If you’re old enough to recall the classic TV series Mission: Impossible (or its recent movie spin-offs), you’ll recall that part of the fun was figuring out how the team of super agents was going to complete an assignment that seemed so ... impossible. In the end, they always triumphed—with Peter Graves’s hair looking perfect. But I wonder how they’d accomplish this mission?

(Tape player starts.) Your mission—if you choose to accept it—is to produce a substance that can shorten, lengthen, or hold itself at any length in between. It needs to support a load many times heavier than itself and cannot be made out of wood or metal, nor joined by nuts and bolts. It must be created from anatomical matter. That’s right—flesh and blood. Oh, one last detail: the essential parts and pieces that will produce this material’s amazing attributes can only be visible with an electron microscope. This tape will self-destruct in five seconds.

This might be one mission they would choose not to accept. After all, to produce a single fiber of muscle tissue—let alone an entire myofascial unit with a muscle belly, fascial components, and tendons—stands apart as one of nature’s most miraculous achievements. With all of this in mind, let’s take a look at five short stories that shed some light on the properties, roles, and actions of this impossibly spectacular material.

Story 1
On the Bus
Skeletal muscle tissue can perform its functions because it possesses four properties:
• Excitability.
• Contractility.
• Extensibility.
• Elasticity.
Singularly amazing, these attributes, when combined, lend muscle tissue an almost superhero-like quality seen in no other tissue of the body.
Found in both muscle and nerve cells, excitability is the capacity to respond to stimuli. Chemical, electrical, or mechanical stimuli can all lead to muscle activation (Image 1). For instance, when standing on a crowded bus, you reach up to grasp the overhead railing. This is possible because your shoulder and arm muscles respond to the electrochemical stimuli sent from your somatic motor nerves.
As the flexors of your fingers curl your digits around the rail, they demonstrate contractility, a muscle’s ability to develop tension when stimulated (Image 2). It may seem counterintuitive, but a muscle’s contractions may result in a muscle shortening, elongating, or staying the same length.
 In order for your fingers to grasp the bar, your flexor muscles need to shorten, which means other muscles have to lengthen. This property is extensibility, the capacity of muscle tissue to stretch without being damaged. On the bus, this is displayed by the extensors of your fingers, which passively lengthen to allow the flexors to shorten (Image 3). Extensibility allows a muscle to maintain functionality while adopting a range of lengths and shapes.
As you release your grip, both groups of muscles  display elasticity, a muscle’s tendency to return to its original length after being stretched. Your hand flexors are stretched and lengthened while your finger extensors contract and shorten (Image 4). When you relax the flexors, the elastic property of the stretched extensors returns both muscle groups to their resting lengths.

Story 2
No Muscle Is an Island
Now, let’s go into the studio and see how the five muscle roles—agonist, synergist, antagonist, neutralizer, and supporter—perform on the stage of the human body. We’ve chosen a straightforward scene that involves our actor reaching out to grasp a banana. Specifically, this focuses on flexion of the right shoulder. As we’ll see, however, no muscle is an island and even simple actions can prove to be complex.
Playing the role of agonist, and producing the main action, will be the deltoid’s anterior fibers. The script instructs the arm to raise to a nearly horizontal level so the hand can reach the fruit (Image 5). Spanning from the clavicle to the humerus, the muscle belly is in place.
 This should be a cinch, as the anterior deltoid has done this countless times. One take, and it will be a wrap.
Our director leads the scene: “‘Grasp the Banana.’ Take One. ACTION!”
The anterior deltoid fibers concentrically contract, but wait ...
“No, no, CUT.”
What went wrong? Well, when the anterior deltoid fibers engaged, instead of just raising the arm, the belly pulled both of its tendinous ends toward its center, which is exactly what a muscle will do when left to its own devices. So the clavicle depressed while the humerus barely lifted (Image 6).
It looks as though our agonist will need some synergists to assist in producing this main action. For these roles, we’ve hired the biceps brachii, the clavicular portion of the pectoralis major, and the coracobrachialis.
Our director resumes: “Here we go. Quiet on the set. ACTION.”
The deltoid contracts, the two synergists begin to hoist and ...
“CUT.”
What’s wrong now? Even with the efforts of these three strong muscles, they can barely lift the arm without the inhibition—or relaxation—of surrounding muscles (Image 7).
“Bring in the antagonists! Now, listen up, posterior fibers of the deltoid, latissimus dorsi, and teres major: your job is to relax and lengthen as the agonist and synergists pull the arm into flexion. Got it? OK. Take Three and ACTION” (Image 8).
Looking good, but ...
“Hold it, HOLD IT. What’s with all the scapular movement?”
It turns out that the agonist (deltoid) and synergists (biceps, pectoralis major, etc.) produce so much pull on their “stable ends” that the scapula (and, by proxy, the clavicle) are being yanked up and over the shoulder.
Who can we hire as neutralizers (or fixators) to reduce these undesired actions? How about muscles on the medial and lateral aspects of the scapula, like the lower trapezius and rhomboids? If they contract, they’ll hold the shoulder blade in place (Image 9).
Our director knocks back a shot of antacid to soothe his stomach before yelling, “Take Four. ACTION.”
Now we’re talkin’. The arm is slowly rising ...
“STOP. Why is the torso beginning to lean toward the arm? That’s not in the script.”
Well, as the arm lifts, the body’s center of gravity shifts, pulling the trunk forward and to the right.
“This was supposed to be a simple scene!”
 We need some supporters to help prop up the spine. Since the right arm is being raised, the left erector spinae and quadratus lumborum will need to be involved.
And while we’re at it, we’ll need further stability of the body’s core, courtesy of the abdominal obliques and transverse abdominis. Consistent with the principle “stability before mobility,” they’ll actually engage before the agonist. And if we really want to do this scene properly, particular muscles of the pelvis, thighs, and even gastrocnemius also will kick in as supporters (Image 10).
Our director takes a deep breath. “OK, this is it, people. Take Five. ACTION.”
The supporters engage,
the trunk remains upright,
the scapula is stabilized,
the antagonists relax,
the anterior deltoid leads the action,
the synergists assist it,
and the hand grasps the banana (Image 11)!
“CUT and PRINT. Take lunch everyone. Then hurry back for our next scene—‘Dancing the Watusi.’”

Story 3
Factors that Affect a Muscle’s Role
The role that a muscle plays in any given movement—major, minor, or none whatsoever—will depend on four factors. Let’s take a closer look.
A muscle’s size will affect its part in joint motion. For instance, due to its small stature, the anconeus won’t star in elbow extension. It will sing in the chorus as a synergist while the larger triceps brachii dominates the action.
The shape and design of a joint will determine a muscle’s role. For example, one of the reasons the quadriceps group can extend the knee is due to the structure of the tibiofemoral (knee) joint. Its design affords that motion. Yet, as hard as the quads (or any other muscle) might try, they’ll never abduct the knee because it doesn’t bend that way.
A muscle’s location in relationship to the joint axis will have a big impact on its role. The supraspinatus, positioned on top of the glenohumeral (shoulder) joint, is in a perfect position (in respect to the joint axis) to abduct the shoulder. The infraspinatus, located further around the posterior surface of the joint, will be unable to pull off that action. Its orientation, instead, will enable it to extend and rotate the joint.
What emerges from number 3 is that a muscle’s line of pull­—the direction of force exerted on a joint by the muscle—will be a major factor governing a muscle’s role in a particular movement.
Put another way, the action a muscle can perform depends on its line of pull in relation to the joint(s) it crosses. If you combine a muscle’s line of pull with the articulation it moves around, you can determine its action(s)­—with a combination of actions potentially emerging from one line of pull. Let’s look at four examples.

A Our trusty brachialis offers us a simple illustration of line of pull. It has parallel fibers that cross a hinge joint and thus produces only one action—flexion of the elbow. Since the elbow joint pivots around one axis, a clear line of pull can be drawn along the brachialis’s belly (Image 12).
Note that the direction of force the brachialis exerts on the joint isn’t going to create, say, elbow extension. If we link its line of pull with the joint structure it rotates around, we can deduce that it will flex the elbow.

B A second example is seen with the coracobrachialis. Unlike the straightforward brachialis, it is situated at an oblique angle on the body and crosses a different type of articulation—the triaxial glenohumeral (shoulder) joint. This structural arrangement allows the muscle to generate a line of pull of two actions (flexion and adduction of the shoulder) (Image 13).

C Third, the jack-of-all-trades gluteus medius has convergent fibers that span the triaxial hip joint. It has a line of pull that produces hip flexion, extension, abduction, and medial and lateral rotation (Image 14).

D Lastly, muscles that cross more than one joint, such as your extensor carpi radialis longus (ECRL), have the potential to create a line of pull in different planes at different joints. For instance, since the ECRL crosses the wrist and elbow, it extends and abducts the wrist, while also flexing the elbow (Image 15).

Factors That Affect Muscle Actions
Review a muscle’s action list in Trail Guide to the Body (Books of Discovery, 2014) and you’ll be excused for classifying it as a flexor, adductor, or otherwise. This might, however, cause you to believe that the muscle creates those actions under all circumstances. But if you recall from our discussion on the roles of muscles, this classification scheme is only in reference to the anatomically neutral starting position.
The biceps brachii, for instance—a flexor and supinator of the elbow and forearm, respectively—actually has little involvement with elbow flexion when the forearm is pronated. The same is true for its ability to supinate when the elbow is extended. Only when these actions are resisted does the biceps often kick in.
This is just one example. Aside from resistance (or a lack thereof), a muscle’s involvement in a joint action might also depend on the speed and direction of the motion, as well as the starting position of the joint. Movement, it turns out, is more complex than its depiction in those anatomy books.

Story 4
Reverse Actions
A muscle (via its tendons) can pull on any bone it attaches to. This means that your deltoid can technically tug the scapula and clavicle just as much as it can swing the humerus. In truth, that doesn’t happen, as each muscle demonstrates a biomechanical preference to pull one bone more than another.
Thus, a muscle’s origin is the attachment to the more stationary bone, while its insertion is the connection to the more mobile bone. For example, the majority of your brachialis contractions pull the distal forearm (the insertion) toward the proximal humerus (its origin). This is logical as the humerus attaches to your stable torso, while your forearm and hand are freer to move (Image 16).
This disposition can switch, however, with the origin and insertion swapping roles to produce a reverse muscle action. In the case of your brachialis, this is easily seen when you grab a bar to do a chin-up (Image 17). Now, the insertion is stationary and the humerus (the origin) must move toward the hand. Please note that the same action—flexion of the elbow—occurs, but now with the (typically stable) origin moving toward the (usually mobile) insertion.

Blood, Muscles, and Waste
Myofascial units (a muscle belly and its fascial elements) are outfitted with an ample blood supply. Yet, all of those tiny blood vessels are useless if the belly is pinched off. Here’s the story.
A muscle serving as a neutralizer, fixator, or longstanding postural unit spends a great deal of time isometrically contracted. Even when you are relaxed, its tone can be excessive. This chronic holding pattern not only prevents arteries from delivering fresh blood, but also restricts veins and lymph vessels (which depend on the pumping action of muscle contractions) from fulfilling their tasks. Soon, a vicious cycle ensues: built-up waste material begins to irritate the nerves. This further tightens the muscles, exacerbating the ischemia (loss of blood supply). Finally, your client lies down on the table where you discover that certain tissues feel like angry harp strings.

Story 5
Passive and Active Insufficiency
We’ll finish with a great party trick! First, the setup: muscles suffer diminished capabilities in positions of extreme shortness or length. This topic deserves additional consideration for bi- and multiarticular muscles such as the flexors and extensors of your wrist and fingers.
As we discussed earlier, a muscle­—being both contractile and extensible—can assume quite a range of lengths. The extent that it can change in length is called its excursion (Image 18). Generally, a muscle can shorten or elongate by half of its resting length. Thus, if a belly’s resting length is 6 inches, it can shorten to 3 inches or stretch to 9 inches.
When a bi- or multiarticular muscle is in a position of maximum excursion (length), you are susceptible to encountering passive insufficiency.
All right, now for that soiree stunt. Begin with your wrist in neutral (not flexed or extended), and make a tight fist. Easy, right?
Now, let’s try it again, but this time fully flex your wrist and then—without letting the wrist extend—try to make a fist (Image 19). You can’t fully clench, can you? This is because of the excursive limit of your antagonist finger extensors. Since they span the posterior knuckles as well as the flexed wrist, their maximum tautness prevents your fingers from fully curling.
You can switch the muscles’ roles, too. Begin at neutral and then extend your wrist. Note how your fingers naturally flex a bit (and how difficult it is to fully extend them with the wrist extended). This is due to the passive insufficiency of the wrist flexors (Image 20).
To further explain: passive insufficiency occurs when an action is inhibited because the antagonist muscle cannot lengthen sufficiently to allow the desired movement to occur. It is stretched too far. This is an issue mostly for actions involving bi- or multiarticular muscles because there is the possibility that the antagonist faces a stretch-intensive position at the multiple joints it crosses.
There’s something else at work here: active insufficiency. This occurs when an action is weakened or incomplete due to excessive shortness of the multi-joint agonist. Returning to your partial fist (Image 19), the flexion of your wrist and finger joints places your flexor muscles in an ultrashortened position. They can’t shorten any further (due to the limitations of the sliding filament mechanism). And let’s not forget that in positions of such extreme shortness (or elongation), muscles become weak. Thus, your flimsy, incomplete fist is due to both insufficiencies.

Andrew Biel, LMP, is the author of Trail Guide to the Body: How to Locate Muscles, Bones and More (Books of Discovery, 2014) and the president of Books of Discovery. This is an excerpt from his forthcoming book Trail Guide to Movement: Building the Body in Motion (Books of Discovery, 2014). He lives outside of Lyons, Colorado, with his wife, Lyn Gregory, and two children, Grace and Elias.

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