Shoulder Series Part 1: Shoulder Anatomy and Biomechanics for the Fitness Professional

Shoulder Series Part 1: Shoulder Anatomy and Biomechanics for the Fitness Professional

As fitness professionals, managing client injuries is a part of the job that can be rather daunting. Rarely do I ever sit down with a client for an assessment without at least one or two orthopedic issues come up. From varying ends of the severity scale, you are sure to encounter some injury that you are going to have to address and/or work around. While as a fitness professional you are not responsible for treating or rehabbing an injury, it is imperative that you have an underlying understanding of what you can do to help that individual reach their fitness-related goals in the safest possible way. Having the knowledge of pertinent anatomy, biomechanics, and their implications as it relates to exercise technique and programming will help you to better serve your clients. During this four-part series, I am going to dive in and take a closer look at the shoulder. In part 1, we will discuss the pertinent anatomy, biomechanics, and some common problems that you are likely to see walk into the gym.

When looking at the biomechanics of the shoulder, the motion comes from both the scapulothoracic (ST) joint and the glenohumeral (GH) joint. It is the synergism of these joints and the muscles acting on them that produce smooth and full movement while we perform pressing and pulling movements in the gym. Understanding the movement and function at both the scapula and the upper arm in isolation allows you to better understand shoulder biomechanics as a whole. In addition, being able to differentiate scapular musculature and movement from upper arm musculature and movement will impact the way that you assess, program, and ultimately train your clients. Conceptually, taking a deeper look at the function of the rotator cuff and the overall biomechanics of the scapular musculature will aid in your understanding of the mechanics that can potentially cause injury.

Function of the Rotator Cuff

The rotator cuff is undoubtedly the most criticized culprit as the cause of shoulder pain. The rotator cuff is comprised of four muscles;  three muscles (supraspinatus, infraspinatus, and teres minor) work to externally rotate the shoulder while one muscle (subscapularis) works to internally rotate the shoulder. So, what is it that causes this group of small muscles to gain all the notoriety for being the linchpin of shoulder pain or injury? The rotator cuff plays a vital role in both the stabilization of the GH joint and allows for full shoulder range of motion. While the muscles that comprise the rotator cuff produce movements of internal rotation and external rotation, the main function of the rotator cuff is joint centration.

Joint centration is a neuromuscular strategy in which your Central Nervous System employs synergistic muscular activity to place the joint in the most mechanically advantageous position. Simply put, the rotator cuff places the head of the humerus in the best position in relation to the glenoid fossa of the scapula to allow for the bigger muscles to produce movement. The idea of joint centration for the rotator cuff is important for two different tasks of the shoulder when it comes to movement. First and foremost, “packing the shoulder” is a term that places the head of the humerus in the most centered, stable, and safe position to do any exercise of the upper body. Packing the shoulder is a combination of external rotation from the rotator cuff and a downward rotation of the musculature of the scapula. Secondly, a force couple between the supraspinatus and the other three rotator cuff muscles occurs to pull the head of the humerus medially (supraspinatus) and inferiorly (infraspinatus, teres minor, and subscapularis) to allow room for the humerus to not run into the acromion of the scapula. If the rotator cuff (or scapular musculature) does not work properly, the head of the humerus will come into contact with the underside of the acromion of the scapula and can lead to impingement, which will be discussed in a later article.

Scapular Biomechanics

The scapula (often referred to as the shoulder blade) sits behind the rib cage to form the scapulothoracic (ST)  joint. While it is not a true articulation, the ST joint can, and should, move throughout a range of motions to allow for the shoulder complex as a whole to obtain full range-of-motion. There are 17 muscles that act on the scapula to produce the movements of elevation/depression, protraction/retraction, and upward/downward rotation. The muscles that act on the scapula do not act on the humerus directly, but function to put the scapula in the most efficient position to indirectly allow for movement of the arm. Similar to the rotator cuff, the most important function of the scapular musculature is to pack the shoulder and allow for full overhead range-of-motion.

Previously, we talked about “packing the shoulder” from a rotator cuff perspective, but involvement from the rotator cuff alone will not be enough to have the shoulder function optimally. In order to achieve this stable, close-packed position, the scapula must be downwardly rotated and depressed. To achieve this position, the lower trapezius, and pectoralis minor work to depress the scapula while the rhomboids work to downwardly rotate the scapula. This achieves this close-packed and stable position for the shoulder to optimally perform many pressing and pulling movements. In addition, the latissimus dorsi can potentially aid in the stabilization of the scapula while working to maintain this close-packed position. While the lats do not directly attach to the scapula, they sit over the top of the inferior border of the shoulder blade and, when activated, can help in depression and downward rotation. This is important when you are asking your shoulder to remain in that close-packed position while deadlifting or performing heavy carries, the size and strength of the lats can do a lot to maintain this position when the loads exceed the capacity of the smaller scapular musculature. Simply put, when the load gets heavy enough, the rhomboids and lower traps are not strong enough to maintain that position without the extra help from the lats.

Providing coaching cues to help clients achieve this position, such as, “put your shoulder blades in your back pocket,” can go a long way to help clients achieve a stable and effective shoulder position for movement. Oftentimes people will mistake scapular retraction for this downward rotation and depression. Instead of driving their shoulder blades down, they attempt to pinch them together. This does not allow for the GH joint to be placed in an optimal position and also will cause a decrease in performance as the scapular retractors are not strong enough to support heavy loads during heavy sets of deadlifts.

Conversely, the scapula also plays a large role in the ability for you to go overhead. In order to achieve full flexion and abduction of the shoulder, a 2:1 ratio of movement between the GH joint and the ST joint must occur. So, of the 180 total degrees of motion, 120 degrees comes from the GH joint and 60 degrees comes from the ST joint. The scapula must upwardly rotate to allow for there to be enough room for the head of the humerus not to impinge on the acromion process when the arm moves overhead. To achieve this upward rotation, a combination of action between the upper trapezius, lower trapezius, and serratus anterior must occur. It is imperative that all three of these muscles act in unison or true upward rotation will not occur.


Muscle Action
Upper Trapezius Elevation and Upward Rotation
Lower Trapezius Depression and Upward Rotation
Serratus Anterior Protraction and Upward Rotation

As you can see from the chart above, while all three of these muscles produce upward rotation of the scapula, they also produce other movements. Namely, the upper trapezius and lower trapezius are responsible for opposing actions (elevation and depression). If you have a client that has overactive upper traps and inhibited or weak lower traps, the resultant motion is the elevation of the scapula and a disorganized upward rotation. This results in the ever common shrugging of the shoulders to allow for them to get their arms overhead.

While anatomy and biomechanics can be a daunting task to learn and understand, it sets the framework for assessment, programming, and the execution and performance of training. In the later sections of this series, we will take this foundational anatomy knowledge and discuss how it applies to specific aspects of assessment, common problems/injuries that you will see, and considerations for programming and exercise performance.

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Jordan is an Athletic Trainer and strength coach based out of Richmond, VA. He earned a Master of Science degree in Kinesiology with an emphasis in Sports Medicine from Indiana University and a Bachelor of Arts degree in Athletic Training from Franklin College. Jordan holds certifications from the BOC and NATA as a Certified Athletic Trainer (ATC) and the NSCA as a Certified Strength and Conditioning Specialist (CSCS). Jordan’s goal is to bridge the gap between rehabilitation and performance by combining both rehabilitative and strength and conditioning principles. Contact: [email protected] or at

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