Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Muscular System
Accessory structures of muscles — Muscle work and strength

Muscles act on skeletal levers, Setting them in motion or maintaining body parts in specific positions. Every movement typically involves multiple muscles. Muscles acting in the same direction are called synergists, while those acting in opposing directions are termed antagonists.

Muscles exert a specific force on skeletal bones, performing mechanical work—either dynamic or static. During dynamic work, skeletal levers change 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. Such Muscle contraction without A change in length is called isometric contraction. Muscle contraction accompanied by a change in its length is termed isotonic contraction.

Taking into account the point of application of muscle force to the skeletal lever and other characteristics, biomechanics distinguishes between first-Class levers and second-class levers (Fig. 32). In a first-class lever, the point of application of muscle force and the point of resistance (body weight, load mass) are located 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) is located on one side of the axis of the atlanto-occipital articulation, while the point of application of the occipital muscle force to the Occipital bone lies on the opposite side of the axis. Equilibrium of the head is achieved when the torque 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) balances the torque of the gravity force 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 load application).

Fig. 32. Action of muscles on skeletal levers:

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

In a second-class lever, both the point of application of muscle force and the point of resistance (gravity) are located on the same side of the fulcrum (joint axis). Biomechanics identifies 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, the human FOOT. The force arm of the triceps surae muscle (the distance from the calcaneal tuberosity to the fulcrum—the heads of the Metatarsal Bones) is longer than the force arm of body weight (from the axis of the ankle joint to the fulcrum). This lever provides a mechanical advantage in applied muscle force (longer lever arm) and a disadvantage in the speed of displacing body weight (shorter lever arm). In the second type of second-class lever, the muscle force arm is shorter than the resistance arm (the application of gravity). 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 gravitational force is applied. In this case, there is an advantage in the speed and range of hand movement (longer lever arm) and a disadvantage in the force acting on the skeletal lever (shorter force application arm).

The muscle's force of action is determined by the mass (weight) of the load that the muscle can lift to a certain height upon maximum contraction. This force is generally referred to as the lifting force of a muscle. The lifting force 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-sectional area of the muscle. For the morphofunctional characterization of muscles, The concepts of Anatomical and physiological cross-sections exist (Fig. 33). 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 magnitude of the cross-sectional area of the muscle at its widest point. In a muscle with longitudinally arranged fibers (such as strap-like or fusiform muscles), the anatomical and physiological cross-sections are equal. When A large number of short muscle fascicles are oriented obliquely, as seen in pennate muscles, the physiological cross-section exceeds the anatomical one.

Fig. 33. Anatomical (solid line) and physiological (dashed line) cross-sections of muscles of various shapes: 1 — strap-like muscle; 2 — fusiform muscle; 3 — unipennate muscle

The rotational force of a muscle depends not only on its physiological or anatomical cross-section or lifting capacity, but also on the angle of muscle attachment to the bone. The greater the angle at which a muscle attaches to the bone, the greater mechanical effect it can exert on that bone. Anatomical pulleys serve to increase the angle of muscle attachment to bones.