3D Muscular System: Five Common Muscle Actions

You are always moving. No matter how still you think you are, there are muscles contracting—you're never truly still. The muscles in your digestive track are working; your cardiac muscle (your heart) is still beating; the muscles that facilitate respiration are helping you breathe; and that rapid eye movement that happens in deep sleep is giving your ocular muscles a major workout.

But I’m not going to talk about the muscles that work when you think you are still. It's the muscles that you purposefully move that have my interest. Think of how much you move during the day. During your morning routine, even hunting for your favorite cereal in a cupboard requires a lot of muscle action.

Let's take a look at five common muscle actions. 

Wrist Extension

Have you ever been so hot that you fanned yourself with your hand? Give it a try right now—fan yourself. The wrist can move in all directions, but notice how it moves when you do the fanning motion. It moves back and forth, like a door on its hinges. This action is called wrist extension. Extension is the act of extending a flexed joint—in the case of your fanning hand, bending your hand forward toward you is flexing the wrist, but extending the wrist brings your hand back to a normal, straight position.

Watch the video below to better understand the motion. And keep in mind the muscle of wrist extension. (Notice that their names offer a hint about their actions.) 

Wrist extension muscles:

2–5 Digit Extension

Remember what I said about the difference between flexion (bending) and extension (straightening)? Most joints in your body flex and extend, including your fingers. If you curl your fingers into a fist, you're flexing them. When you straighten them out (like uncurling your fingers from a steering wheel), you're extending the fingers at the interphalangeal joints.

Watch the video below to see how the fingers extend. Note that the thumb has its own extensor and is not part of these! 

2–5 digit extension muscles:

Horizontal Shoulder Adduction

The next time you're on the receiving end of a very enthusiastic, one-armed hug from your uncle, pay attention to the motion of his arm. Horizontal shoulder adduction is the action in which the arm moves back toward the center of the body (like when your uncle drags you in for a hug). The anterior deltoid, the pectoralis major, and the coracobrachialis are involved in this action.

Watch the video below to see these muscles interact to adduct the shoulder horizontally. 

Scapula Elevation

Here's one you've probably done a million times since you first learned the phrase "I don't know." When you shrug your shoulder, you're elevating the scapula. Elevation is exactly what it sounds like: it's the act of lifting something. Shrugging lifts the scapula, clavicle, and humerus. In fact, shrug your shoulders now. Pretend I just asked you the answer to the Hodge conjecture (which no one knows, by the way). When you shrug your shoulders, your muscles contract to hold your shoulders up, and then relax when you lower them.

Watch the video below for a better visualization. 

Knee Flexion

You flex your knees all day long. When you walk, run, cross your legs—or do most things with your legs, really. Knee flexion is an interesting action because you wouldn't be able to walk without doing it.

Remember, flexion is the opposite of extension. Instead of straightening a joint, you're bending it back and forth. When you flex your knee, many muscles come into play: the gastrocnemius, the biceps femoris, the semimembranous, the semitendinosus, the gracilis, the Sartorius, the popliteus, and the plantaris.

Watch the video below to see these muscles interact to flex the knee.  

Video footage from Muscle Premium. 

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