Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal System
Features of the musculoskeletal system and human body biomechanics
Anatomical mechanisms of statics and dynamics

Analyzing the Anatomical and physiological features of The Musculoskeletal System through the lens of mechanics is of great applied significance and forms the subject of a specialized science—biomechanics. Data from biomechanics are used to optimize labor movements, physical education practices, and athletic training. For example, when studying movement structures, it is taken into account that any movement involves only specific Muscle groups rather than all of them (Fig. 1.66). These findings serve as the foundation for therapeutic physical culture and prosthetic design.

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Fig. 1.66. Muscles of the Lower Limb involved in lowering and raising the body on one leg:

1 — gluteus maximus; 2 — rectus femoris; 3 — gastrocnemius; 4 — soleus; 5 — plantar muscles

The Study of the mechanics of the living human body begins with determining its center of gravity.

During relaxed standing, the total center of body gravity (Fig. 1.67) in men is typically located 1.5 cm posterior to the anteroinferior edge of the body of the fifth lumbar vertebra, whereas in women it is located 0.5 cm anterior to the anteroinferior edge of the body of the first sacral vertebra and 3 cm lower than in men.

THE POSITION OF the center of gravity depends on a person's physical characteristics—their posture, constitution, sexual and age-related differences (muscular development, skeletal mass, fat deposition, etc.). In children, the center of gravity is positioned higher than in adults; in weightlifters, it is lower than in gymnasts, and so forth. The lower the center of gravity, the more stable the body. Conversely, the longer the legs, the higher the center of gravity and the lower the body's stability; therefore, The ratio of leg length to torso length plays a definite role in movements that involve backward torso extension (swaying).

Fig. 1.67. Position of the vertical line of the total center of gravity and the strained muscle groups:

A — in a relaxed standing position ("at ease"); B — in an erect standing position ("attention"). The circle with a dot indicates the body's center of gravity. Black dots represent the projections of the frontal axes of the leg joints

The perpendicular dropped from the center of gravity—the so-called gravity vertical—is projected onto the support area, which is defined as the plantar surface of both feet and the space between them.

The support area increases when the feet are set apart. Following the laws of physics, human body equilibrium is more stable the larger this area is and the more centrally the gravity vertical is projected within it. Equilibrium is disrupted as soon as this vertical extends beyond the BOUNDARIES OF THE support area.

The Human Body is not a monolithic entity; it consists of individual segments connected in a mobile manner. Maintaining equilibrium is related to structural features that ensure the mutual reinforcement of these segments. To keep the body in an upright position, the Skeleton and Muscles that counteract gravity are of primary importance. The connections between body segments, predominantly joints, are arranged such that gravity acts on their frontal axes and induces flexion or extension of body parts. The mechanisms countering this force act on the same axes but in the opposite direction.

The position of the human body can be either static or dynamic. Examples of the former include standing and sitting, while walking, running, and jumping belong to the latter. Both positions are characterized by a specific posture, or carriage.

Posture. Every individual has a characteristic posture or carriage, meaning the position of the body during standing, sitting, walking, and working. Posture expresses the body's balance within its surrounding environment and is typically maintained by the static work of muscles. The anatomical basis of posture comprises the shape of the spine and chest along with the degree of development of various trunk muscle groups. Posture is equally determined by functional factors, such as muscle tone and the state of The Nervous system. Combined, these factors dictate the position of the HEAD, shoulder girdle, arms, torso, pelvis, and legs. Posture characterizes an adult's individuality just as much as voice timbre or handwriting. Let us examine two extreme types of posture: correct (good) and poor.

With correct, or upright, posture, the physiological curves of the spine have a uniform undulating appearance. The head is held straight or tilted slightly backward, and the torso is vertical. The chest protrudes slightly relative to the abdomen (Fig. 1.68, A). The shoulders are squared and kept at an equal height, the shoulder girdle is moderately lowered, and the arms hang loosely along the sides of the torso. The legs are straightened at the knees, the heels are brought together, and the toes are turned outward.

With poor posture, the head is thrust forward and the Neck Muscles are overstrained. Lumbar lordosis and thoracic Kyphosis are exaggerated (round back). The abdomen protrudes, while the chest appears sunken (Fig. 1.68, B). The shoulders are pulled forward. The legs are extended at the knee joints.

Fig. 1.68. Various body postures:

A — correct, or upright; B — slouching. The circle indicates the total center of gravity

Posture is considered an innate human trait. It represents a unique skill—a specific combination of conditioned Reflexes that maintain the habitual body position. An individual maintains their inherent posture without conscious tension in particular muscle groups. Posture begins to form in childhood and changes throughout life under the Influence of Environmental factors. Correct posture holds immense physiological significance and is essential for normal development. By favoring The activity of the entire Organism, especially Internal Organs like the Lungs and Heart, correct posture ensures increased working capacity. Conversely, poor posture disrupts normal bodily development and reduces physical efficiency. Physical education is critically important during middle school age. The key role here is played by balanced exercise and the harmonious development of all muscle groups. By the age of 18, posture stabilizes, after which correcting its deficiencies becomes quite difficult.

The potential for children to develop poor posture has specific anatomical and functional prerequisites. Static muscles in children develop and grow slower than dynamic muscles; therefore, it is harder for children than for adults to maintain a correct body position during prolonged standing or sitting, such as during lessons. Growing tired quickly, children unconsciously seek to relieve certain groups of trunk muscles. This easily turns into a habit, leading first to poor posture and subsequently to weakened back muscles and the curvature of the growing, and thus deformation-prone, spine. Prolonged sitting in the classroom fatigues the nervous system, weakens back muscles, and can cause developmental disorders in the spine. Guided by the age-related anatomical and physiological Features of the musculoskeletal system, it is necessary to prevent the onset of posture disorders by applying various sets of muscle exercises. Introducing labor education lessons as early as the primary grades will undoubtedly have a positive impact on student posture. It should also be borne in mind that physical exercises selected inappropriately for a child's age or uncontrolled enthusiasm for sports lead to harmful overloads on the body, especially given that in adolescents, the growth of the muscular system lags behind the skeleton, and The Heart lags behind the musculoskeletal system.

Analysis of muscle work in the standing position. During calm, symmetrical standing (the "at ease" position), the body leans slightly backward (Fig. 1.67, A). The gravity vertical passes anterior to the transverse axes of the atlanto-occipital joints and the spine, posterior to the transverse axes of the hip joints, and anterior to the knee and ankle joints.

Equilibrium in a standing person is maintained by the contraction of skeletal muscles opposing gravity. All of their work is of a static nature. The head is kept from leaning forward through the contraction of the neck muscles, while the upper torso is supported by the work of deep back muscles, primarily the erector spinae.

Spinal curves are exceptionally important for maintaining body equilibrium. For instance, the cervical lordosis, convex forward, together with the head's ligamentous apparatus (such as the nuchal ligament), allows the head to be held vertically without significant muscular strain. Extremely favorable conditions for maintaining an upright torso are also created by lumbar lordosis.

Intervertebral discs play a vital role in maintaining upright posture and balance. When a person is standing, their gelatinous nuclei are subjected to high pressure, yet due to their elasticity, they maintain constant equilibrium in the spine, thereby conserving muscular effort. In children, the relative thickness of the intervertebral discs is greater than in adults, while the degree of compression of the gelatinous nuclei is lower. Overall, the spinal Column is highly flexible, and its upright posture is maintained primarily by the tension of the deep back muscles. Consequently, maintaining an upright posture is more difficult for children than for adults.

Since the line of gravity passes approximately 2 cm behind the hip joint, the body is constantly prone to backward tilting at this joint. Counteracting this gravitational force requires the active tension of the iliopsoas, sartorius, rectus femoris, and tensor fasciae latae muscles. The work of these muscles is significantly facilitated by the powerful iliofemoral ligament, which lies anterior to the joint and can withstand a load of up to 350 kg. By stretching, it counteracts the force of gravity, thereby reducing active muscle exertion and preventing fatigue.

The line of gravity passes 1.5 cm anterior to the knee joint. To secure the leg in an upright position, the tension of the two heads of the gastrocnemius muscle on the posterior aspect of the lower leg is sufficient. Standing is also facilitated by the ligaments located on the sides and within the knee joint, as well as by the shape of the articular surfaces. Because the articular surface of the distal femoral epiphysis extends onto its posterior aspect, flexion at the knee joint occurs easily: the epiphysis slides first on its distal surface and then on its posterior surface over the articular fossa formed by the menisci and the Tibia. Extension, however, stops with only a slight forward movement from the longitudinal axis of the limb. This arrest occurs when the articular surface of the Femur abuts against the anterior edge of the corresponding tibial surface, the menisci become wedged between them, and the ligaments become taut.

The line of gravity passes 2.5 cm anterior to the transverse axis of the ankle joint. The body is primarily prevented from falling forward in this joint by the triceps surae muscle. Its superficial heads—the gastrocnemius muscle, which is pennate in shape and contains a high proportion of red fibers—function as a postural muscle. Its tendon (the Achilles tendon), one of the strongest in the body, can withstand a load of 400 kg. Gravity is also counteracted by the deep posterior muscles of the lower leg: the tibialis posterior, flexor digitorum longus, and flexor hallucis longus. A mechanism that facilitates upright standing is the shape of the articular surfaces of the ankle joint. When the leg is extended at this joint, the anterior, wider part of the talar trochlea becomes somewhat more firmly wedged between the fork-like malleoli.

Thus, in the "at ease" position, when the entire torso is leaned slightly backward, postural mechanisms play a major role in maintaining balance: the tension of the iliofemoral ligament in the hip joint and the specific Structural Features of the knee and ankle joints. As a result, significant muscle activity is not required to stabilize these joints.

In the "attention" position, unlike the "at ease" position, the body is shifted forward. Consequently, the line of gravity passes anteriorly not only to the knee and ankle joints but also to the hip joints, reaching the support area near its anterior boundary (Fig. 1.67, B). To prevent the body from falling, the muscles located posterior to the transverse axes of these joints must remain in a state of continuous tension. The workload of the gluteus maximus is particularly high, as its tension prevents the torso from falling forward at the hip joint. In the lower joints, the conditions for maintaining balance are the same as in the "at ease" position. However, because the line of gravity is deflected further forward from the knee joints in the "attention" position than in the "at ease" position, the action of the gastrocnemius muscle alone is no longer sufficient to stabilize these joints, and the Posterior Thigh Muscles must also contract.

When standing, a person relatively rarely distributes their weight evenly on both legs. A symmetrical standing posture is highly fatiguing because it requires the contraction of numerous muscles on both sides of the body. People generally prefer asymmetrical standing, placing more load on one leg than the other. In this case, the pelvis tilts, and the lumbar spine curves toward the less-loaded limb; the center of gravity shifts, but its vertical line remains within the base of support. Most muscles on the unloaded side of the body are relaxed during asymmetrical standing.

Sitting at a desk. Sitting (Fig. 1.69) is not classified as a work or athletic movement, but it is analyzed here due to its importance in school and daily life.

Fig. 1.69. Sitting:

1 — upper fibers of the trapezius muscle; 2 — deep back muscles; 3 — anterior and 4 — posterior fibers of the deltoid muscle; 5 — biceps brachii; 6 — brachialis, 7 — long and 8 — short heads of the triceps brachii; 9 — brachioradialis; 10 — radial and 11 — ulnar wrist extensors; 12 — pectoralis major muscle

A — overcoming work; B — yielding work; C — holding work; unshaded areas in the figure represent muscles not actively involved in the work (see p. 103)

When utilizing body stability while sitting to balance the weight of the torso and head, holding work is required from the spinal extensors, transversospinales, splenius muscles of the Head and Neck, and the upper fibers of the trapezius muscles. At the Hip and knee joints, the legs are bent at a 90° angle or slightly more, while at the ankles they are extended. The feet rest on the footrest of the desk or table. However, due to the large supporting areas of the thighs (posterior surface) and feet (entire sole), the leg muscles are relaxed. The shoulder girdle assumes a horizontal position. The arms are slightly abducted and flexed at the shoulder joints, bent at the elbows at a 90° angle, with the forearms resting on the desk surface and the hands aligned with them. The large supporting area of the forearms relieves the arm muscles from tension.

Maintaining body balance while sitting is easier than while standing. This is due to the increased base of support and the lowering of the body's overall center of gravity, which provides greater stability. Balance is maintained by the static contraction of certain muscles.

Proper positioning of the torso and legs is of vital importance while sitting. It depends on the ratio between the height of the desk surface and the student's height, i.e., the correct Selection of furniture. A normal posture is not a rigidly straight body position, but one that is slightly inclined forward, with a mild kyphotic curve of the back.

However, prolonged sitting with a forward body lean leads to compression of the chest cavity and breathing difficulties, as well as disruptions in Blood Circulation and METABOLISM. Therefore, the slightly curved posture should be periodically altered to an upright position, which is facilitated by resting against the back of the chair, thereby reducing tension in the back muscles.

When performing fine motor tasks, such as writing, the hands and forearms should rest On the surface of the desk or table.

When sitting at a desk, body weight rests primarily on the ischial tuberosities. To a lesser extent, it is distributed between the feet, the back (with the lumbosacral region leaning against the back of the bench), and the forearms. The head is inclined forward by no more than 15 cm from the vertical. The chest must not rest against the edge of the table.

During writing, all flexor muscles of the fingers of the right hand contract to perform the static work of holding the pen. Dynamic work is performed by: the flexor and extensor muscles of the wrist, which contract sequentially and subtly to move the hand against the resistance of the paper; and the flexors and extensors of the elbow joint, the deltoid muscle, and its antagonistic adductor muscles (such as the pectoralis major and others), which abduct and adduct the arm at the shoulder joint.

Thus, the majority of muscles remain in a relaxed state while sitting at a desk. However, the holding, static work of certain muscles—primarily the deep back muscles—as well as the compression of Tissues around the joints, which impairs BLOOD AND Lymph circulation, causes significant fatigue.