How do the rotator cuff muscles support movement in the body?
In our previous article we went over the most commonly reported movements of the rotator cuff muscles. In this article we’ll dig a little deeper and take a tour through what researchers report about how the rotator cuff muscles support movement in the body in real life. We’ll look at how the rotator cuff muscles work together to do an important job and what other movements they can assist with beyond their most basic actions.
We’ve previously written about the origin, insertion, and main actions of the rotator cuff group of muscles. But, you already know that individual muscles don’t act alone when we’re doing organic movements in daily life. There are always additional muscles that act as synergists to create a particular movement and other muscles that act as antagonists that resist a movement. Additionally some muscles assist in creating an action rather than act as the prime mover. In this article we’ll dig a little deeper and look more specifically at where and how the rotator cuff muscles contribute to movement.
Why do we have a rotator cuff at all?
The glenohumeral joint allows for a greater range of movement than any other joint in the body (Chaitow and DeLany, 2008). This particular ball and socket joint has a shallow socket (the glenoid fossa of the scapula), where the ball (the head of the humerus) sits much more outside the surface area of the socket than in our other ball and socket joint, the hip joint.
All this available mobility is important to allow us to reach our arm in every direction and place our hands in position to do all kinds of vital everyday movements from eating to driving to brushing our hair to using tools. All of this mobility could be problematic however, if we didn’t have muscles that also contribute to stability of the shoulder joint. This is why the healthy functioning of our rotator cuff muscles is so important.
Researchers have suggested that the existence of our rotator cuff is directly related to supporting the movements that we do. Those researchers who looked at the evolution of the four muscles that make up the rotator cuff found that quadruped species (like cats and dogs) do not have a true rotator cuff (Sonnabend and Young, 2009). The four individual muscles are still present, but their tendons do not come together to form the blended tendons that create the “cuff” over the glenohumeral joint that we’re familiar with as the rotator cuff.
The true rotator cuff was only found in more advanced primates (Sonnabend and Young, 2009). The researchers have suggested that the reason for the anatomical change that occurred in advanced primates was due to their habit of frequent overhead reaching actions as well as the action of reaching out to the side. These are both actions that quadrupeds do not do (Sonnabend and Young, 2009).
What does the rotator cuff do?
What do the four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) do exactly? The rotator cuff muscles contribute to maintaining the balance of tension around the glenohumeral joint and maintaining the head of the humerus securely in the glenoid fossa (Dark et al., 2007; Sangwan et al., 2015). You could think of the rotator cuff muscles like guy wires that keep a central object, the head of the humerus, in balance by maintaining just the right amount of tension from each direction.
One group of researchers found that the amount of force that subscapularis can generate as an internal rotator is nearly equal to the combined force of the other rotator cuff muscles, two of which are external rotators (infraspinatus and teres minor) (Keating et al., 1993). This supports the idea that the rotator cuff muscles are important for maintaining the balance of tension in all directions around the shoulder joint.
The four rotator cuff muscles are often listed as stabilizers of the glenohumeral joint. But what does that mean? The most current research suggests that the rotator cuff muscles actually do something more subtle than just stabilize. They prevent translation of the head of the humerus when we are moving the shoulder joint (Sangwan et al., 2015). What do we mean by the term translation? In this case, translation refers to the movement of an object (the head of the humerus) in a line in the same plane of movement, in contrast to rotation where the object rotates around a point.
Preventing translation is subtly different than simply stabilizing. While we do need to keep the head of the humerus securely in the shoulder socket, we also need to allow for movement in all directions. When the rotator cuff muscles prevent translation, they’re allowing the head of the humerus to move, without letting it go too far in any one direction.
Maintaining this balance of tension is important in the long-term for just generally keeping our shoulder joint healthy. It can also be particularly important if you want to do sports or activities where you frequently do overhead movements away from the body (Alizadehkaiyet et al., 2015) or where you would be weight-bearing with the upper body. Balance between the force these muscles have on the humeral head allows the humeral head to stay securely in place while still allowing us to use the arm and other shoulder girdle muscles to do strong and/or dynamic movements with the upper body (Dark et al., 2007).
Movements and activity of the rotator cuff muscles
If we think of the main job of the rotator cuff muscles as maintaining the balance of tension around the head of the humerus in all types of actions, then it wouldn’t be hard to imagine that they might contribute to other movements besides the commonly reported ones, depending on the position that the arm and scapula were in.
When the arm is reaching up and out to the side (abduction), multiple rotator cuff muscles can contribute to this action, depending on the position of the arm. When the humerus is internally rotated and we abduct the arm, then infraspinatus joins in with supraspinatus to assist this action (Otis et al., 1994). When the humerus is externally rotated and we abduct the arm, the subscapularis works with supraspinatus to support that action (Otis et al., 1994). Hik and Ackland, 2019 reported that the rotator cuff muscles can also contribute weakly to depressing or elevating the humerus.
Both the intensity of the movement we’re asking the body to do (Alizadehkaiyet et al., 2015; Dark et al., 2007) and the position of the body (Wattanaprakornkul et al., 2011) impact which muscles are recruited to do an action and how much of the effort they contribute to. Wattanaprakornkul et al., 2011 found that supraspinatus and infraspinatus (as well as serratus anterior, deltoids, and trapezius) were more active when the shoulder was doing flexion, but subscapularis (as well as latissimus dorsi) were more active when the shoulder was doing extension.
You could also imagine that in more dynamic movements, where more overall power is needed, the rotator cuff muscles might contribute more activity generally. But, in simple and more passive movements, they might be recruited less or hardly at all.
Why is it important to use the rotator cuff for the right job?
Research suggests that part of maintaining a healthy shoulder girdle is recruiting the best muscles for the task or sport. It’s certainly possible, however, through overuse, improper form, or fatigue, for the body to recruit muscles that are not ideal as the primary source of power for that activity. In healthy study participants, Happee and Van der Helm, 1995 found that most of the energy used for the actions of flexion and extension came from the bigger thoracoscapular muscles that contribute to these actions, not the smaller rotator cuff muscles, which contributed only a small percentage of energy to those actions.
Muscle fatigue can change which muscles are recruited for an action and may result in recruiting less than ideal muscles to create a movement (Alizadehkaiyet et al., 2015). In fact, Alizadehkaiyet et al., 2015 reported that insufficient strength in the muscles that do external rotation was one contributing factor to shoulder injury in sports requiring repeated overhead actions. In order to maintain that balance of tension around the head of the humerus that we’ve been talking about, there needs to be sufficient activity from each of the muscles creating that tension. If individual muscles are fatigued, then that can affect the balance of tension between them.
The rotator cuff group of muscles are a special group of muscles with an important job. They allow us to do so many movements, both large and small, that are a part of our day to day activities and the sports that we enjoy. While they may not be the largest or most powerful muscles in the body, they have a vital role to play every time we move our arms.
Alizadehkaiyet, O., D.H. Hawkes, G.J. Kemp, and S.P. Frostick. 2015. Electromyographic analysis of the shoulder girdle musculature during external rotation exercises. The Orthopaedic Journal of Sports Medicine. 3(11): Pgs. 1-9.
Chaitow, L. and J. DeLany. 2008. Shoulder, Arm, and Hand. In Clinical Application of Neuromuscular Techniques. Volume 2 (Second Edition).
Otis, J.C., C.-C. Jiang, T.L. Wickiewicz, M.G.E. Peterson, R.F. Warren, and T.J. Santner. 1994. Changes in the moment arms of the rotator cuff and deltoid muscles with abduction and rotation. The Journal of Bone and Joint Surgery. 76-A(5):667-676.
Wattanaprakornkul, D., I. Cathers, M. Halaki, K.A. Ginn. 2011. The rotator cuff muscles have a direction specific recruitment pattern during shoulder flexion and extension exercises. Journal of Science and Medicine in Sport. 14:376-382.