Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

Special Part
Myology, myologia [the study of muscles] - General Overview
Muscle Action

Muscle work is examined from the perspective of the general laws of mechanics, where work is defined as the displacement of a body or a load over a distance under METABOLISM/18.html">The Influence of a force acting upon it. Numerically, work is determined as the product of the muscle force and the distance over which the load is displaced by this force, and is expressed in kilogram-meters. Muscle work is subdivided into overcoming, yielding, and holding work. A muscle performs overcoming work when it overcomes resistance, displaces the body, lifts a load, or moves a body across a surface. During yielding work, the muscle remains in a state of tension, progressively stretches, and yields to the action of another force. This type of muscle work is essential and necessary to ensure smooth and elastic movements. During holding work, the muscle maintains a degree of tension that counterbalances another force, meaning the load is held in a fixed position without spatial displacement.

Muscle work is also classified into static and dynamic. During static work, a portion of the Muscles contracts and attempts to counterbalance the moment of gravity or resistance force, which is observed when aligning or maintaining the posture of the body and its segments. In this state, the muscle neither shortens nor lengthens, but merely develops tension. Static muscle work is necessary to maintain an upright body position, or posture. Dynamic muscle work is divided into overcoming and yielding.

The types of muscle work often alternate during the execution of movements. An example is the abduction of the arm to the side, where the deltoid muscle performs overcoming work; holding the arm in a horizontal position involves static (holding) muscle work, while slowly bringing the arm back to the torso entails yielding work. Thus, a specific type of work comes to the forefront depending on The Nature of each movement.

The operation of The Musculoskeletal System is viewed as a system of levers. A lever is any rigid body capable of rotational motion around an axis, upon whose arms two opposing forces act: the motive force (of Muscle contraction) and the resistance force. Depending on the magnitude of the motive and resistance forces, the lever may achieve equilibrium or motion. Depending on the arrangement of the muscle contraction site and the resistance force relative to the axis of rotation, first-, second-, and third-Class levers are distinguished.

A first-class lever, or lever of equilibrium, is a two-armed lever. In this system, the two forces are located at the ends of the lever and act in the same direction. Examples of the lever of equilibrium include the atlanto-occipital joint and the hip joint (Fig. 116).

A second-class lever, or lever of force, is a single-armed lever. In this lever, the applied forces have opposite directions. The motive force is applied to the long arm of the lever, while the resistance force is applied to the short arm. An example of the lever of force is the ankle joint, where one force acts upward and the other downward (Fig. 117). The pressure generated at the axis of Rotation of the lever corresponds to the difference between the acting forces.

A third-class lever, or lever of speed, is a single-armed lever. The lever of speed differs from the lever of force in that the muscle traction force, applied close to the fulcrum, creates a shorter arm compared to the second arm, at the end of which the gravitational force acts. An example of the lever of speed is the elbow joint (Fig. 118). When flexion is performed at the elbow joint, the long force arm—the forearm—covers a greater range of motion than the short force arm extending from the radial tuberosity to the elbow joint. Thus, by acting on the short arm, the muscle gains in speed and distance, but loses in force.

A couple of forces. To execute rotational movements around a joint, a couple of forces is required, meaning a system of two equal, parallel forces directed in opposite directions (Fig. 119). The shortest distance between the lines of action of the forces is called the arm of the couple. This shortest distance is the perpendicular dropped from the point of application of one force to the line of action of the other. An example of a couple of forces is the flexion of the forearm at the elbow joint by the biceps brachii. One force is the muscle force, and the other is the joint resistance exerted by the humerus, directed in the opposite direction.

Parallelogram of forces. Positioned at an angle to one another, muscles pull a bone in two different directions; the resultant of these forces is represented by the diagonal of a parallelogram constructed upon these forces. For instance, the line of pull of each of the major adductor Muscles of the arm (the pectoralis major and latissimus dorsi) does not coincide with the direction of movement during arm adduction (Fig. 120). Furthermore, there is no line of muscle pull that entirely coincides with the direction of adduction when this movement occurs in the frontal plane.

Thus, two muscles, by forming a parallelogram of forces between them, substitute for the absent muscle required to execute this movement. The rule of the parallelogram of forces applies not only to two muscles, but also to several other muscles pulling the bone in various directions.