Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Musculoskeletal System
Muscular System
Structure and Functions of Skeletal Muscles

Skeletal Muscles represent the active component of The Musculoskeletal System and are composed of striated Muscle fibers. Attached to the skeletal bones, muscles contract (shorten) to set the bone levers in motion. They maintain the posture of the body and its parts in space, move bone levers during walking, running, and other movements, perform chewing, swallowing, and respiratory movements, participate in speech articulation and facial expressions, and generate heat.

The Human Body contains about 600 muscles, the majority of which are paired. In adults, Skeletal Muscle mass accounts for 35–40% of total body weight, whereas in newborns and infants, muscles comprise up to 20–25% of body weight. In elderly and senile age, the mass of Muscle tissue does not exceed 25–30%.

Skeletal muscles possess properties such as excitability, conductivity, and contractility. Under METABOLISM/18.html">The Influence of nerve impulses, muscles are capable of being excited and entering an active state. During this process, excitation rapidly spreads (is conducted) from nerve endings (effectors) to the contractile structures of the muscle fibers. As a result, the muscle contracts and sets the bone levers in motion.

Muscles comprise a contractile part—the belly, formed by Striated muscle tissue—and tendinous ends—tendons—which attach to the skeletal bones. However, in some muscles, tendons blend into the Skin (facial muscles) or attach to the Eyeball. Tendons are formed of dense regular Connective Tissue and exhibit high tensile strength. In limb muscles, tendons are narrow and long. Many ribbon-like muscles feature broad tendons known as aponeuroses.

Muscle shape. Fusiform and ribbon-like muscles are the most common (Fig. 26). Fusiform muscles are predominantly located in the limbs, where they act on long bone levers. Ribbon-like muscles vary in width and typically participate in forming the walls of the trunk, abdominal, and thoracic cavities. Fusiform muscles may have two bellies separated by an intermediate tendon (digastric muscle), or two, three, or even four heads of origin (biceps, triceps, quadriceps muscles). Muscles are further classified as long and short, straight and oblique, circular and square. Muscles can have a pennate Structure, where muscle bundles attach to the tendon from one, two, or multiple sides (resembling bird feathers), forming unipennate, bipennate, or multipennate muscles. Pennate muscles, constructed from numerous short muscle bundles, possess considerable strength and are thus powerful muscles; however, they can only contract over a short distance. Conversely, muscles with a parallel arrangement of long muscle bundles are not particularly strong, but they can shorten up to 50% of their length. These are dexterous muscles, found where movements require a wide range of motion.

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Fig. 26. Muscle shapes:

1 — fusiform, 2 — ribbon-like, 3 — digastric, 4 — biceps, 5 — unipennate, 6 — bipennate, 7 — broad, 8 — sphincter

According to their function and effect on joints, muscles are categorized into flexors and extensors, adductors and abductors, sphincters and dilators. Muscles are also classified by their Location in the human body: superficial and deep, lateral and medial, anterior and posterior.

Accessory apparatus of muscles

Muscles perform their Functions with the aid of accessory structures, which include fasciae, fibrous and osteofibrous canals, synovial sheaths, synovial (mucous) bursae, and pulleys.

Fasciae are connective tissue sheaths of muscles (Fig. 27). They compartmentalize muscles by forming intermuscular septa and eliminate friction between adjacent muscles. In cases of Hemorrhage or abscess rupture, fasciae limit the spread of Blood or pus beyond the fascial sheath. They are subdivided into propria (deep), superficial, and deep fasciae. In areas subject to continuous mechanical stress, fasciae may acquire a tendinous structure (resembling the aponeuroses of broad muscles).

Canals (fibrous and osteofibrous) are found in regions where tendons span across multiple joints (in the hand and FOOT). These canals serve to hold tendons in a fixed position During Muscle contraction. The walls of such canals are formed of dense Fibrous connective tissue, occasionally incorporating bone. Inside these canals, synovial sheaths are typically present to eliminate tendon friction against the canal walls. Synovial sheaths consist of a synovial membrane, one layer of which lines the canal walls while the other surrounds and fuses with the tendon. Both layers merge at their ends to form a closed cavity containing a small amount of fluid (synovia), which lubricates the sliding synovial membranes.

Fig. 27. Osteofascial sheaths of muscles in the lower third of the right thigh:

1 — fascia lata of the thigh, 2 — fascial sheath of flexor muscles, 3 — Femur, 4 — sciatic nerve, 5 — femoral artery and vein, 6 — fascial Sheath of the sartorius muscle, 7 — medial intermuscular septum of the thigh, 8 — osteofascial sheath of extensor muscles, 9 — lateral intermuscular septum of the thigh

Synovial (mucous) bursae perform a function similar to that of synovial sheaths. These bursae are closed sacs filled with synovial fluid or mucus, located in areas where a tendon passes over a bony prominence or over the tendon of another muscle.

Pulleys refer to bony prominences (condyles, epicondyles) over which a muscle tendon is reflected, thereby increasing its angle of insertion onto the bone and enhancing the mechanical force of the muscle acting upon the bone.

Muscle work and strength

Muscles act upon bone levers, Setting them in motion or maintaining body parts in a specific position. Every movement typically involves multiple muscles. Muscles acting on a joint in the same direction are called synergists, whereas those acting in opposite directions are antagonists.

Muscles exert a certain force on skeletal bones and perform work—either dynamic or static. During dynamic work, bone levers alter their position and move through space. During static work, muscles contract, but their length remains unchanged, holding the body (or its parts) in a fixed position. This type of muscle contraction without A change in length is termed isometric contraction.

Taking into account the point of application of muscle force to the bone lever and other characteristics, biomechanics distinguishes first-class and second-class levers (Fig. 28). In a first-class lever, the point of muscle force application and the point of resistance (load mass, body weight) lie on opposite sides of the fulcrum (the joint). An example of a first-class lever is the HEAD, which rests on the atlas (the fulcrum). The weight of the head (its facial portion) lies on one side of the atlanto-occipital articulation axis, while the point of application of the occipital muscle force to the Occipital bone lies on the other side. Equilibrium of the head is achieved when the rotational moment of the applied force (the product of the occipital muscle force and the lever arm length, equal to the distance from the fulcrum to the point of force application) equals the rotational moment of gravity of the anterior part of the head (the product of gravity and the lever arm length, equal to the distance from the fulcrum to the point of gravity application).

In a second-class lever, both the point of muscle force application and the point of resistance (gravity) lie on the same side of the fulcrum (joint axis). Biomechanics distinguishes Two Types of second-class levers. In the first type of second-class lever, the lever arm of the applied muscle force is longer than the resistance arm. For instance, in the human foot, the lever arm of the triceps surae muscle force (the distance from the calcaneal tuberosity to the fulcrum—the metatarsal heads) is longer than the lever arm of body weight (from the ankle joint axis to the fulcrum). This lever provides an advantage in applied muscle force (longer lever arm) at the expense of the speed of moving the body weight (shorter lever arm). In the second type of second-class lever, the lever arm of the applied muscle force is shorter than the resistance arm (gravity application). The lever arm from the elbow joint to the insertion site of the biceps tendon is shorter than the distance from this joint to the hand, where the load weight is applied. In this case, there is an advantage in the speed and range of hand movement (longer lever arm) at the expense of the force acting on the bone lever (shorter force application lever arm).

Fig. 28. Diagram of Muscle Action on bone levers:

I — first-class lever (lever of equilibrium), II — first type of second-class lever (power lever), III — second type of second-class lever (speed lever). A — fulcrum, B — point of force application, C — point of resistance

The contractile force of a muscle is determined by the weight it can lift to a specific height. This is the lifting capacity of the muscle, which depends on the number and thickness of its muscle fibers. In humans, muscle strength ranges from 5 to 10 kg per 1 cm² of the physiological cross-section of the muscle. For a morphofunctional description of muscles, The concepts of Anatomical and physiological cross-sections are used. The physiological cross-section of a muscle is defined as the sum of the cross-sectional areas of all muscle fibers within that muscle. The anatomical cross-section is the area of the muscle's cross-section at its widest point. In muscles with longitudinally arranged fibers (such as strap-like or fusiform muscles), the anatomical and physiological cross-sections are identical. However, when numerous short muscle bundles are arranged obliquely, as seen in pennate muscles, the physiological cross-section exceeds the anatomical one.

Muscle Tone

Muscles Attached to the skeletal bones are constantly in a state of tension known as muscle tone. This tone is maintained by nerve impulses continuously arriving from the Brain. The nerve impulses reaching the muscle trigger the depolarization of the presynaptic membrane of the nerve terminal, which contains a vast number of acetylcholine-filled vesicles. Consequently, Acetylcholine is released from the synaptic vesicles into the synaptic cleft, increasing the permeability of the postsynaptic membrane (the muscle fiber) to Na⁺ and K⁺ ions. The influx of positively charged ions into the muscle fiber generates a postsynaptic electronegative potential on its membrane. This creates a potential difference across the muscle fiber, exciting it and generating an Action Potential. This potential propagates along the muscle fiber and triggers its contraction. Returning the muscle fiber to its initial state is achieved through the action of the enzyme cholinesterase, which breaks down acetylcholine.

Muscle tone enables the human body to maintain a specific posture in space and preserves the ready-to-move state required for initiating various movements and actions.

Muscle Fatigue

Fatigue is defined as a temporary decrease in performance capacity that is restored following rest. Muscle fatigue is caused by excessive physical loads and improper work rhythms (excessively rapid, extremely heavy, or slow monotonous work). Under these conditions, metabolic byproducts (such as lactic acid) accumulate in the muscle, inhibiting the function of muscle fibers and depleting their energy reserves (Glycogen). Following rest, muscle performance is restored—especially after active rest, which involves changing the nature or type of activity.

REVIEW AND SELF-Control Questions:

1. Name the functions and properties of skeletal muscles.

2. What do you know about the Classification of Muscles, and what is it based on?

3. Describe the Accessory structures of Introduction/39.html">Muscles and their functions.

4. What types of muscle work do you know? Give Examples.

5. What is meant by muscle strength, and what factors does it depend on?

6. Provide an anatomical and functional description of first- and second-class levers in biomechanics.

7. What muscle condition is referred to as muscle tone?

8. What causes muscle fatigue, and what type of rest is most effective for restoring muscle performance?