Skeletal Muscles: How the Structure Determines Function


By Elizabeth A. MurrayMount St. Joseph University

Muscle tissues that help move the bones of the skeleton with the help of its attachments are called skeletal muscles. These are voluntary muscles tissues that can be controlled consciously. Let’s have a look at the anatomy of these muscles and, also, how they work.

Illustration of elbow flex
The skeletal muscle tissues contract and flex depending on their structure to help the elbow move. (Image: VectorMine/Shutterstock)

Structure Determines Function

The packing of muscle cells into groups called fascicles gives skeletal muscle a “grain”—a striated appearance of directionality that is very helpful in understanding a muscle’s action. Muscles do come in different shapes and sizes, and those shapes not only have names, but also relate to the action of that muscle.

Muscles and the cells within them shorten to execute movement, so the ability to see the directionality, or grain, within a muscle is key to understanding how it acts on the bones it’s connected to. Because all muscles aren’t shaped the same way, they don’t function the same way—since in anatomy, structure determines function.

Anatomical Structure of a Muscle

In order to understand how muscles work on the gross level, we also need to consider their attachments. Generally, each muscle has a tendon of attachment at either end, and the fleshy mass between the two is called the muscle’s belly. So, let’s consider a typical fusiform muscle—that means one that tapers at both ends, like the biceps brachii of the arm.

The tendon of attachment to the bone that the muscle does not move is called the muscle’s origin. The tendon of attachment to the bone that the muscle moves is known as its insertion. In the case of the biceps brachii, there are actually two origins; as the bi- of its name implies, it’s a two-headed, or two-bellied, muscle. It has what’s known as a long head and a short head, and the origins of both of those are on the scapula.

Workings of the Elbow Muscles

Saying that those are the origins of the biceps brachii doesn’t mean the scapula cannot move—it just means that moving the scapula is not the job of the biceps brachii rather there are many muscles that act on the scapula.

Now, the tendon of insertion for the biceps brachii attaches to the radius in the forearm, and although the biceps brachii crosses both the shoulder joint and the elbow joint, its main role is to flex the elbow.

Since the biceps brachii crosses the front of the elbow, and given muscles shorten to cause movement, if the biceps brachii passes anterior to the elbow joint and inserts on the radius just distal to the elbow joint, it will flex the elbow. Knowing where a muscle is relative to a joint is key to knowing its action, especially when the grain of that muscle is considered.

This article comes directly from content in the video series How We Move: The Gross Anatomy of MotionWatch it now, on Wondrium.

Biceps Brachii Versus Brachialis

But the biceps brachii is not the only muscle that flexes the elbow. There is another muscle, lying deeper than the biceps brachii, called the brachialis. The brachialis originates on the humerus, by a broad, flat attachment—along the anterior surface of the humerus, more so than a defined tendon. And it inserts on the ulna of the forearm.

Illustration of flexing of elbow
Both the biceps brachii and the brachialis are instrumental in helping with the movement of elbow. (Image: Alila Medical Media/Shutterstock)

So, to review, is its job to move the humerus where it originates, or the ulna where it inserts? Right, moving the ulna is its role. When we flex the elbow, the brachialis actually does more of the work than the biceps brachii, due to the positioning of its attachments, but the biceps often gets more credit, since it is more superficial and pumps up beneath the skin, due to its fusiform shape.

So, it’s not just the shape or grain of a muscle, but its attachment position relative to the joint that matters—and all are a part of the leverage system.

Role of Triceps Brachii

It must be remembered that joint movements had partners—flexion of the elbow is bending it, and we’ve seen that the biceps brachii and brachialis work together to do that. So, which muscle extends the elbow? The triceps brachii is in the posterior arm, and, as is evident from the prefix tri-, it has three bellies, or three heads, as they are often called.

One of the three heads originates on the scapula, while the other two originate on the humerus—by broad, flat attachments, like the brachialis had, rather than a visible tendon.

So the job of the triceps brachii is not to move either the scapula or the humerus. Triceps brachii inserts on the olecranon process of the ulna—that pointy prominence on the back of your elbow. Because the triceps inserts on the ulna, its job is to move the ulna—and because it is positioned posterior to the elbow joint, the triceps brachii functions to extend the elbow.

Muscles in Groups

The example above illustrates some basic principles of muscle shape, position, and attachments, and how muscles work in groups. The main muscle that causes a given action is called the prime mover or agonist for that action. And a muscle that assists in a given action is termed the synergist for that movement. A muscle that must relax in order for a prime mover and its synergists to act is called an antagonist to the given movement.

So, in our example, the prime mover for flexion of the elbow is the brachialis, and the biceps brachii is its synergist, or its assistant. The triceps brachii must relax in order for elbow flexion to occur, so it is the antagonist in elbow flexion.

On the other hand, if we discuss extension of the elbow, then the roles reverse, and the triceps brachii becomes the prime mover, and the biceps brachii and brachialis must relax, so those two muscles now become the antagonists to triceps brachii.

The antagonist’s role is not just in relaxing; it also helps prevent injury by not allowing excessive action of the prime mover. The antagonistic pairs often flank either side of the joint they move, and an understanding of origins and insertions is crucial to understanding muscle action.

Common Questions about Skeletal Muscles

Q: Why does the structure of a muscle determine its function?

Muscles come in different shapes and sizes, and those shapes relate to the action of that muscle. Because all muscles aren’t shaped the same way, they don’t function the same way—hence, structure of the muscle determines its function.

Q: What is a muscle belly?

Generally, each muscle has a tendon of attachment at either end, and the fleshy mass between the two is called the muscle’s belly.

Q: What is a skeletal muscle?

Muscle tissues that help move the bones of the skeleton through its attachments are called skeletal muscles.

Keep Reading
The Skeleton: Roles and Functions
The Different Types of Joints in Our Body
Skeleton System: The Features of Our Bones