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

Musculoskeletal System
Features of the musculoskeletal system and human body biomechanics
Analysis of certain occupational and athletic movements

Human physical and mental activity is manifested through movement. Walking, running, writing, and human labor are all made up of regularly combined and recurring movements.

Movements occur in response to internal and external (physical, biological, and social) stimuli. These are reflex Reactions of the Central Nervous system to incoming signals from both the proprioceptors of the musculoskeletal apparatus and the intero- and exteroceptors of other Sensory systems.

Every complex movement has a specific Structure, which refers to a system of simple movements integrated into a unified whole (such as extension, flexion, and rotation in joints, as well as pronation and supination of the arms and legs). A movement cannot be successfully executed without the consistent and systematic performance of these components. Movement structure is determined by the interaction of forces acting on the body from the outside and generated within it.

External and internal forces that determine The structure of movement.

External forces (relative to The Human Body) that affect the structure of movements include: body weight, environmental resistance, support reaction, friction, and the inertia of external objects (Fig. 1.70).

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Fig. 1.70. External forces determining the structure of movement:

F₁ — gravity; F₂ — support reaction; F₃ — environmental resistance; F₄ — friction force

Internal forces manifest primarily through The activity of The Musculoskeletal System. These include: Muscle tension force, the passive resistance of Tissues, and internal reactive forces.

Muscle traction is involved in all human movements. It is greater when Muscles are more strongly contracted. As muscle tension decreases and relaxation occurs, the traction force drops.

During the action of multi-joint muscles, the contraction of some can induce a concurrent change in the tension of others. This phenomenon is known as muscle coordination. For instance, when the leg is flexed at the hip joint by the iliopsoas and other muscles, their antagonists—the biarticular extensors (semitendinosus, semimembranosus, and biceps femoris)—are stretched. Due to this stretch, excitation arises within them, and by contracting, they bend the leg at the knee joint. Meanwhile, the gastrocnemius muscle relaxes and does not prevent the tibialis anterior from extending the ankle joint via its muscle tone.

A paradoxical action sometimes occurs in the functioning of multi-joint muscles. In such cases, muscle traction produces a movement opposite to the usual one. For example, during standing—when the legs are fixed to the support by the body's weight—the biarticular extensors of the leg at the hip joint, upon contraction, act on the knee joint not as flexors (their primary function), but as extensors, because it is the upper end of the lower leg that shifts backward rather than the lower, fixed end.

The passive resistance of tissues is manifested in movement limitation created by ligaments, joint capsules, and muscle viscosity, which slows down their stretching and contraction. Bones also provide resistance by acting as levers that transmit movement to other mobile segments of the body. Multi-joint muscles can also restrict movement depending on the specific alignment of the body segments they connect. For instance, flexing the leg at the hip joint is significantly easier when the knee is bent. If the knee joint is extended, hip flexion is hindered due to the insufficient length of the Posterior Thigh Muscles; when the knee is straight, their points of origin (on the ischial tuberosity) and insertion (on the lower leg) end up significantly far apart from each other. This phenomenon is known as "passive insufficiency" of muscles.

Internal reactive, or reflected, forces are inertial forces. Because the body is a chain of mobile segments, the inertia of one segment can be transmitted to others. For example, after throwing a discus, the entire body experiences a jolt in the opposite direction through the muscles that decelerate the arm's movement.

Mechanisms of movement. Most of the large JOINTS OF THE trunk and limbs are multiaxial. This means that A wide variety of movements (degrees of freedom of movement) are possible within them; however, in each specific case, movement occurs in only one direction, around a single axis. To turn a joint into a motor mechanism, muscle tension must be applied to the bones it connects. The muscle that sets the joint in motion performs overcoming (concentric) work. However, the contraction of any muscle is accompanied by the stretching of its antagonist. The resulting elastic resistance forces against this contraction, along with subsequent reflex tension, prevent the movement from being executed with full range and maximum speed; the antagonist performs yielding (eccentric) work, which slows down the movement. As a result of the interaction between overcoming and yielding, dynamic (or isotonic) muscle work, movement is carried out in the joint at a precisely regulated speed. The tension of Muscles Attached to bones in a direction perpendicular to the movement regulates that movement. Thus, a multiaxial joint becomes a definite motor mechanism—that is, a connection with a single capacity for precisely regulated movement.

In most movements, not all muscles perform dynamic work. Many muscles contract to firmly anchor PARTS OF THE Skeleton, thereby providing the foundation for the dynamic work of other muscles and maintaining human posture. Such supportive, static (isometric) muscle tensions manifest in Three types of work:

✵ holding work, when the muscle exerts its tension against gravity;

✵ strengthening work, when gravity acts vertically on the joint and the muscle contracts to resist its Separation;

✵ fixing work, when the joint becomes immobilized through the simultaneous contraction of antagonist muscles.

As a result of The Nervous System constantly coordinating dynamic and static muscle work, the musculoskeletal system is always maintained in a state that ensures the normal interaction of the Organism with the environment.

During any activity, not all muscles are active: some are in a relaxed state and maintain only their natural muscle tone.

Training and automation of movements. When a person learns a new, complex movement, they recruit an excessive number of muscles for its execution. Therefore, when mastering any movement, a person's attention must first be directed toward inhibiting the activity of unnecessary muscles. With further practice, The sequence of executing the components of a complex movement is established and consolidated. Once a movement is automated, it is carried out with the participation of only those cortical areas (area 6) that have extensive connections with efferent centers (subcortical ganglia, etc.) (see Chapter 3). Only the Main Components of the movement remain under the control of consciousness, which is associated with the activity of the Cerebral Cortex as a whole. The more refined the motor skill, the higher the degree of its automation. This is achieved through the repeated repetition of the movement, which trains the motor apparatus and leads to The formation of new neural connections in the CEREBRAL CORTEX AND subcortical regions.

Classification of movements. Human movements can be divided into cyclic, acyclic, and variable.

Cyclic movements consist of sequentially repeating phases (stages). In each cycle, one phase precedes another, serving as its foundation. These movements are characterized by uniformity, constancy, continuity, and relatively easy automation. They predominantly include locomotor movements—such as walking, running, swimming, and cross-country skiing—as well as many labor-related movements, such as sawing and planing.

Acyclic movements are single-action events. They consist of separate, dissimilar, sequentially executed, but non-repeating components. These movements include physical exercises—such as throwing a projectile or jumping—and certain work-related movements, such as lifting heavy loads.

Non-steady movements represent a combination of cyclic and acyclic motions. A long jump serves as a classic example, where the primary acyclic movement is preceded by a preparatory cyclic motion in the form of a run-up.

Walking. Walking is one of the fundamental dynamic states of the human body. It is a complex progressive cyclic movement characterized by the alternate loss and recovery of body balance. Walking consists of the body alternately supporting itself on both legs (the double-support phase) and on a single leg (the front and rear step phases). Thus, unlike other locomotor movements such as running, the body never loses contact with the supporting surface during walking. Walking begins by shifting the vertical line of gravity beyond the anterior boundary of the support area, which results in a loss of balance. One leg is brought forward through the contraction of the anterior muscle groups of the thigh and lower leg to establish a new support area, while the body is prevented from falling by the tension of the gluteus maximus muscle of the opposite, supporting leg. When the forward-reaching leg contacts the supporting surface (with the heel), the front step phase ends, and the double-support phase begins. The forward progression of the body now continues due to inertia and the push-off from the ground by the second leg, which has remained behind; this marks the beginning of the third phase, known as the rear step. The push-off is performed first by the heel, which lifts off the ground as a result of the contraction of the triceps surae muscle of the lower leg, and subsequently by the toes, driven by the contraction of the flexor hallucis longus. Having undergone a new forward displacement, the body is once again thrown out of balance. Consequently, the contraction of the hip flexors of the "rear" leg carries the latter forward; once the swinging leg passes the supporting leg (the vertical moment), it enters the phase of a new front step.

Thus, In addition to forward progression, walking also involves vertical movement up and down within the same sagittal plane, caused by the rolling motion of the FOOT from heel to toe. Furthermore, displacement also occurs in the frontal plane. This is executed at the hip joint of the supporting leg through the contraction of its abductor muscles (the gluteus medius and gluteus minimus). As a result, the torso tilts toward the supporting leg, lifting the moving leg off the ground and preventing it from dragging, a phenomenon often observed in elderly individuals with weakened musculature.

Running. The primary difference between running and walking lies in the absence of the double-support period—that is, the phase where the body rests simultaneously on the leg already brought forward and the "rear" leg that has not yet left the ground. A more forceful push-off by the "rear" leg replaces the double-support moment with a flight phase suspended in the air.

Like walking, running is classified as a cyclic movement of maximum intensity (Fig. 1.71). A full cycle is considered a double step (a step with the left leg followed by a step with the right), consisting of two support phases—push-off and landing—separated by two flight phases. Each phase conditions the subsequent one, utilizing the momentum generated by the preceding movements.

Running develops muscle strength, as well as the precision and speed of movements in the joints; it refines neural coordination mechanisms and significantly boosts METABOLISM. Moreover, it intensifies the activity of the respiratory and cardiovascular systems, making it an excellent tool for physical conditioning.

Swimming. Swimming is a locomotor, cyclic movement performed in Water (Fig. 1.72). It takes place in an environment unusual for humans and in an unnatural horizontal posture, during which the effective weight of the body is reduced by the weight of the displaced water.

During swimming, static muscular efforts are minimal, whereas the dynamic load is quite high. This is due to the inherent difficulty of maintaining balance in water and the fact that the push-off occurs against a fluid medium.

The force of gravity acting vertically downward and the water pressure directed vertically upward form a "force couple," which tends to impart rotational movements to the body. Equilibrium is achieved when the body's overall center of gravity and its center of buoyancy (which lies higher) align on the same vertical axis. To accomplish this, the arms are extended forward beyond the HEAD.

The high density of water and the difficulty of pushing off against it account for the relatively low speed of movement. However, the horizontal body position minimizes the surface area facing resistance, although this posture remains unnatural for humans and complicates movement coordination.

Let us examine the breaststroke swimming technique. It consists of four phases—deltoid muscle bundles; 12 — biceps; 14 — long head and 15 — short heads of the triceps; 23 — gluteus maximus and 24 — gluteus medius; 25 — tensor fasciae latae; 26 — posterior thigh muscles; 27 — rectus femoris; 28 — broad heads of the quadriceps; 29 — gracilis; 30 — gastrocnemius; 31 — soleus; 32 — anterior lower leg muscles; 33 — lateral lower leg muscles. Hatching — see p. 98 — which occur over the course of a single respiratory cycle: I — arm stroke, II — preparatory movements, III — leg kick, IV — body glide (Fig. 1.72).

Fig. 1.71. Sprint running (100 m):

1 — upper bundles of the trapezius muscle; 2 — pectoralis major; 3 — serratus anterior; 5 — latissimus dorsi; 7 — external oblique abdominal muscle; 10 — anterior and 11 — posterior

Swimming is one of the most beneficial sports. The diverse movements, which engage almost all the Muscles of the body, promote overall physical development, train the Respiratory Muscles and The Heart, harden the body, increase resistance to colds, and strengthen the nervous system.

Teaching a child to swim can begin at the age of 5. By this age, the musculoskeletal system reaches a stage of development where executing swimming movements becomes entirely feasible.

A. Phase I — Push-off. This phase is characterized by an intensive forward swing of one leg and a powerful sweep of the arms (bent at the elbows): the arm corresponding to the swinging leg moves backward, while the opposite arm moves forward. Pushing off with the takeoff leg (the left leg in the illustration) requires the greatest muscular effort and maximum pressure against the support, resulting in a powerful propulsion of the body forward and upward. Flexion of this leg at the ankle joint is performed by the posterior muscle groups of the lower leg.

Through the contraction of the posterior thigh muscles and the gluteus maximus, the leg extends at the hip joint. As a result of the action of the hip abductor muscles (the gluteus minimus and medius), the pelvis rotates to the left, and the right leg is brought forward. The power of the swing depends on the initial contraction force of the iliopsoas and quadriceps muscles. Subsequently, the leg advances further, extending at the knee joint purely by inertia.

The push-off by the rear (left) leg induces a Rotation of the torso to the right. The eccentric work of the back muscles (the erector spinae and left vertebral rotators) as well as the Abdominal muscles (the right external oblique and left internal oblique) brakes this rightward rotation, which is counterbalanced by the crossed movement of the arms. Arm movement is achieved through the alternating (right and left) rotation of the shoulder girdle around the vertical axis and the arms at the shoulder joints around the frontal axis. During the backward swing of the right arm, the pectoralis minor, rhomboids, levator scapulae, latissimus dorsi, triceps brachii, and posterior deltoid bundles contract. During the forward swing of the left arm, the serratus anterior, upper trapezius bundles, biceps brachii, coracobrachialis, anterior deltoid bundles, and pectoralis major contract.

At the elbow joints, the right arm completes its extension while the left completes its flexion. The palms are turned toward the body due to the tension exerted by the pronators, supinator, and brachioradialis muscles.

B. Phase II — Flight — begins as the takeoff leg (left) is pulled forward, while the swing leg lowers for landing. The arms begin to move in the direction opposite to that of The first phase. The torso rotates slightly to the left.

The takeoff leg (left), now free, is drawn forward through the concentric work of the iliopsoas, rectus femoris, sartorius, and tensor fasciae latae. The gluteus maximus and the posterior thigh muscle group perform eccentric work. The knee joint flexes without muscular intervention, driven by inertia, and the lower leg advances with a minimized resistance surface area. The angle of knee flexion is regulated by the tension of the quadriceps muscle. Later, the posterior lower leg muscles relax, while the anterior ones transition to concentric action.

The swing leg (right) begins to land under the Influence of the concentric action of previously stretched muscles: the gluteus maximus (extending the leg at the hip), the broad heads of the quadriceps (extending it at the knee), and the posterior lower leg muscles (pointing the toe for landing).

The shoulder girdle is held in a slightly elevated position by the action of the upper bundles of the trapezius muscles and the serratus anterior. This achieves the highest possible position of the overall center of gravity and facilitates the movement of the upper extremities.

Arm movement occurs exclusively at the shoulder joints via concentric action: in the right arm — the anterior deltoid bundles, coracobrachialis, and biceps; in the left arm — the posterior deltoid bundles and the long head of the triceps. Flexion begins at the elbow joints of the right arm, and extension begins in the left arm.

III. Phase III — landing — begins with the forefoot, which lengthens the stride; the body weight is supported by the right foot, which has become the weight-bearing foot. The Hip and knee joints of this leg are slightly flexed. The left leg is swung forward and upward from the supporting leg. The Structural Features of the foot — its arches, ligamentous apparatus, and muscles — serve to absorb the Shock of landing. At the same time, the flexors of the foot and toes, as well as the muscles of the posterior and lateral compartments of the lower leg, experience significant tension.

Concentric work is performed by the hip abductor group (the gluteus medius and minimus, and the tensor fasciae latae). As they contract, the pelvis tilts to the right at the hip joint, facilitating the subsequent forward swing of the recovery leg. All other muscles of the right leg that cause movement in the hip and knee joints perform eccentric work, preventing the leg from buckling further under The Influence of body weight.

The left (unweighted) leg is swung forward through the concentric action of all its hip flexors: the iliopsoas, rectus femoris, and sartorius muscles. The progressively increasing flexion at the knee joint, occurring without muscular effort, reduces the surface area of the leg opposing air resistance and facilitates its transition from the "trailing" to the "leading" stride. The muscles of the posterior and lateral compartments of the lower leg, which were actively engaged during both preceding phases, are now relaxed and thus resting. The ankle joint is dorsiflexed by the concentric action of the anterior muscle group — the tibialis anterior, toe extensors, and the extensor hallucis longus — preventing the toe from dragging on the ground.

Flexion continues in the right elbow joint as the arm is brought forward. The contraction of the anterior fibers of the right deltoid muscle and the posterior fibers of the left arm acting at the shoulder joints counterbalances the rotation of the torso around the vertical axis.

Fig. 1.72. Muscles involved in swimming:

2 — pectoralis major; 5 — latissimus dorsi; 6 — Deep Muscles of the back; 10 — anterior and 11 — posterior fibers of the deltoid muscle; 12 — biceps brachii; 13 — brachialis; 14 — long and 15 — short heads of the triceps brachii; 23 — gluteus maximus and 24 — gluteus medius; 25 — tensor fasciae latae; 27 — rectus femoris; 28 — vastus muscles of the quadriceps; 32 — anterior compartment of the lower leg; 33 — teres major. For hatching, see p. 98. A. Phase I — arm pull — is achieved by abducting the arms, thereby increasing the propulsive surface area. The middle and posterior fibers of the deltoids abduct and extend the arms at the shoulder joints. The elbows are not fully extended (triceps brachii) and are pronated by the contraction of the pronator teres and quadratus. The latissimus dorsi muscles become tense. The torso arches via the contraction of the deep back muscles, lifting the head, neck, and partially the chest above the water level. Inhalation takes place.

The legs are extended with their muscles relaxed. The rectus femoris begins to develop the muscle tension characteristic of the following phase.

B. Phase II — preparatory movements. The arms are adducted at the shoulder joints through the intensive action of the latissimus dorsi and pectoralis major, and flexed at the elbows (biceps brachii and brachialis). The legs are flexed at the hip and knee joints, while the ankle joints are plantarflexed at a 90-degree angle. Simultaneously, the legs are abducted to at least shoulder width. The head is submerged for exhalation.

Flexion and abduction of the legs at the hip joints result from the action of the iliopsoas, sartorius, rectus femoris, tensor fasciae latae, gluteus medius, and gluteus minimus muscles. Flexion at the knees of the hip-flexed legs occurs passively due to external forces — water pressure. Consequently, the posterior muscle groups of the thighs and lower legs remain relaxed. The anterior compartment muscles of the lower legs plantarflex the ankle joints.

C. Phase III — leg kick. This phase involves the greatest muscular effort, resulting in a vigorous forward propulsion of the body. The arms continue to be adducted at the shoulders (pectoralis major, latissimus dorsi, and teres major) and are sharply thrust forward (via the contraction of shoulder flexors — the anterior deltoid fibers, coracobrachialis, and biceps — and the contraction of elbow extensors — the triceps (short heads) and anconeus). The legs are sharply brought together, as if pushing off and repelling against the water, extending at the hip and knee joints while remaining extended at the ankles.

Leg adduction is executed by the adductor magnus, longus, and brevis, along with the gracilis and pectineus muscles. Simultaneously, the gluteus maximus extends the legs at the hips, and the vastus muscles of the quadriceps extend them at the knees. The ankle joints are maintained in extension by the tension of the anterior lower leg muscles.

D. Phase IV — gliding. The arms, torso, and legs are extended in a horizontal position, minimizing frontal water resistance and facilitating smooth glide. The arm and trunk muscles are under static tension, while the leg muscles are relaxed.