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

Musculoskeletal System
Features of the human musculoskeletal system and body biomechanics
Specific characteristics of the human musculoskeletal system

The Specific features of the human physical type evolved in connection with an upright body posture, bipedalism, and labor activity.

The force of gravity of The Human Body acts in a vertical direction, which has directly affected the shape and Structure OF THE Skeleton, joint connections, and The Muscular System. This same factor has created a predisposition to conditions specific to humans, such as spinal curvatures, flat feet, ptosis of abdominal viscera, and the like. Adaptations to vertical statics can be traced in The structure of all skeletal regions: the spine, Skull, and limbs.

The superior and inferior surfaces of the vertebral bodies are parallel; however, under METABOLISM/18.html">The Influence of increased loads (for example, in heavy physical labor or weightlifting), the vertebral bodies are compressed in their ventral part and acquire a wedge-like shape. The sizes of the vertebral bodies increase from the cervical region to the sacrum, which is caused by the increasing load upon them. Of the five sacral vertebrae, three (and frequently even a part of the fourth) rest against the pelvic bones; in animals with a horizontal body position, only a single vertebra serves as a support in the sacrum (two in monkeys). The force of gravity during bipedalism largely determines the human-specific spinal curves (Fig. 1.59).

The foramen magnum and the atlanto-occipital joint of the skull have shifted closer to the center of its base (Fig. 1.39), which makes it possible to hold the HEAD with less muscular effort and rotate it with ease.

The human lower limbs, which bear a heavy static load and perform a locomotor function, have a more massive skeleton (compared to the upper limbs); they are straightened at the knee joints, their joints and ligaments are powerfully developed, the widened pelvis (Fig. 1.60) Supports the abdominal viscera, the widely spaced hip joints contribute to trunk stability, and the supinator Muscles are more strongly developed compared to the pronators. Human legs are longer than the arms, whereas in apes, conversely, they are shorter (Figs. 1.61, 1.62); however, this trait develops in humans only after birth.

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Fig. 1.59. Spinal shapes and head position in a chimpanzee (A) and a human (B)

Fig. 1.60. Skeletons of the pelvis and FOOT: of an ape (1, 2) and a human (3, 4)

Fig. 1.61. Body proportions:

1 — human skeleton; 2 — gorilla skeleton

Fig. 1.62. Skeletons in the same body position: 1 — chimpanzee; 2 — human

Fig. 1.63. Musculature in the same body position: 1 — chimpanzee; 2 — human

The musculature of the legs possesses greater strength, yet at the same time less diversity and a restricted range of motion. Strong gluteal muscles, the quadriceps femoris, and the triceps surae fix the leg during standing. These muscles are relatively weakly developed in anthropoids (Fig. 1.63).

The foot features a longitudinal spring arch that distributes the weight falling onto the foot from above, dampens shocks and impacts during walking, adapts the foot to uneven ground, and imparts smoothness to the gait and resilience to standing. In a newborn infant, the arch of the foot is not pronounced; it forms later as the child begins to walk.

The foot lacks a grasping function, although embryological and anatomical data (the presence of weak muscles that abduct and adduct the big toe) indicate that digit I of human ape-like ancestors possessed some ability to oppose the other digits. The first metatarsal bone and the Phalanges of digit I are the most massive, whereas the lateral side of the foot arch is weakened, primarily due to the Skeleton of the digits, especially digit V. The latter is clearly reduced and in 40% of cases consists of only two phalanges (due to the loss of the middle phalanx, which fuses with the terminal one). Partial reduction of the skeleton of digit V while the skeletons of the other digits are fully developed is a human trait not found in animals. The phalanges are shortened, while the remaining BONES OF THE foot are massive and larger than the homologous bones of the hand. The tarsus constitutes about half the length of the entire foot (Fig. 1.60).

Muscles play a substantial role in maintaining the arches of the foot. By attaching to the plantar aponeurosis and ligaments, the muscles tension them and impart stability. Therefore, when standing—when many leg muscles are tensed—the arches of the foot are not flattened, and are often even more pronounced. In particular, the transverse arch of the foot is supported by the fibularis longus Muscle. It attaches not only to the first metatarsal bone, as in monkeys, but also to the medial cuneiform bone, which reinforces the transverse arch.

Professional strain on the foot causes clearly noticeable morphological changes in its skeleton, arising as an adaptation to working conditions. An example is the foot of experienced ballet dancers. In classical dance, when standing and moving on pointe, the entire body weight falls upon the first three digits. This leads to a specific remodeling of the foot skeleton, especially in the region of these digits.

Excessive load on the foot in individuals whose occupations involve prolonged standing or walking (textile workers, postal workers, etc.) can lead to pathological changes, such as the flattening of the arches of the foot, or Flatfoot. This causes persistent pain in the arch region, leading to rapid fatigue and loss of working capacity. Flatfoot may develop in school-age children under frequent physical overload (carrying heavy schoolbags, the habit of carrying younger siblings, etc.).

The decisive condition for transforming the forelimb of the ape-like human ancestor into a hand was its emancipation during anthropogenesis from the function of locomotion and support. The hand became an organ of labor, performing a completely new function compared to the forelimbs of animals.

Adaptations of the hand to vertical statics were expressed in the lightening of its Skeletal structure, A change in the relative length of its parts, and fine Differentiation of the musculature. The human hand acquired exceptional mobility, which is ensured by the shape of the rib cage, long clavicles, THE POSITION OF the scapulae (on the DORSAL SIDE OF the rib cage), The Nature of The connection between the shoulder girdle and the trunk, and the Structural Features of the shoulder joint. Thanks to the clavicle, the shoulder joint is set off from the trunk, and through its articulation with the Sternum, the skeleton of the arm rests upon the Skeleton of the Trunk.

The rib cage is thickened in the sagittal direction and does not impede arm movement. In most animals, it is narrow and compressed laterally, and the forelimbs therefore move predominantly only in the sagittal plane.

The Pectoral Girdle is attached to the trunk primarily by muscles, which fix its position through simultaneous contraction.

The range of motion in the human elbow and shoulder joints is greater than in apes. The forearm is shorter than the upper arm (the reverse is true in anthropoids) (Fig. 1.61; 1.62; 1.63), which allows the arm to execute fast and precise movements (such as strikes) and requires less effort when lifting loads. Rotational Movements of the radius—pronations and supinations—performed in conjunction with the hand at the proximal and distal radioulnar joints, play a crucial role in arm function. These movements already appear in primitive terrestrial vertebrates, become more refined in apes, but are most highly developed in humans (especially supination), which is associated with the robust development of pronator and supinator muscles.

In connection with the adaptation of the hand for labor, its musculature exhibits a predominance of flexors over extensors, pronators over supinators, and adductors over abductors in terms of both quantity and differentiation.

The robust Development of the brachioradialis muscle is of significant importance to hand function. Positioned parallel to the radius, this muscle sets the hand in an intermediate position between pronation and supination, leaving the palm facing toward the trunk.

The human hand is a highly specialized structure. It consists of numerous articulating bones bound together by a strong ligamentous apparatus into an arched formation (Fig. 1.26, B). The hand is simultaneously exceptionally strong and elastic. Numerous muscles provide not only a wide range of motion but also versatility, speed, and precision in movement. The human wrist is more strongly developed than that of anthropoids, although The Hand as a whole is shorter due to the phalanges (Fig. 1.62; 1.63; 1.64). The limited and minor movements between the Carpal Bones collectively grant the wrist the high degree of plasticity necessary for the manipulative tasks of the hand as an operational organ.

The high mobility of the first digit (thumb) and its ability to oppose the other digits, including the fifth digit (pinky), are ensured by the saddle shape of the first carpometacarpal joint. The function of the thumb is so critical that if it is lost, the hand virtually ceases to be an organ of labor. In human ancestors—the Neanderthals—this joint was flattened, and thumb opposition did not achieve the perfection seen in modern humans. Although Neanderthal hands were strong, they were not yet capable of fine motor movements, as evidenced by the crude workmanship of primitive stone tools. In anthropoids, the thumb also opposes the others, but it is short, poorly developed, and incapable of the delicate yet forceful movements required for labor activities.

In newborns, opposition of the thumb to the other digits is imperfect, although their grasping reflex is striking. Infants have relatively longer phalanges and a shorter wrist compared to adults. Thus, the structure of a newborn's hand exhibits certain ape-like traits that subsequently disappear.

Under the influence of chronic occupational stress, functional hypertrophy of specific structures within the hand skeleton may occur, altering their shape and proportions. For instance, drivers exhibit an increased width of the metacarpals and phalanges without significant thickening of their compact layer, along with elongation of digits III–V and their corresponding metacarpals. Weightlifters show an increased thickness of the compact layer of the metacarpals and an elongation of the first digit and its metacarpal. In adolescents attending trade and music schools, the systematic impact of physical labor and constant practice playing musical instruments delays the epiphyseal synostosis of the metacarpals and phalanges, leading to their elongation. A striking example of thumb elongation resulting from prolonged, intensive occupational stress beginning in childhood is undoubtedly found in the hand of the brilliant Italian violinist Niccolò Paganini (1782–1840) (Fig. 1.65).

Fig. 1.64. The human hand:

1 — chimpanzee; 2 — orangutan; 3 — human

Fig. 1.65. Cast of Paganini's hand: The thumb reaches the middle of the second phalanx of the index finger (normally it reaches the distal third of the first phalanx)