Part 2: The Muscles

There are over 400 skeletal muscles present in the human body. While many names of the muscles are long and sound confusing, there is a "code" to muscle names that will come in very handy.

Before we look at diagrams, let's talk about this "code."

Look through (skim) the section from our textbook: Naming Skeletal Muscles. Then answer question 5.

Part 3: Intro to Origins, Insertions, and Actions

Read the following excerpt from the textbook, then answer questions 14-15.

To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle must also be attached to a fixed part of the skeleton. The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. During forearm flexion—bending the elbow—the brachioradialis assists the brachialis.

Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. To lift a cup, a muscle called the biceps brachii is actually the prime mover; however, because it can be assisted by the brachialis, the brachialis is called a synergist in this action (Figure 11.2). A synergist can also be a fixator that stabilizes the bone that is the attachment for the prime mover’s origin.

A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect; and (2) they control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb. The antagonist of the biceps brachii is the triceps brachii, as it performs the opposite action (extension of the lower arm.)

For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of the thigh are activated (and would be called the agonists of knee extension). However, to flex the knee joint, an opposite or antagonistic set of muscles called the hamstrings is activated.

As you can see, these terms would also be reversed for the opposing action. If you consider the first action as the knee bending, the hamstrings would be called the agonists and the quadriceps femoris would then be called the antagonists.

The sternocleidomastoid muscle is named after its origin and insertion. It originates at the sternum (sterno-) and clavicle (-cleido-) and inserts at the mastoid process (-mastoid). Examine the picture below of the right sternocleidomastoid muscle.

Remembering that the origin of a muscle is the bone that remains stationary (it does not move!) and that the insertion is the bone that is pulled on when the muscle contracts (the bone that moves), answer questions 16-18.