Human Anatomy - H. I. Koliadenko 2009

Myology (The Study of Muscles)
General Information
Biomechanics and Coordination of Movement

Muscle contraction is associated with The conversion of chemical energy into mechanical work. If a muscle, upon contracting, maintains a load while under tension, this type of work is referred to as static (for example, sitting or standing). If muscle contraction alternates with relaxation, causing The Human Body or its individual Organs to move through space, such work is termed dynamic.

The Displacement of the body in space involves the movement of its Skeleton, which is set in motion by the active component of The Musculoskeletal System—the Muscles. A muscle is a viscoelastic body; when stretched, it stimulates receptors that send impulses to the Central Nervous system, which in turn counteract the stretching of the muscle.

Bone movements obey the laws of mechanics and can be analyzed as lever systems. Every lever possesses two arms. One arm is acted upon by the force of body mass, while the other is acted upon by muscle traction. Consequently, the first is designated as the force arm of body mass, and the second as the muscle traction arm.

Just as in mechanics, the living Organism exhibits levers of the First and Second classes. First-Class levers are two-armed, with the force of body mass and muscle traction directed in the same direction (downward). An example of a first-class lever is the Articulation of the HEAD with THE Vertebral Column at the atlanto-occipital joint. The lever arms are located on opposite sides of the joint. The anterior arm is acted upon by the gravity of the facial part of the head, whereas the posterior arm is influenced by the Muscles of the neck, head, and back attached to the Occipital bone. The head maintains an upright position (equilibrium) due to the equality of the moments of body mass force and the muscle traction of the lever (the Skull). In effect, the force of the occipital muscles balances the mass of the head. Therefore, a first-class lever is also called a lever of equilibrium, or rest. Another example of an equilibrium lever is the pelvis balancing on the heads of the femurs.

The second-class lever is also two-armed, but in this case, the force of body mass acts downward, while muscle mass force acts upward. An example of such a lever is the forearm. By tensioning the biceps brachii (muscle traction force), which attaches near the elbow joint (the fulcrum), the mass of the forearm is overcome, and work is performed at high speed; hence, this lever is additionally known as a speed lever. Another example of a second-class lever is the FOOT when a person stands on tiptoe. Here, the entire foot acts as the lever. The fulcrum is located at the heads of the Metatarsal Bones, and the point of application for muscle traction is the calcaneal tuberosity, to which the triceps surae attaches via the calcaneal (Achilles) tendon, lifting the entire body mass upward. In this instance, as in the previous one, the two forces act in opposite directions; however, unlike the speed lever, the muscle traction arm is longer than the mass force arm. For this reason, this lever is conventionally termed a power lever.

Characterizing body positions or movements from the perspective of biomechanical laws is essential for understanding the function of the musculoskeletal system. This requires knowledge not only of lever mechanics within individual body chains, but also of how other external forces affect the body. The most significant external forces for the anatomical characterization of human postures and movements are gravity, ground reaction force, inertia force, friction, and environmental resistance.

The force of gravity (or gravitational force) is equal to body mass applied at the Location OF THE total center of mass (TCM) and is directed downward.

The ground reaction force is equal in magnitude to body mass but acts in the opposite direction. As long as gravity and the ground reaction force are balanced, the body remains in equilibrium. If gravity exceeds the ground reaction force, the body falls; conversely, when the ground reaction force exceeds body mass, the body is propelled away from the support surface (such as during trampoline jumping).

During walking, jumping, or running, the ground reaction force is directed toward the body at an angle, causing it to be resolved into two components—vertical and horizontal—According to the parallelogram rule. The vertical component of the ground reaction force is directed upward and interacts with gravity, whereas the horizontal component (friction force) influences body displacement, either facilitating movement or, conversely, acting as a brake.

Environmental resistance acts on the body during locomotion through various media (air, Water). This force depends on the frontal surface area of the body's resistance profile, speed, and the density of the external medium. Reducing the frontal surface area (for example, cycling in an aerodynamic tucked position) decreases environmental resistance.

Internal forces include muscle traction, which increases in magnitude with greater muscle contraction and tension.

Muscle coordination—such as the contraction of the muscles on the anterior surface of the arm—induces relaxation in the antagonistic muscles located on the posterior surface, alongside passive tissue resistance provided by ligaments and joint capsules, muscle viscosity, inertial forces, and other factors. The human body, like any physical object, remains in a state of equilibrium when its center of mass is positioned such that the perpendicular dropped from the center of mass falls within the base of support. If the line of gravity moves outside the support boundary, the body either falls or begins to move. A falling body is restrained by muscular tension; if this muscular force is insufficient, a fall occurs.

The center of mass of the human body is the point of application of the resultant of all mass forces from individual body segments—the head, trunk, and limbs. The center of mass is located at the level of the second sacral vertebra, slightly above the Pubic Symphysis, and measures 5–10 mm. In men, the center of mass is 1–2 cm higher than in women of the same height. The center of mass does not remain permanently fixed; it shifts depending on age, sex, body mass, and other variables. It also fluctuates throughout the day, driven by the functional states of the cardiovascular, digestive, and respiratory systems, as well as the body's spatial orientation.