Almost all skeletal muscles either originate or insert on the skeleton. When a muscle moves a portion of the skeleton, that movement may involve flexion, extension, adduction, abduction, protraction, retraction, elevation, depression, rotation, circumduction, pronation, supination, inversion, eversion, lateral flexion, or opposition. Before proceeding, you may want to review the discussions of planes of motion and Figures 9-2 to 9-5.

Actions can be described by one of two methods. The first, used by most undergraduate textbooks and references such as Gray's Anatomy, describes actions in terms of the bone affected. Thus a muscle such as the biceps brachii is said to perform "flexion of the forearm." The second method, of increasing use among specialists such as kinesiologists, identifies the joint involved. With this method, the action of the biceps brachii muscle would be "flexion at (or of) the elbow." Both methods are valid, and each has its advantages. We shall primarily use the latter method.

When complex movements occur, muscles commonly work in groups rather than individually. Their cooperation improves the efficiency of a particular movement. For example, large muscles of the limbs produce flexion or extension over an extended range of motion. Although these muscles cannot develop much tension at full extension, they are generally paired with one or more smaller muscles that provide assistance until the larger muscle can perform at maximum efficiency. At the start of the movement, the smaller muscle is producing maximum tension while tension production by the larger muscle is at a minimum. The importance of this smaller "assistant" decreases as the movement proceeds and the efficiency of the primary muscle increases.

On the basis of size and range of motion, muscles are described as follows:

• An agonist, or prime mover, is a muscle whose contraction is chiefly responsible for producing a particular movement. The biceps brachii muscle is an agonist that produces flexion at the elbow.
• Antagonists are muscles whose actions oppose that of the agonist under consideration. The triceps brachii muscle is an agonist that extends the elbow. It is therefore an antagonist of the biceps brachii muscle, and the biceps brachii is an antagonist of the triceps brachii. Agonists and antagonists are functional opposites; if one produces flexion, the other will produce extension. When an agonist contracts to produce a particular movement, the corresponding antagonist will be stretched, but it will usually not relax completely. Instead, it will contract eccentrically, with the tension adjusted to control the speed of the movement and ensure its smoothness.  You may find it easiest to learn about muscles in agonist/antagonist pairs (flexors/extensors, abductors/adductors) that act at a specific joint. This method highlights the functions of the muscles involved, and it can help organize the information in a logical framework. The tables in this chapter are arranged to facilitate this approach.
• When a synergist contracts, it helps a larger agonist work efficiently. Synergists may provide additional pull near the insertion or may stabilize the point of origin. Their importance in assisting a particular movement may change as the movement progresses. In many cases, they are most useful at the start, when the agonist is stretched and unable to develop maximum tension. For example, the latissimus dorsi muscle is a large trunk muscle that extends, adducts, and medially rotates the arm at the shoulder joint. A much smaller muscle, the teres  major muscle, assists in starting such movements when the shoulder joint is at full flexion. Synergists may also assist an agonist by preventing movement at another joint and thereby stabilizing the origin of the agonist. Such synergists are called fixators.