Shoulder impingement is arguably the most diagnosed musculoskeletal shoulder issue, especially in the functional fitness, weightlifting, and tactical worlds requiring repetitive overhead movements and heavy loading. Unlike most other musculoskeletal diagnoses, shoulder impingement is more of a biomechanical syndrome and movement dysfunction than an actual anatomical pathology. This is important to understand because, despite having the same diagnosis, those suffering from shoulder impingement can have varying impacted structures, symptoms, limitations, and causes. The goal of this article is to provide this physical therapist’s general overview of shoulder impingement and potential causes. Future posts will focus on possible treatment strategies based on identified impairments and limitations.

When researching shoulder impingement, you may see more specific diagnoses including internal versus external impingement. For this series, we will discuss external impingement, which is predominantly seen in these communities.

Simply, impingement, like the word “impinge” implies, is a mechanical encroachment or invasion of space in the shoulder, primarily in the sub-acromial space between the ball-and-socket joint and the acromion (boney roof over the joint). This encroachment can increase friction and pinch the structures running through this space. These include the joint capsule (the bag that wraps around the ball-and-socket joint), rotator cuff muscles and tendons, bursa (fluid filled sacs to reduce friction), and one of the two bicep tendons. The body is usually resilient enough to withstand the occasional pinch and rub, but repetitive exposure overtime develops associated pathology like tendonitis, bursitis, and microtears.

Fortunately, shoulder impingement is dependent on motion and is not a constant, always present, encroachment unlike a large disc bulge pinching a nerve. Typically, the sub-acromial space is unimpacted when the shoulder is resting, and most folks dealing with impingement are fine if the arm stays at or below shoulder height. Why is this? Well, like mentioned earlier, impingement is a mechanical syndrome; therefore, symptoms are dependent on shoulder motion (aka shoulder mechanics). Let me further elaborate.

The shoulder joint is more than just the ball-and-socket joint. It also consists of the acromioclavicular joint (aka AC Joint, where collar bone meets the shoulder blade) and the scapulothoracic joint (the shoulder blade floating along the rib cage). For you to raise your arm overhead, motion occurs in all three, especially the ball-and-socket and scapulothoracic joints. Initial movement is primarily the ball rotating and gliding in the socket; however, that only accounts for about 50-70% of the movement, depending on which way it is being raised. The rest of the movement comes from the shoulder blade gliding along the rib cage and rotating in order to point the socket upward. Shoulder impingement occurs when this combination of joint movements isn’t clean and the humerus (arm bone) jams up against the acromion, pinching all the structures between. Let’s take a moment to review some potential reasons this occurs in different individuals and why treatment needs to be individualized based on the person’s impairments and not the diagnosis alone.

Mobility, or lack of, is often a contributing factor to shoulder impingement. We will see folks strong as ox in their readily available range of motion; unfortunately, they oftentimes lack the mobility to easily get into a full overhead position with pressing, snatching, and pull-ups. Common contributors include tight muscles connecting directly to the humerus (ex: lats, pec major) and limited extension of the thoracic spine (mid/upper back). Additionally, postural limitations can play a big impact as it impedes the scapula’s ability to glide and rotate. Ideally, at rest while sitting or standing upright, the shoulder blade is vertical. With a rounded upper back and forward-dumping shoulder (commonly seen with a slouched position), the scapula is no longer vertical and now tilted and rotated forward. This puts the shoulder in a disadvantaged position as the scapula is now unable to fully point the socket upward when raising the arm overhead. To illustrate this, sit/stand with “perfect” posture then raise your arm overhead. Now, sit/stand with “bad” posture (think Hunchback of Notre Dame) and do the same. Feel the difference?

Not everyone with shoulder impingement needs mobility work because not everyone is “tight”. Plenty of weightlifters and CrossFit athletes have the available range of motion to get into these overhead positions; however, they may lack the strength and stability to support the movement, especially under load. Insufficient strength can alter the movement mechanics and rhythm of shoulder elevation or prevent unwanted micro movements within the joint. An example of altered mechanics may be decreased strength around the shoulder blade, impairing its ability to glide and rotate smoothly and timely along the rib cage. If the rhythm is off, the scapula will not be able to move the acromion out of the way in time as the humerus continues to rise, resulting in impingement. If the person has good strength and mechanics to get overhead, they may lack stabilizing strength while in that position under load. For example, the rotator cuff muscles ensure the ball is and stays seated in the socket. Weakness here can create micro movements within the joint that can result in impingement. So, as you can see, not  everyone with shoulder pain is tight and needs to perform banded distractions from the pull-up bar.

The last contributing factor to shoulder impingement I want to quickly address is general positioning and control. Individuals that fall into this category have the readily available range of motion and strength to support the movement but have some possible movement flaws that place their shoulders in a vulnerable position. For example, I may see an athlete who only has pain at the end range of overhead pressing. When reviewing their press, they press beyond the vertical position (bar directly overhead with bar, head, shoulders, torso stacked in line when viewed from the side) and actually end with the bar more rearward, placing more strain on the shoulder by jamming the humerus into the acromion. We will see similar issues with snatches and overhead squats, especially in the bottom of the squat position. Another example is kipping pull-ups where the athlete generates too much swinging momentum with their torso and lower body resulting in excessive overhead flexion. Unlike the mobility and strength impairments previously discussed, these are treated with proper coaching, cuing, and consistent clean repetitions under lighter loads.

In summary, shoulder impingement is a mechanical syndrome and not an anatomical diagnosis. It occurs when repetitive strain is applied to the structures located in the sub-acromial space. Contributing factors can vary and include mobility limitations, strength deficits, and movement flaws.

I hope you have found this to be useful. Stay tuned for future articles with ways to address these impairments!

Dr Pat Casey PT, DPT, OCS, CSCS