There are almost 700 skeletal muscles of the human body, governed by a few basic principles involving muscle movements or muscular activity.

(i) Skeletal muscles produce movements by pulling on bones or tendons. The tendon gives a very firm anchorage. The point where a muscle is connected to an immovable or relatively fixed bone is called the point of insertion. It is nearer to the central portion of the body. The movable bone is connected by a tendon at the point of insertion and is away from the central portion of the body.

(ii) The bones serve as levers and joints act as fulcrums for the levers; Figure 21.6 makes this clear. Muscles can only contract a short distance, but since they are attached near a joint, the movement at the opposite end of a limb is greatly magnified. The biceps muscle of the arm may contract only 89 to 90 mm, but the hand will move about 60 cm.

(iii) The muscles that move a part of the body usually do not lie over that part. For example, the biceps muscle that moves the lower arm does not lie over the lower arm (it is attached to it by tendons). But lies in the upper arm (Fig. 21.6). Similarly, flexors of the wrist are located in the lower arm and not in the wrist.

(iv) The skeletal or voluntary muscles act in pairs rather than singly. One of the muscles produces contraction while the other allows relaxation. Flexion results when contraction causes two bones to bend toward one another, while extention results from contraction of antagonistic muscles, resulting in an increase in angle between two bones. Such pairs of muscles are called antagonistic. Often antagonistic muscles are in groups, e.g., both the brachialis and the biceps muscles flex the arm at the elbow and antagonize the triceps, but only when the palm is facing upwards. Of such pairs or groups of antagonistic muscle, one is usually much stronger than the other. The biceps, which flex the arm are larger and more powerful than the triceps which extends it. The gastrocnemius and soleus muscles of the calf which extend the foot at the ankle are stronger than the peroneus muscles which flex it.

(v) When the body is at rest, the proportion of antagonistic skeletal muscles remain in a state of contraction, called muscle tone, which holds the body in rigid position. If the person becomes unconscious, or is asleep, muscle tone is lost as the muscles relax completely.

(vi) For every action there is an opposing or restoring action (antagonists muscles). Rarely does one muscle contract alone, but is assisted by another . Synergists are muscles that supplement or assist one another, thereby reducing undesired action or unnecessary movement.

(B)Involuntary muscles. The involuntary or smooth muscles occur in the (intestines, gall bladder, urinary bladder ureter), arteries, iris, diaphragm of the eye etc.. They are abundant in sluggish invertebrates like molluscs. Structurally they are different from voluntary or skeletal muscles. They are usually under automatic rather than voluntary control. In the alimentary canal etc. their distribution in circular and longitudinal muscles, allows an antagonistic action.

Involuntary muscle produces relatively slow but prolonged contractions, or rhythmic contractions as in peristalsis.

Muscles are often given names that describe their actions (levator, flexors, adductor extensors, abductors, depressors, sphincters, supinators, pronators.).

(ii) By shapes (biceps, delltoid rhomboideus, trapezius).

(iii) By positions (temporalis, pectoralis, gluteus, rectus femoris, tibialis anterior).

(iv) By attachments, the origin being named before the insertion (geniohyoid, sternomastoid stylohyoideus.) While most muscles were first named in the human, the terms are not always appropriate when applied to homologs in other animals.