Straightforward Shoulder Anatomy and Biomechanics for Personal Trainers

Straightforward Shoulder Anatomy and Biomechanics for Personal Trainers

Managing client injuries is a part of the job that can be daunting for personal trainers. I rarely sit down with a client for an assessment without at least one or two orthopedic issues coming up. You are sure to encounter clients with injuries that vary in severity, and you will have to address or work around them. Though personal trainers are not responsible for treating or rehabbing injuries, 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. You will better serve your clients if you have the knowledge of pertinent anatomy, biomechanics, and how they affect exercise technique and programming.

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.

Intro to the Rotator Cuff

When looking at the biomechanics of the shoulder, the motion comes from 2 joints: 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 exercises in the gym. Understanding the movement and function at both the scapula and the upper arm in isolation allows you to better understand the biomechanics of the whole shoulder. 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.

The #1 Culprit of Shoulder Pain

The rotator cuff is undoubtedly the most common cause of shoulder pain. The rotator cuff is comprised of four muscles: three (supraspinatus, infraspinatus, and teres minor) externally rotates the shoulder, while one (subscapularis) internally rotates the shoulder. So, what 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 the stabilization of the GH joint which allows 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.

Shoulder Anatomy and Biomechanics Made Simple

Joint centration is a neuromuscular strategy in which your Central Nervous System helps muscles work together 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 with the glenoid fossa of the scapula so bigger muscles can produce movement. Joint centration for the rotator cuff is important for two different tasks of the shoulder when it comes to movement.

First, “packing the shoulder” 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.

Second, force between the supraspinatus muscle and the other three rotator cuff muscles pull the head of the humerus toward the midline and inferiorly to allow room for the humerus. If the rotator cuff does not work properly, the head of the humerus will come into contact with the underside of the acromion of the scapula which can lead to impingement.

The scapula (AKA shoulder blade) sits behind the rib cage to form the scapulothoracic (ST) joint. The ST joint moves through a series of motions to allow the whole shoulder complex to reach 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 function to put the scapula in the most efficient position to indirectly allow for movement of the arm. Like 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” with a focus on the rotator cuff muscles. 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 (lats) can potentially aid in the stabilization of the scapula while working to maintain this close-packed position. The lats do not directly attach to the scapula, they sit over the top of the inferior border of the shoulder blade. When activated, they help in depression and downward rotation. This is important when you are asking your shoulder to remain in the 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 muscles. 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.

Coaching Notes

Providing coaching cues 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. Clients often mistake scapular retraction for downward rotation and depression. Instead of driving their shoulder blades down, they attempt to pinch them together. This does not put the GH joint to be placed in an optimal position and causes a decrease in performance, because the scapular retractors are not strong enough to support heavy loads during heavy sets of deadlifts.

The scapula also plays a large role in the ability for you to go overhead. A 2:1 ratio of movement between the GH joint and the ST joint must occur to achieve full flexion and abduction of the shoulder. 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 enough room for the head of the humerus, so it does not impinge the acromion process when the arm moves overhead. To achieve this upward rotation, a combination of actions 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 happen.


Upper TrapeziusElevation and Upward Rotation
Lower TrapeziusDepression and Upward Rotation
Serratus AnteriorProtraction and Upward Rotation

As you can see from the chart above, all three of these muscles produce upward rotation of the scapula, but they also produce other movements. For example, 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 motion presents as the elevation of the scapula and a disorganized upward rotation. This results in the too-common shrugging of the shoulders to allow them to get their arms overhead.

How to Apply Your Anatomy and Biomechanics Knowledge

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 remaining sections of this series, we will take this foundational anatomy knowledge and discuss how it applies to specific aspects of assessment, common problems and injuries 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: or at

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