Key Points

• A muscle is a simple pulling machine, and when it contracts, it pulls equally on both attachments. The attachment that moves is simply the one with less resistance to moving. 

• Origin/insertion terminology creates a rigidity in thinking that can obscure the simplicity of muscle function.

The vast majorityof time, I think about the fundamentals of muscle structure and function and how to teach it as the basis for manual and movement therapy. And the longer I have taught, the more I have come to dislike some of our foundational kinesiology terminologies. 

And, strange as it may seem, my difficulties lie with the two most fundamental ways that we describe muscles—how we name their attachments and how we name their actions. Or, in other words, how we think about their anatomy and physiology, structure and function. This is especially problematic when we realize that what we ask of our students when they are learning the muscular system are precisely these two things! 

Muscle Attachments . . . Origins and Insertions?

Let’s begin with muscle attachments, which we usually name as origin and insertion. So, what exactly is wrong with naming attachments as origin and insertion? Well, first of all, I don’t believe that muscles think of their own attachments as origin and insertion. Now, muscles don’t have self-consciousness or the ability to think at all, but if they did, I am certain they would identify their attachments as simply this one over here and that other one over there. They would think of them based on where they are located. In other words, as proximal and distal, or superior and inferior, etc. (Image 1) 

Why do I say this? Because when a muscle contracts, it doesn’t treat one attachment any differently than the other. If we were to have a conversation with a muscle about its function, I imagine it would go as follows: 

Us: “Which attachment will move when you contract?” 

Muscle: “I don’t know.”

Us: “Do you want one of the attachments and not the other to move when you contract?” 

Muscle: “No. I don’t have a preference.”

Us: “OK. Well, which attachment should move when you contract?”

Muscle: “Look, when I contract, I just pull in toward my center. I have no idea which attachment will or should move. If you want to know that, you need to talk to the nervous system.”

So why do we teach attachment terminology as origin/insertion? I believe it is because of habit and momentum. It began this way many years ago, so we continue to do so. We learn it in school because our teacher learned it in school, and our teacher learned it in school because their teacher learned it in school, etc. (see Suzy sidebar).

Why Origin/Insertion Terminology?

So why might attachments have originally been described and taught as origin and insertion? Well, let’s first define what an origin and insertion are. Most often, the origin is defined as the attachment that stays fixed when a muscle contracts, and is assigned to the proximal attachment. And the insertion is defined as the attachment that moves, and is assigned to the distal attachment. This is true in open-chain kinematics but is not true in closed-chain kinematics (see Open-Chain and Closed-Chain Kinematics sidebar). 

Open-chain kinematics are common in the upper extremity where the hand is usually free to move. But in the lower extremity, the foot is often in a closed-chain position against the ground, so closed-chain reverse actions often occur in the lower extremity. 

So perhaps the origin should be defined as the attachment that is usually fixed, and the insertion as the attachment that usually moves. And it is typically thought that the proximal attachment is usually fixed, and the distal attachment is usually mobile, so the proximal attachment is assigned the role of origin and the distal attachment is assigned the role of insertion. But if we are going to go that far, why not just describe the attachments as proximal and distal? It is far simpler. After all, origins and insertions can switch which one stays fixed and which one moves. But the proximal attachment is always the proximal attachment, and the distal attachment is always the distal attachment. 

Problems with Origin/Insertion Terminology

The first problem with origin/insertion terminology is that it adds one more thing that the overwhelmed student must learn: the muscle’s name, where the attachments are located, and what its actions are . . . and now, the student must also learn which attachment is the origin, and which is the insertion. 

But origin/insertion terminology creates a much greater problem, and one that is insidious. It tends to create a rigidity of thinking, in which the proximal attachment named as the origin is always thought of as being fixed when a muscle contracts, and the distal attachment named as the insertion is always thought of as moving when a muscle contracts. Not only is this possibly wrong, but worse, it obscures the true simplicity of muscle function. That is, a muscle is a simple pulling machine, and when it contracts, it pulls equally on both attachments. The attachment that moves is simply the one with less resistance to moving in that situation. 

In the lower extremity, closed-chain, reverse-action movements of the origin are more common than the open-chain distal insertion moving. During the gait cycle, each foot is on the ground and stable 60 percent of the time, thereby causing the more proximal attachment at the joint to move instead of the more distal one. An excellent example of this is the leg (instead of the foot) moving at the ankle joint. When the foot is planted on the ground during the stance phase of the gait cycle from foot-flat to midstance, it is the leg that dorsiflexes relative to the foot (and not the foot that dorsiflexes relative to the leg) (Image 2).

Another excellent example is extension at the knee joint. It is typical when working with origin/insertion terminology to think of the leg moving at the knee joint. If I were to say that the knee joint extends, most students and therapists would likely think of the leg extending at the knee joint (Image 3A). But the more crucial action is the extension of the thigh at the knee joint. Indeed, it is why the quadriceps femoris (if considered to be one muscle) is the largest muscle in the human body—to extend the thigh at the knee joint. This is the joint action needed to get up from a seated position (Image 3B). 

Closed-chain reverse actions are not restricted to the lower extremity. They are also very common in the upper extremity. Whenever the hand is gripping a stable object, the proximal body part moves toward the distal one—for example, if we grab a banister to help pull ourselves up the stairs, hold onto a bar to do a chin-up, or sit and someone takes our hand to help us up. In each of these cases, we flex our arm at the elbow joint relative to the forearm (Images 4A and 4B). 

Adverse Effects

Can thinking in origin/insertion terms have adverse effects on the practice of manual and movement therapy? The answer is yes; it can in many scenarios. Let’s look at one example to see the implications: postural assessment and muscle function across the hip joint. The functional muscle groups across the hip joint are usually named as flexors and extensors in the sagittal plane, abductors and adductors in the frontal plane, and medial (internal) and lateral (external) rotators in the transverse plane. These names are based on the distal thigh/femur moving. The question is: How useful is this way of describing the muscle groups for a manual therapist? It may well turn out the answer is not very. 

Hip Joint Flexors/Anterior Tilters: When a new client presents and we perform a postural assessment exam with the client standing in front of us, if the client’s hip flexors are tight (locked short/overly facilitated), they will not be able to stand with their thigh(s) up in the air in flexion; they cannot do this and remain standing. So, the tightness in the hip flexors will play out by pulling their proximal attachment, the pelvis, into anterior tilt (Images 5A–5C). Understanding this is critically important because an excessively anteriorly tilted pelvis results in a compensatory hyperlordotic lumbar spine, which then shifts body weight posteriorly, loading the lumbar facet joints and increasing the likelihood of low-back pain and dysfunction. And the hyperlordotic lumbar spine then causes postural distortion compensation patterns above, all the way up to the thoracic spine, shoulders, neck, and head. 

Hip Joint Extensors/Posterior Tilters: Similarly, tight hip joint extensors in standing posture will result in a posteriorly tilted pelvis, resulting in a hypolordotic lumbar spine, thus increasing load on the disc joints and creating compensatory patterns up the body (Images 6A–6C). 

Hip Joint Abductors and Adductors: Examining functional groups across the frontal plane, we see that abductors of the thigh are depressors (lateral tilters) of the pelvis on that side; and adductors of the thigh are elevators of the pelvis on that side (Images 7A–7E). And just as postural distortions of the pelvis in the sagittal plane result in compensatory sagittal-plane distortions of the spine above, frontal plane obliquity of the pelvis will result in a frontal-plane distortion pattern of the spine; in other words, a scoliosis (Image 8). 

Hip Joint Lateral and Medial Rotators: Examining these biomechanics in the transverse plane, lateral rotators of the thigh are contralateral rotators of the pelvis, and medial rotators of the thigh are ipsilateral rotators of the pelvis, with compensatory distortion patterns up the spine.

Implications and Recommendation Changing how we discuss attachments has tremendous implications for manual and movement therapy. Understanding the importance of reverse actions, in which the origin moves and the insertion stays fixed, allows us to reason that any postural distortion pattern we find in the client’s spine might well be due to a postural distortion of the pelvis, which might result from an imbalance of hip joint musculature. This points us toward examining and assessing this critical musculature. But learning origin/insertion terminology tends to blind us to this possibility, allowing us to miss this critical step in assessment, resulting in ineffective treatment. 

My recommendation? Let’s retire origin/insertion terminology and replace it with a far simpler terminology, one that promotes a clearer understanding of muscle function. Let’s just name the locations of the attachments—proximal/distal, superior/inferior, medial/lateral, etc. 

And in the case of the hip joint, when we name the functional groups of muscles across the hip joint, we might name them as pelvic anterior and posterior tilters (instead of flexors and extensors), pelvic depressors and elevators (instead of abductors and adductors), and contralateral and ipsilateral pelvic rotators (instead of lateral/medial rotators). 

Muscle Function . . . Joint Actions? 

This leads us to muscle function. In muscle atlases, we describe muscle function by what the muscle does when it concentrically contracts and shortens. This creates two major problems. 

The first is that it creates a certain bias regarding muscle function—that a muscle functions primarily to create movement, which is not necessarily true. Much of the time, a muscle functions as fixator/stabilizer to isometrically contract to stop movement by stabilizing one body part so another muscle (mover/agonist) can more efficiently move another body part (Image 9). And a muscle can function as an antagonist to eccentrically contract to slow down movement, such as when slowing/controlling the lowering of a body part by gravity toward the ground. 

But it is another aspect of concentric joint actions that is even more troubling. We do not actually describe the concentric functional movement of the muscle. Most muscle contractions create oblique-plane movements (that occur across two or more cardinal planes). But instead of describing this oblique-plane movement, we break it down into its cardinal-plane components and name these cardinal-plane component movements as joint actions. In other words, a joint action is a cardinal-plane movement, and this cardinal-plane movement is often simply a component of the actual functional movement created by the muscle. We then list these joint actions for that muscle in our muscle atlases and the student/therapist comes away with the likely impression that the muscle can create each of these cardinal-plane component joint actions. But it cannot.¹

To better understand this, let’s compare joint action terminology to geography terminology. If a person is walking northeast, in geography terms, we can combine the terms north with east, to create the term northeast, and if we say that the person is walking northeast, it is understood that the person is walking neither purely north nor purely east, but rather diagonally northeast.

However, in muscle joint action terms, we are not allowed to combine cardinal-plane components into one oblique-plane motion pattern. A muscle like the coracobrachialis that moves the arm into both sagittal-plane flexion and frontal-plane adduction cannot be said to flexoadduct or adductoflex; the terms flexion and adduction cannot be combined. So, we must say the coracobrachialis flexes and adducts. The unfortunate result of this terminology is that many students and therapists come to believe the muscle can create the pure sagittal-plane movement of flexion and it can also create the pure frontal-plane movement of adduction. In reality, the coracobrachialis cannot. It must do a combination of the two: flexion with adduction—in other words, the oblique-plane motion of flexoadduction (Image 10).

This distinction in understanding can become important during palpation of the coracobrachialis. We usually ask a target muscle for palpation to contract/engage, so that it becomes hard and can be discerned from adjacent soft tissue. But what motion do we ask the coracobrachialis to perform during the palpation assessment? Flexion or adduction would engage it, but flexoadduction would better engage it, yielding a more precise palpation assessment. 

And we can use this understanding to better stretch the coracobrachialis. Given that it flexoadducts, we would most efficiently stretch it with extensoabduction—in other words, extension and abduction combined. 

Perhaps an even more elucidating example would be the supraspinatus. This is a favorite of mine to ask during a continuing education workshop. I ask the class, “What is the motion caused when the supraspinatus contracts and shortens?” And 99 percent of the workshop participants respond that the supraspinatus creates abduction of the arm at the shoulder joint. To this, I respond it is not possible. I then lead them to the answer by pointing out its line of pull is within the plane of the scapula, and the plane of the scapula is approximately 30 degrees off the frontal plane toward the sagittal plane, so the supraspinatus cannot do pure abduction. It must do a combination of abduction (in the frontal plane) with flexion (in the sagittal plane). This is a movement within the plane of the scapula, what some texts call scaption; scaption is essentially abductoflexion.

So how do we best palpate the supraspinatus? Ask the client to abductoflex the arm instead of doing pure abduction. That will best engage it to make it contract and pop during the palpation protocol so we can better feel it and discern it from adjacent musculature (Image 11). And how do we stretch it? Not by doing the opposite of abduction, in other words, adduction; but by doing the opposite of abductoflexion, which would be adductoextension. So, we bring the client’s arm into both adduction (in the frontal plane) and extension (in the sagittal plane) (Image 12). The coracobrachialis and supraspinatus are two good examples. But there are countless others that could be explored for how the use of joint action terminology affects how we perform manual therapy assessment and treatment skill sets. 

My point is that using cardinal-plane joint actions as the terminology to describe muscle function is problematic and leads many students and experienced therapists to have a rigid and inappropriate understanding of how muscle function works, and, therefore, ineffective applications of this understanding to manual and movement therapy skill sets, with possible adverse effects for our clients. 

Where Do We Go From Here?

For naming muscle attachments, my goal would be to eliminate the automatic designation of origin/insertion terminology. Instead, name them by their location: proximal/distal, superior/inferior, medial/lateral, anterior/posterior, etc. (or even perhaps combine these terms to name attachments as superomedial to inferolateral, posterolateral to anteromedial, etc.). If we continue with the use of the terms origin and insertion, we do so only when we are examining a specific kinesiologic circumstance in which we want to name which attachment in that scenario is fixed/stable and which one is mobile/moving. 

As for joint action terminology? I realize this becomes more problematic to entirely jettison this terminology. I recommend we begin the description of the function of a muscle by first stating its motion pattern, which will likely be an oblique-plane motion pattern; for example, flexoadduction for the coracobrachialis and abductoflexion for the supraspinatus. Then, after exploring the oblique-plane motion pattern, lay out a list of the cardinal-plane component joint actions that comprise it. This way, it is clear to the reader that joint actions are simply components of a larger oblique-plane motion pattern for the muscle. And when the motion pattern for a muscle is simply a cardinal-plane motion, such as the brachialis muscle whose function is to flex the elbow joint, which is a pure sagittal-plane motion, then state that its motion pattern is a cardinal-plane joint action.

I believe that adopting these terminology changes will allow for a simpler yet more profound understanding of muscle function. And this will promote greater critical reasoning that will allow us to be empowered to creatively apply our hands-on assessment and treatment skill sets, thereby becoming more effective therapists. 

Note

1. A muscle can create a cardinal-plane movement if its one line of pull is within that cardinal plane, or if the muscle has multiple lines of pull, with one or more of its lines of pull being within a cardinal plane. But the typical muscle has one line of pull, and that line of pull is within an oblique plane, so when it concentrically contracts, it must create the oblique-plane movement; it cannot create the separate cardinal-plane component joint actions. 

Suzy

I am reminded of the story of Suzy, a 5-year-old girl who sees her mother place a large ham for dinner onto a cutting board and cut off three inches from each end before placing it in a tray and putting it in the oven. She asks her mother why she does this and her mother replies that her mother taught her to do this. So, they drive down the street to where Suzy’s grandmother lives and they ask her why she taught Suzy’s mom to do this, and she replies because her mother taught her to do it this way. So, they go downstairs to where Suzy’s great-grandmother lives, and they ask her why she taught Suzy’s grandmother to do this. She replies: “Well, when I was young, the oven wasn’t large enough to fit the ham, so we had to cut off some from each end.” The point here is that there might have once been a good reason to do or learn something a certain way, but that reason might no longer exist; yet we continue to teach and learn this way just because we have always taught and learned this way.

Open-Chain and Closed-Chain Kinematics

Open-chain and closed-chain kinematics describe motions wherein the distal segment of a chain of body parts is either free to move in the air or fixed against a stable surface or object. This distal segment is the hand in the upper extremity (shoulder girdle, arm, forearm, hand) or the foot in the lower extremity (pelvic girdle, thigh, leg, foot), or even the head in the axial body (trunk, neck, head). If it is free to move in the air, it is described as open-chain, and therefore easily able to move. If it is fixed against a stable surface or object, it is described as closed-chain, and not easily able to move.

For example, when I concentrically contract flexors across the elbow joint, they can either flex the forearm at the elbow joint or they can flex the arm at the elbow joint. If my hand is in the air and not fixed against a stable surface/object, then the forearm (along with the hand, which must move with it) offers less resistance to moving than does the arm (as well as the rest of the upper body, which must move with it). So, the forearm flexes at the elbow joint (Image A). In other words, the insertion/distal attachment/forearm moves toward the origin/proximal attachment/arm. This is the classically thought of joint action that occurs when a muscle contracts. 

But if my elbow joint flexors contract, and my hand is fixed/stable, holding onto a chin-up bar for example, then because my hand and forearm cannot move, my arm must move instead. And I get the closed-chain movement of flexion of the arm at the elbow joint; the arm moves toward the forearm instead of the forearm moving toward the arm (Image B). Some sources describe these closed-chain movements as reverse actions. These closed-chain/reverse actions result in the proximal attachment moving toward the distal attachment, or in other words, the origin moves toward the insertion.

Hip Joint Functional Muscle Groups

Open-Chain (Insertion Moves)

Flexors

Extensors

Abductors

Adductors

Lateral rotators

Medial rotators

Closed-Chain (Origin Moves)

Anterior tilters

Posterior tilters

Depressors 

Elevators 

Contralateral rotators

Ipsilateral rotators 

Muscle Contractions

The function of musculature is to contract. Concentric contractions are shortening functions of a muscle that cause movement of one or both of the muscle’s attachments. Eccentric contractions are lengthening contractions that slow down/modify movement caused by another force, usually gravity; and isometric contractions are same-length contractions that by virtue of not changing their length, help to stabilize an attachment of another muscle so that the other attachment of that other muscle can move more efficiently.