Human Anatomy and Physiology - N. I. Fedyukovich 2003

Muscular System
Accessory Structures and Muscle Function

Muscles are equipped with various structures (accessory apparatus) that create favorable conditions for their contraction. The accessory apparatus includes fasciae (ligaments), tendon sheaths, synovial bursae, and muscular pulleys of sesamoid bones. Fascia is a Connective Tissue sheath of a Muscle that forms a casing for it, separates muscles from one another, reduces friction, and provides support for the muscle belly during contraction. A distinction is made between deep (proper) and superficial fasciae. Each region has its own deep fascia (for example, the arm, the forearm), but if muscles lie in several layers, they have a deep fascia. Superficial fascia is located under the Skin and covers the entire group of muscles, while deep fascia lies deeper and surrounds specific muscles and muscle groups. Intermuscular septa usually run between muscle groups. Muscles subjected to heavy loads have a denser fascia reinforced with tendinous fibers (for example, the Fascia of the thigh, the fascia of the leg), whereas muscles with light loads have a loose, delicate fascia. In some areas, thickening of the fasciae is observed: tendinous arches located over the underlying neurovascular bundles. In the region of certain joints (ankle, wrist), the fascia thickens to form a fibrous bridge—a retinaculum—which guides the direction of tendon movement.

The tendon sheath facilitates the unobstructed movement of tendons; it has a closed, slit-like cavity bounded by two layers and filled with fluid.

In areas where tendons or a muscle pass over a bone or another muscle, there are synovial bursae, which perform the same Functions as sheaths. A synovial bursa is shaped like a flat connective tissue sac containing fluid. On one side, the wall of the bursa fuses with the mobile organ (muscle), and on the other, with the bone or tendon.

If a synovial bursa lies between a tendon and a bony prominence covered with Cartilage, a so-called muscular pulley is formed, which alters the direction of the tendon, serves as its support, and increases the leverage of the applied force. The same function is performed by sesamoid bones (the Patella, the pisiform bone).

By contracting under METABOLISM/18.html">The Influence of nerve impulses, muscles act on bones through joints and alter their movement. In a uniaxial joint (pivot, hinge), movement occurs around only one axis. If muscles surround a joint on two sides and act in two directions, flexion and extension or adduction and abduction occur. Muscles acting in opposite directions are called antagonists, while muscles acting in the same direction are called synergists.

Since a muscle is attached to bones, its ends draw closer together during contraction; thus, the muscle performs work. This changes THE POSITION OF the body or its parts in space and overcomes gravity. In this regard, a distinction is made between overcoming, holding, and yielding muscle work.

Overcoming work is performed when the force of Muscle contraction changes the position of the body or its part, overcoming opposing forces.

Holding work refers to work in which muscle force maintains the body or a load in a specific position without movement in space.

Yielding work is defined as work in which muscle force yields to the gravity acting on a body part (limb) and the load it holds.

Bones connected by joints act as levers when muscles contract. Depending on the relative position of the acting forces and the fulcrum, two classes of levers are distinguished.

A first-Class lever is double-armed if the fulcrum is located between the points of force application, for example, the Articulation of the spine with the Skull (Fig. 55)

Fig. 55. Lever of balance:

А — fulcrum; Б — point of force application; В — point of resistance

A second-class lever is single-armed. It comes in two types. The first type—the lever of force—occurs when the arm of muscle force application is longer than the resistance arm (Fig. 56).

Fig. 56. Lever of force:

А — fulcrum; Б — point of force application; В — point of resistance

In the other type of single-armed lever—the lever of speed—the arm of muscle force application is shorter than the resistance arm, where the opposing force, gravity, is applied (Fig. 57). Muscle strength depends on anatomical, physiological, and other factors.

Fig. 57. Lever of speed:

А — fulcrum; Б — point of force application; В — point of resistance

A distinction is made between the Anatomical and physiological cross-sections of muscles, which reflect their functional characteristics. The anatomical cross-section is the cross-sectional area of the muscle at its widest part, taken perpendicular to its long axis. The physiological cross-section is the sum of the cross-sectional areas of all the muscle fibers that make up the muscle. The former parameter characterizes the length and thickness of the muscle, while the latter characterizes its strength.