Skeletal Muscles

A typical skeletal muscle is an elongated mass of tissue consisting of millions of individual muscle fibers interconnected by connective tissue fibers. This entire formation is enclosed in a strong, smooth connective tissue membrane and therefore can move relative to neighboring muscles and other structures with a minimum of friction. The two ends of a muscle are usually attached to two different bones, although some muscles run from bone to skin or even from one area of ​​skin to another, such as the facial muscles used in speech and facial expressions.

The end of the muscle that remains relatively motionless during its contraction is called the beginning of the muscle, and the one that moves is called the attachment; the thickened part between both ends is called the abdomen. The origin of the biceps is located at the shoulder, and the attachment is on the radius bone of the forearm; When the biceps contracts, the shoulder remains motionless and the arm bends at the elbow. Muscles never contract alone; they always act in groups.

No matter how hard you try, you will not be able to contract the biceps alone - you can only bend your arm at the elbow, which involves contracting not only the biceps, but also a number of other muscles. Further, the muscles can only pull, but not push. Therefore, they usually form pairs of antagonists: one pulls the bone in one direction, the other in the opposite direction.

The names flexor and extensor are applied to muscles to indicate the type of movement they produce. Thus, the biceps, which flexes the arm, is a flexor, and the triceps brachii muscle, which counteracts it, starts at the scapula and the upper part of the humerus and is attached at the other end to the ulna, extends the arm at the elbow, i.e. is an extensor. Similar pairs of oppositely acting flexors and extensors are found at the wrist, knee, ankle and other joints.

Antagonists are also the adductor and abductor muscles, which move parts of the body towards or away from its central axis; pronators and supinators, turning, for example, the hand with the palm up or down; sphincters and dilators, compressing or expanding one or another opening.

Even at the moment when the muscle is not contracted to perform any movement, it is not in a state of complete relaxation. While a person is conscious, all his muscles are slightly contracted; this phenomenon is called tone. A certain body position is maintained by partial muscle contraction