Students and therapists often get a fixed idea of a muscle’s functional joint action(s). We look up the muscle we want to learn about in our muscle atlas textbook, and we read that it either does or does not do a certain joint action. And we’re done, right? Not necessarily.
Muscle function is actually much more fluid than this. After all, ultimately, a muscle’s joint action is based on its line of pull relative to the joint it crosses. And this relative line of pull can change if the position of the bones of the joint changes. To understand this better, let’s look at a few examples in the body, beginning with a muscle that could be called a de-rotator.
Coracobrachialis—A De-Rotator
The coracobrachialis attaches from the coracoid process of the scapula to the medial side of the shaft of the humerus (Image 1A). Because it crosses the glenohumeral (GH) joint anteriorly and medially, it flexes and adducts the arm at the GH joint. But because it attaches on the medial side of the humeral shaft, it does not have the ability to rotate the humerus at the GH joint. Or at least, it does not have the ability to create rotation when the body is in anatomic position, which is the position we usually use to state a muscle’s joint action(s).
However, if the humerus is first laterally rotated, then the coracobrachialis wraps around the shaft of the humerus, and its humeral attachment ends up instead being located more anterolaterally relative to the trunk. If the coracobrachialis contracts and shortens from this position, it will pull on the humerus, rotating it medially back to anatomic position (Image 1B). So, the coracobrachialis is a medial rotator of the arm at the GH joint, but only if the arm is first in a position of lateral rotation.
If, instead, the humerus is first medially rotated relative to anatomic position, then the coracobrachialis has to wrap around the shaft of the humerus in the opposite direction, and its humeral attachment ends up being located more posterolaterally relative to the trunk. If the coracobrachialis contracts and shortens from this position, it will pull on the humerus, again rotating it back to anatomic position. But this motion is lateral rotation of the humerus at the GH joint (Image 1C). So, the coracobrachialis can also be a lateral rotator of the arm at the GH joint, but only if the arm is first in a position of medial rotation.
Because the coracobrachialis medially rotates a laterally rotated arm back to anatomic position, and laterally rotates a medially rotated arm back to anatomic position, it can be called a de-rotator. It eliminates the position of arm rotation regardless of whether the arm is laterally or medially rotated. So, is the coracobrachialis a medial rotator, a lateral rotator, or not a rotator at all? The answer is yes, yes, and yes, depending on the position the humerus is in when the motion starts.
This is an important concept to understand, because our clients do not always begin motions from anatomic position. So, when we try to understand if the coracobrachialis of the client is being used, and perhaps overused and injured during a motion of their body, we have to consider the fact that the coracobrachialis might have engaged for medial rotation or lateral rotation, even though our textbook might not state that it can rotate the arm. And certainly, this concept can be widened to all muscles that can change their joint action when there is a change in the posture of the body. So, let’s look at a few more examples.
Brachioradialis—Pronator and Supinator
The brachioradialis attaches from the lateral supracondylar ridge of the humerus (immediately proximal to the lateral epicondyle) to the styloid process of the radius (Image 2). Therefore, it crosses the elbow joint anteriorly and can flex the forearm at the elbow joint.
But can it also pronate or supinate the forearm (at the radioulnar joints), or does it have no pronation/supination capability? The answer again is yes and yes, depending on the position of the forearm when the motion started. From anatomic position (Image 3A), which is a position of full supination, the brachioradialis can pronate the forearm, but only to a position that is approximately halfway between full supination and full pronation (Image 3B). Why? Because this is the position that brings the radial styloid as close as possible to the lateral supracondylar ridge of the humerus. After all, fundamentally, a muscle’s joint action is its concentric contraction, which means that it shortens and brings the muscle’s attachments as close to each other as possible.
But if the forearm begins in a position of full pronation (Image 3C), then the brachioradialis would have the ability to supinate the forearm, once again to a position that is approximately halfway between full supination and full pronation, bringing the radial styloid as close as possible to its humeral attachment (Image 3B).
So, the brachioradialis can be a pronator or it can be a supinator. Or, if the forearm were to start in the position that is approximately halfway between full supination and full pronation, then the brachioradialis would have no pronation/supination capability at all.
Anterior Scalene—Contralateral Rotator or Ipsilateral Rotator?
The anterior scalene attaches from the transverse processes of the cervical spine (C3–C6) to the first rib—near the border of the rib with its costal cartilage (Images 4A–4C). There is a lot of controversy as to whether the anterior scalene is a contralateral or ipsilateral rotator of the cervical spine; some textbooks state that it contralaterally rotates and some state that it ipsilaterally rotates. So, is it a contralateral rotator, an ipsilateral rotator, or not a rotator at all? Of course, once again, the answer is yes, yes, and yes.
Let’s examine this question by looking at the rotation capability of the right anterior scalene. Beginning with the fundamental concept that when a muscle concentrically contracts, it shortens and brings its attachments closer together, we can see that from anatomic position, as seen in Image 4A, when the right anterior scalene contracts, it will bring its transverse process attachments toward the right first rib attachment, which would result in rotating the cervical spine to the left, as seen in Image 4B. So, the right anterior scalene is a left rotator. In other words, from anatomic position, the anterior scalene is a contralateral rotator. However, looking at Image 4B, we see that the right anterior scalene rotates the cervical spine to the left, but it is important to realize that it can only rotate the cervical spine to the left approximately 45 degrees, which is the position in which its attachments are closest to each other.
If, instead, the neck were to first be in a position of left rotation beyond 45 degrees, as seen in Image 4C, the right anterior scalene would actually rotate its cervical spine attachment to the right, approximating the attachments toward each other as seen in Image 4B. So, from a position of left rotation that is greater than 45 degrees, the right anterior scalene is a right rotator (in other words, an ipsilateral rotator). And what if the cervical spine were first in a position of 45 degrees of rotation toward the opposite side of the body, as seen in Image 4B? Then, the anterior scalene would have no rotation capability at all.
Other Examples
There are many other examples in the human body of muscles that change their joint action when the position of the body changes. Image 5 shows the clavicular head of the pectoralis major. From anatomic position, it is an adductor of the arm at the GH joint because its line of pull is below the (anteroposterior) axis of motion for frontal plane motion (see right side of the client’s body). But if the arm were first abducted approximately 100 degrees or more, the line of pull moves to be above the axis, so the clavicular head of the pectoralis major becomes an abductor of the arm (see left side of the client’s body).
Images 6A–6B depict the adductor longus. From anatomic position, the adductor longus is a flexor of the thigh at the hip joint (in addition to being an adductor). And if the thigh is in extension, it is also a flexor (Image 6B). But if the thigh is in full flexion, then the line of pull of the adductor longus moves posterior to the (mediolateral) axis of sagittal plane motion and the adductor longus becomes an extensor (Image 6A). The advantage to this ability to change its joint action is that when walking or running, the adductor longus contributes to both flexion and extension of the thigh depending on the position during the gait/running cycle.
Perhaps one of the most well-known examples of a muscle that can change its joint action is the piriformis (Image 7). From anatomic position (Image 8A), the piriformis is a lateral rotator of the thigh at the hip joint because its line of pull passes posteriorly to the axis of motion in the transverse plane. But if the thigh is first flexed approximately 60 degrees or more (Image 8B), the piriformis becomes a medial rotator because its line of pull is now anterior to the axis of motion. For a video demonstration, visit www.learnmuscles.com/mb to watch “Transverse Plane Motions of the Piriformis.”
Conclusion
Learning about muscles that change their actions when the position of the body changes might seem trivial or esoteric. But it is actually fundamental to not only our understanding of muscle function, but also our ability to apply our hands-on assessment and treatment skills. For example, understanding the rotation capability of the anterior scalene informs how we perform our orthopedic assessment test for the anterior scalene syndrome version of thoracic outlet syndrome. It explains why Adson’s test for this condition has the client ipsilaterally rotate to stretch the anterior scalene, but why the alternative anterior scalene syndrome orthopedic test, Halstead’s test, has the client contralaterally rotate the neck instead. Either end range will stretch the anterior scalene. As another example, understanding the piriformis’s change of function determines how we stretch it. In anatomic position, we need to medially rotate the thigh to stretch it, but if the thigh is sufficiently flexed, we now need to move the client’s thigh into lateral rotation to stretch it.
Understanding and being able to figure out muscle function, regardless of the position of the body, empowers us to be able to critically think through, understand, and see muscle function, which in turn allows us to understand postural and movement dysfunctions, as well as the mechanics of hands-on assessment and treatment techniques. Armed with this knowledge, we will be better equipped to help treat our clients and have a successful clinical orthopedic manual therapy practice.
On-Axis and Off-Axis Attachments of a Muscle
In order to understand the concept of whether a muscle can or cannot create rotation (long axis rotation), we need to see where the muscle attaches relative to the axis of motion for rotation. Images A and B show a generic muscle that crosses a hypothetical joint. Image B shows this muscle attaches from one bone (designated as fixed) to the other bone (designated as mobile), and attaches (“on-axis”) directly over the axis of motion for rotation. Therefore, when this muscle contracts and shortens, it would pull the mobile bone toward the fixed bone, but it would not create any rotation. But in Image C, the muscle is instead shown attaching “off-axis,” to one side. Now, the muscle would pull the mobile bone toward the fixed bone but would also rotate the mobile bone, as represented by the arrow seen in the figure. And in Image D, the muscle is now seen attaching “off-axis” on the other side, and the arrow in the figure shows that the muscle now has the opposite rotation motion. The ability to create rotation, or any motion for that matter, is based on whether the muscle in question attaches over the axis of motion—in other words, on-axis—or whether the muscle attaches off-axis. Permission Joseph E. Muscolino, Kinesiology: The Skeletal System and Muscle Function, 3rd ed. (Elsevier, 2017).
Rubber-Band Buddies
There is a simple way for two students or therapists to work together to figure out a muscle’s action(s). Working in a pair as “rubber-band buddies,” one is the client and the other is the therapist. A colorful rubber band is placed over the client’s body to represent the muscle in question, holding each end of the rubber band on the body to represent the two attachments of the muscle. Then, while the client holds one end of the rubber band fixed, the therapist moves the other end toward the fixed end, directly along the line of the rubber band. The movement of the body part is the action(s) (or motion pattern) of the muscle. Then, change which end is fixed and reverse the process, and you will have the reverse action of that action of the muscle. (Note: given that a rubber band is elastic, be careful when performing this exercise near the face, because if one end is accidentally let go, the rubber band could hit the client in the face. When working near the face, perhaps a colorful shoelace would be preferable.) For a video demonstration of this technique, visit https://learnmuscles.com/mb.
Critically Thinking to Figure Out Muscle Actions
The ability to understand muscle actions is incredibly simple. It is dependent on a few concepts that most every therapist learned in their first science class at school, but perhaps never applied once the class was over.
1. When a muscle contracts, it pulls.
2. A movement of the body can be described as occurring in a plane.
3. Most movements (axial movements) occur around an axis.
Simply put, when a muscle contracts, it creates a pulling force that can move a body part within a plane, and assuming that the movement is an axial movement (most movements are), then that movement occurs within the plane around an axis. If these concepts sound familiar, then you are empowered to be able to critically think through, understand, and see muscle function. All we need to do is look at the line of pull of the muscle and see what plane it is in and where it is relative to the axis at the joint; this will tell us the joint motion created by the muscle in the plane around the axis. Knowing these fundamental basics of kinesiology allows you to figure out the action or actions of any muscle of the body, regardless of the position the body is in when the muscle contracts.
Joseph E. Muscolino, DC, has been a manual and movement therapy educator for more than 30 years. He is the author of multiple textbooks, including The Muscular System Manual: The Skeletal Muscles of the Human Body (Elsevier, 2017); The Muscle and Bone Palpation Manual with Trigger Points, Referral Patterns, and Stretching (Elsevier, 2016); and Kinesiology: The Skeletal System and Muscle Function (Elsevier, 2017). He is also the author of 12 DVDs on manual and movement therapy and teaches continuing education workshops around the world, including a certification in Clinical Orthopedic Manual Therapy (COMT), and has created Digital COMT, a video streaming subscription service for manual and movement therapists, with seven new video lessons added each and every week. Visit www.learnmuscles.com for more information or reach him directly at joseph.e.muscolino@gmail.com.