs' efficiency is the increased number of alveoli - tiny air sacks where the air physically comes in contact with blood to extract and transfer oxygen. A typical number of alveoli is 480 million. Some research shows that with physical training it can be slightly increased.
The blood. The body uses it to delivers oxygen and nutrition to the muscles. The blood has many different cells, including red blood cells containing a protein - hemoglobin. The oxygen extracted from the air comes in contact and binds with the red blood cells' hemoglobin in the lungs capillaries. The bloodstream carries the red blood cells to the muscle cells. Exercising can significantly increase the number of red blood cells and the mass of hemoglobin in each of them. Such improvement makes oxygen delivery much more effective.
The blood vessels. They carry the blood throughout the body. Inside each muscle, we have tiny blood vessels - capillaries, by which oxygen and nutrition get delivered into the muscle cells. The muscles which are used often and vigorously, develop greater capillary density. It allows higher quantity and speed of oxygen and nutrition delivery and improves carbon dioxide removal.
The muscle. The cells of the muscles have a structural component called mitochondria. The mitochondria produce energy (ATP) using oxygen and nutrients. When ATP runs out, contraction stops. The cells of the muscles which work often and intensely, develop a higher quantity of mitochondria and grow them in size, making energy supply abundant. Thus, the muscle can endure more vigorous physical activity for longer.
Endurance has prime and secondary components. The prime components are the heart, lungs, and blood (red blood cells with hemoglobin). The secondary components are the muscles' capillaries and mitochondria.
The prime components improve with any prolonged physical activity, whether it is running, walking, numerous pushups, or hand-balancing. The more and the bigger muscle groups we use, the better the heart, lungs, and blood improve (running or jogging is a good choice).
The secondary components improve only in the frequently used muscles. If we run often, then the leg muscles will improve their stamina, but not the biceps. Shall we do a hundred biceps curs we will not be successful because the improved functionality of the heart, blood, and lungs will be hindered by an insufficient amount of the capillaries and mitochondrial in the biceps. Therefore, we need to make sure that along with "cardio" exercises on treadmills, we also do drills targeting other body parts’ stamina.
In a healthy body, flexibility is not about the physical quality of the muscle tissue but rather the function of the CNS. It is the CNS that controls how much a muscle can be elongated (stretched) by using biofeedback from the muscle tissue. Here is how it works.
Inside any muscle, joint, or connective tissue, we have nerves - proprioceptors. They connect to the spinal column and comprise the proprioceptive system, which feeds the brain with biofeedback whenever the body parts are touched, bent, moved, exposed to cold, heat, or chemicals. One of the proprioceptors is muscle spindles, located inside the muscles and entwined around the muscle cells. (Pic. 5)
During the muscle stretch, the corresponding spindles are stretched as well and generate electrical impulses sending them into the brain. The intensity of these impulses is directly proportional to the intensity of the stretch. The muscle spindles are also sensitive to how fast we stretch the muscle.
If a muscle elongates too much or too fast, the CNS might trigger the stretching reflex (myotatic reflex), which is a muscle contraction in response to stretching within the muscle. The function of this reflex is to bring the muscle within its "acceptable" length to prevent any damage.
The brain treats a muscle elongation as "acceptable" only if the electrical activity of its spindles stays within a certain threshold. Once the electrical activity exceeds this threshold, the stretching reflex is activated.
The threshold is defined by how the muscles have been used. For example, children tend to be more active, especially in early childhood. Their muscles are often stretched, their strength is tested continuously, the joints move throughout a vast range of motion. As a result, the CNS gets used to the intense muscle spindles' electrical activity and sets the threshold for each muscle accordingly high making it possible for children to be exceptionally flexible by comparison to adults.
On the other hand, when we mature and become adults, a sedentary lifestyle sets in. The muscles are not stretched anymore up to the same extent; consequently, their spindles don't generate intense electrical activity, thereby lowering the threshold. Because the threshold is very low, even a mild muscle elongation makes the CNS trigger the stretching reflex, causing loss of flexibility.
The stretching drills, if done properly, desensitize the CNS to the muscle spindles' activity. It allows us to stretch the muscles with little or no resistance.
Mobility attributes to the joints. The joints are held together by the ligaments, which is a kind of connective tissue made of the protein - collagen. In general, the connective tissue is not meant to stretch as much as the muscles and would typically tear if elongated more than 6% of its length. Also, when damaged, it heals very slowly requiring many more nutrients. In some cases, reconstructive surgery might be required to fix the injury.
However, depending on how often and how much the body parts move, their ligaments' chemical structure might vary, making them rigid or playable. Regular physical activity of a joint will keep its ligaments playable and vice versa. In many cases, to improve the mobility of a specific joint, we need to enhance the flexibility of the corresponding muscles first so that we could bring the joint closer to its current maximum range of motion, which will allow us to challenge it and improve. It needs to be done with caution as to not damage the ligaments.
Coordination refers to our ability to know exactly where our body parts are at the moment and what kind of activity they are performing. The CNS uses the proprioceptive system to be aware of the body's actions. This system is comprised of nerve sensors - the proprioceptors - found primarily inside the muscles, tendons, ligaments, joint capsules, and skin.
When we move the body, the proprioceptors send electrical signals -biofeedback - to the brain. The biofeedback provides the CNS with information about the limbs' velocity, degree of joint movements, amount of muscle load, stretch, and pain. Based on this information, combined with the other channels of sensory input such as vision, smell, and hearing the brain adheres to that or another motor program.
A motor program is a pattern for a movement, stored inside a part of the brain, called the cerebellum. This pattern is simply a set of instructions for the muscles which participate in a chosen action. For instance, when we want to walk, the walking motor program is extracted from the cerebellum and executed. This program contains a set of parameters for each muscle-participant: how strong it needs to contact, for how long, at what time, and when it needs to relax. When the walk starts, the CNS monitors the movements of the limbs via their biofeedback to make sure that they stay within the chosen course of action.
When we attempt a novel movement, the CNS fails to find a corresponding motor program in the cerebellum and has to improvise. During this improvisation, the biofeedback from the proprioceptors is used to form a new motor program, based on our judgment. For example, when we see that our new dance move matches the one on the TV precisely, we are satisfied. Because we are satisfied, the CNS links the biofeedback from the body's proprioceptors to the parameters of the muscles we used, thereby forming a new motor program to be stored in the cerebellum. Next time we perform this move, it feels natural, and we are confident because we know the course of action in advance.
Unfamiliar physical activity feels awkward as the CNS fails to match the body's biofeedback to any existing motor programs. It leaves us in the dark on how to coordinate the body parts resulting in clumsiness or complete absence of control. With training, we learn new patterns and their combinations, expanding our functionality, thereby improving coordination.
Often, movements depend on balance, which is the ability to keep the body or some of its parts motionless in a chosen position. To create balance, we need to make some of the physical forces acting upon the body cancel each other. An example of a balanced body is standing still. When the body changes its position frequently, we need to manipulate the limbs dynamically to maintain stability. It would require the CNS to extract an appropriate motor program every time we move, creating a reliance on coordination. It is during the coordination training, we discover all possible scenarios for different movements, and learn to discriminate between them enriching the cerebellum with new motor programs. During the balance training, we drill the CNS to extract and implement the motor programs fast and with perfect timing.
That is it for now. I really appreciate the time you took to read this article. I hope it was informative and simple to understand. In my next article, I will talk about how we learn skills from the brain's perspective. Also, I would greatly appreciate any comments you might leave here for this post.