Muscle function: how the muscular system works
Muscle function, meaning how a muscle contracts, relaxes and adapts, is one of the simplest but most important fundamentals of strength training. Once you understand why a muscle grows, why it gets fatigued and why recovery matters, every exercise you do in the gym becomes far more purposeful.
The goal of bodybuilding training isn’t just a bigger biceps or a wider back. The idea is a harmoniously developed musculature: a body that moves well, tolerates training load and supports the work of the heart, digestion and breathing.
Muscle function and why muscles matter
Muscle is an extremely economical source of strength. It does work, maintains posture and helps the body cope with changes in the external environment. Strong musculature is therefore one of the guarantees of good health, not just a question of athletic looks.
Weak musculature that isn’t up to the demands of everyday life can make the heart work harder. The development of the abdominal muscles, in turn, is linked to the work of the internal organs: better core tension helps support normal digestion, balances intra-abdominal pressure and aids breathing.
In everyday life, muscle seems permanent. We don’t notice any change from one day to the next. But all it takes is a leg being put in a cast, or a muscle being left unused for some other reason. Then the muscle atrophies quickly: its weight drops and its size visibly shrinks.
The other side is just as important. In response to physical training load, a muscle increases in weight and cross-sectional area. Even moderate training improves blood supply, opens up reserve capillaries, thickens muscle fibers and can increase their working capacity. Strength exercises affect muscle protein metabolism especially quickly.
All movement is governed by the same principle: the musculature does the work, and the nervous system organizes the control. This functional versatility allows a person to handle a huge variety of movement tasks, from walking up the stairs to lifting a heavy barbell.
How muscles work and adapt to training
The body is not a passive machine. It responds to training load, cold, fatigue and rest. This ability is called reactivity. Thanks to it, the body maintains dynamic stability: external influences come and go, but the organism strives to keep itself in working order.
Supercompensation, or over-recovery, is also important in training. When the training load exceeds the usual level and you allow time for recovery, the body can prepare for the next session stronger than before. A simple truth: muscle doesn’t grow only during the workout. It needs effort, food and rest.
With regular training, the body selectively raises exactly the working capacity that is demanded of it. If you do strength training, strength-related working capacity improves. If you do endurance work, the endurance side develops more. This produces morphofunctional specialization, or MFS.
MFS involves the whole organism, but the muscle groups and physiological systems that carry the main training load change the most. As athletic skill rises, specialization becomes more and more precise. Success no longer comes from general fitness alone, but from how well the body performs exactly the work required.
Muscle fibers and muscle types
Every muscle consists of a large number of fibers. These are divided into white, or fast, and red, or slow, muscle fibers. Fast muscle fibers contract quickly and with great strength, but they fatigue quickly. Slow muscle fibers work more calmly and with less strength, but they tolerate prolonged effort better.
Strength training with heavy weights and a low rep count mainly develops the fast muscle fibers. Training with lighter weights and a high rep count loads the slow muscle fibers more. With long-term strength training, the ratio of fast to slow muscle fibers generally doesn’t change, but the volume of both fiber types does.
The share of fast muscle fiber volume can increase more than that of slow fibers. In weightlifters, the area of fast muscle fibers makes up 70% of a muscle’s cross-sectional area. This shows well why the same muscle can look and perform very differently in different athletes.
Muscle hypertrophy means an increase in a muscle’s physiological cross-section. It happens through the thickening of existing muscle fibers. A muscle’s size doesn’t always grow conspicuously, because within a muscle fiber the density of myofibrils can increase while the fat content decreases.
There are two main types of hypertrophy: sarcoplasmic and myofibrillar. Sarcoplasmic hypertrophy means that muscle fibers thicken primarily because of an increase in sarcoplasm volume. This also includes metabolic reserves such as glycogen and creatine phosphate. In myofibrillar hypertrophy, the number and volume of myofibrils increase, which raises the muscle’s maximal strength.
The type of hypertrophy is determined by the nature of the muscle work. Prolonged dynamic exercises with relatively low resistance favor sarcoplasmic hypertrophy more. Exercises with high muscle tension, where the resistance is over 70% of the maximum, favor myofibrillar hypertrophy more.
How muscle works in strength training
Muscle hypertrophy rests on the synthesis and breakdown of muscle proteins. Strength training speeds up both. When the training load is sufficient, some muscle structures break down during the workout and are rebuilt during recovery. That is how a muscle becomes stronger and more capable of work.
Androgens, or male sex hormones, also play a role here. In men they are produced by the gonads and the adrenal cortex, in women only by the adrenal cortex. That is why men’s bodies contain more of them. The age-related development of muscle mass runs in parallel with the production of androgenic hormones.
The first noticeable thickening of muscle fibers appears at the age of 6-7 years. During puberty, at 11-15, intensive muscle mass growth begins in boys and continues later on. In girls, muscle mass development usually ends once sexual maturity is reached.
Artificial administration of androgenic hormones, or anabolics, can trigger intensive muscle protein synthesis during the recovery period. As a result, the mass and strength of the trained muscle increase. This is exactly why there is so much interest in using them in sport, although the topic needs separate, cautious treatment.
The energy for short, intense effort comes mainly from alactic anaerobic processes. To maintain muscle function, the body has to regenerate ATP, that is, carry out ATP resynthesis. One fast source is the energy released when creatine phosphate breaks down.
When adapting to heavy short-term loads, energy is also drawn through glycogenolysis and glycolysis. With maximal effort lasting no more than 6 seconds, no lactic acid forms in the muscles and blood. When the effort lasts 30 seconds, the lactic acid concentration already rises significantly. This shows that the glycolytic mechanism has kicked in.
Phosphorus compounds, enzymes and hormones are important for developing strength abilities. In prolonged and repeated efforts, an athlete’s strength also depends on the energy sources located in the fast muscle fibers. The oxygen contained in hemoglobin and myoglobin helps slow the formation of lactic acid. Strength training raises the amount of hemoglobin more than endurance training does.
Muscle strength depends on the composition of the structural proteins. Muscle proteins ensure contraction and relaxation. Proteins are not the main energy source, but they provide about 12 % of energy. They are renewed constantly: within 30 days, 50 % of the proteins are replaced.
Intensive strength training accelerates the breakdown of proteins and some muscle structures. Recovery takes 2-3 days. Synthesis increases especially for the proteins that break down more during training: the contractile proteins of the myofibrils, myosin and actin. If the strength training load triggers strong protein breakdown, it can lead to a significant gain in muscle mass. This requires sustained effort, 30 seconds and longer.
The muscular system and the types of muscle in the body
The muscular system consists of skeletal muscles and cutaneous muscles. There are over 400 muscles in the body in total. Depending on age, sex and individual characteristics, they make up more than one third of a person’s body weight. In athletes, muscle can account for up to 50 % of body weight.
The most important job of skeletal muscles is to move parts of the skeleton and to enable movement of the body and its individual parts through space. They start from a bone or bones, cross a joint or joints, and attach to another bone or bones via soft tissues. These include fascia, the joint capsule and other tissues.
In a broader sense, cutaneous muscles also belong among the skeletal muscles. Their main function is to change the shape and position of the openings of the head, such as the mouth, eyes, nose and ears. This produces facial expression, which conveys emotions.
Muscles come in a wide variety of shapes. They are classed as long, short and wide muscles. Long muscles are mostly located in the limbs, short ones in the hand, the foot and between the vertebrae of the spine. Wide muscles can be triangular or rhomboid in shape and sit, for example, in the abdominal wall or connect the upper limb to the trunk.
A muscle consists of functional elements and a connective-tissue framework. The functional elements are striated muscle fibers. They can be up to 15 cm long and 0.01-0.1 mm in diameter. Their main properties are contractility, excitability and elasticity.
The connective-tissue framework surrounds the muscle fibers, the muscle bundles and finally the whole muscle. Blood vessels, lymph vessels and nerves run through it. At its ends, the muscle belly passes into a tendon. The tendon is the passive part of the muscle, transmitting the contractile force of the muscle belly to the bones or soft tissues.
A tendon has to have very high tensile strength: 6-12 kg per square millimeter. It is whitish because its blood supply is poor. The muscle belly is reddish-brown because it has a rich blood supply and contains the pigment myoglobin.
Muscles are controlled by nerves that contain afferent and efferent fibers. Afferent nerve fibers, or sensory fibers, receive information about muscle tension and the position of body parts. Efferent nerve fibers carry impulses from the central nervous system to the muscles and end as motor end plates.
Weak impulses arriving in the muscles create a constant state of tension, or tone. From this baseline tone, a muscle begins active contraction or relaxation. Stronger impulses trigger active work by the muscle.
One motor neuron connects through its branches to a number of striated muscle fibers. Together they form a motor unit. The muscle fibers of one unit always contract together and with maximum strength. Since a muscle often contains thousands of motor units, the strength of a contraction depends on how many units are recruited at the same time. Being well trained noticeably improves this ability.
Muscle function in practice: what it means for training
In training, it doesn’t pay to think of a muscle as a single piece of tissue that simply contracts. A muscle needs blood, a nerve signal, energy and a suitable training load. During work, the blood vessels dilate and the amount of blood flowing through the muscle increases. This is called exercise hyperemia.
The smaller a muscle fiber is, the better its contact with the capillaries and the more intense its metabolism can be. This explains why strength, endurance and recovery depend not only on the size of a muscle, but also on how the muscle is built and how it is trained.
In a dynamic contraction, the muscle fibers shorten and the origin and insertion of the muscle move closer to each other. During a static contraction, the muscle fibers do not shorten, but the muscle can be under high tension. Holding a weight, a plank or a pause at the bottom of a squat are simple examples of static tension.
Most exercises use both. In a squat, the descent and the ascent are dynamic work, but the trunk and the hip girdle have to maintain stability at the same time. The same applies to presses, rows and deadlifts. So what actually happens? The body combines movement and stabilization into a single whole.
The maximum strength of a muscle is reflected by its physiological cross-section, that is, the combined cross-sectional area of all its muscle fibers. Research has shown that a muscle with a physiological cross-sectional area of 1 square centimeter can balance about 10 kg. If a muscle’s physiological cross-section is 3 square centimeters, its maximum strength is about 30 kg.
Muscles of the same mass can do equal work, but in different ways. A muscle with a few long fibers has a weaker effect. A muscle with many short fibers can lift a heavier weight, but over a smaller range. That is why the outward shape of a muscle does not tell the whole truth about its capacity.
Muscles never work completely in isolation. They work in groups and coordinate their activity in different combinations. In one movement the same muscle may be the prime mover, in another a supporter. The muscles involved in a movement are called agonists, antagonists, synergists and fixators.
Agonists perform the desired movement. Antagonists are muscles with the opposite action. For example, when the upper arm is adducted, the agonists are the muscles that adduct the upper arm and the antagonists are the muscles that abduct it. When the agonists work, the antagonists have to relax, but this relaxation is controlled. That makes the movement more precise.
Synergists help prevent unwanted side effects of the agonists. Fixators create a stable base. For example, in the initial phase of abducting the upper arm, the deltoid is the agonist and the trapezius is the fixator, because it fixes the scapula.
If you do bodybuilding, it is useful to know a muscle’s name, location, origin, insertion and function. This is not dry anatomy. It helps you choose exercises, understand technique and see why a particular movement loads exactly the muscle you wanted to train.
- Which bones does the muscle act on, and where does it attach?
- Which joints lie in the muscle’s path?
- Which axes of rotation in the joint does the muscle cross, and on which side of the axis does it lie?
- What support does the muscle work from, given that one end is usually more firmly fixed than the other?
Once these questions are clear, exercise technique becomes more logical. You no longer perform a movement just out of habit. You know what should be working, what should be stabilizing and where the technique may start to break down.
FAQ: muscle function – frequently asked questions
What is the difference between fast and slow muscle fibers?
Fast muscle fibers deliver a strong, quick effort but tire quickly. Slow muscle fibers work with less strength but last longer. Strength training affects both, but the nature of the training load determines which one is developed more.
Why does a muscle grow after strength training?
A muscle grows when training provides a sufficient stimulus and recovery allows the body to repair damaged structures. Hypertrophy means the thickening of existing muscle fibers, not just a temporary pump after a workout.
Is static muscle work important in strength training?
Yes. Static work maintains body position and gives a movement a strong foundation. Without it, holding a weight, stabilizing the trunk or using precise technique would not be possible.
Strength training keeps you young! ArtGym
Author: Bodybuilding I J.Loko ref.MM
Source: WHO – physical activity.
