What is Subacromial Impingement?

Subacromial impingement, also known as shoulder impingement syndrome, is a condition that affects countless people—athletes, desk workers, and those of us simply navigating daily life. If you or your clients have ever experienced sharp pain or a nagging ache in the shoulder, especially when lifting or reaching overhead, you’re likely familiar with the discomfort impingement can cause. This common shoulder issue arises when the tendons of the rotator cuff and bursa (a fluid-filled sac that reduces friction) get compressed or “impinged” beneath the acromion, a bony structure at the top of the shoulder.

Subacromial impingement can be incredibly painful, affecting your ability to perform even the simplest tasks. Because it restricts shoulder movement and often causes stiffness, it’s commonly confused with, or even misdiagnosed as, a frozen shoulder. However, there are critical differences between these two conditions, especially when it comes to effective treatment and recovery. As a manual therapist and athletic trainer, I’ve worked with clients across the spectrum—from professional athletes to those dealing with office-related strain—who suffer from this condition. Understanding what’s happening inside the shoulder and knowing how to address it effectively can make a huge difference in managing pain and improving function.

What Exactly is Subacromial Impingement?

In a healthy shoulder, the rotator cuff tendons and bursa glide smoothly beneath the acromion whenever we move our arm. However, subacromial impingement occurs when this space becomes narrowed, causing the tendons or bursa to get pinched. This impingement creates inflammation and irritation in the soft tissues, leading to pain, stiffness, and eventually, limited range of motion.

The shoulder is an extremely mobile joint, allowing us to lift, throw, push, pull, and rotate our arms in multiple directions. But because of its mobility, the shoulder is also vulnerable to injury, and even subtle misalignments or muscle imbalances can create big problems over time. In cases of subacromial impingement, this vulnerability is compounded by the narrow subacromial space. Activities that repeatedly involve lifting the arm overhead—think swimming, tennis, weightlifting, and even certain manual labor jobs—can contribute to or worsen impingement.

The pain of subacromial impingement often starts off as mild discomfort, easily mistaken for muscle soreness, and can gradually worsen over time if left unaddressed. It’s not uncommon for clients to try and “push through” the pain, only to find that it becomes unbearable during common movements, like reaching for an object on a high shelf or putting on a jacket.

The Anatomy Behind Shoulder Impingement

To understand subacromial impingement, it helps to know a bit about the shoulder’s anatomy. The shoulder is a ball-and-socket joint made up of the humerus (upper arm bone), scapula (shoulder blade), and clavicle (collarbone). The acromion is a bony projection of the scapula that sits above the shoulder joint, forming an arch-like structure over the rotator cuff tendons.

The rotator cuff itself is composed of four muscles—the supraspinatus, infraspinatus, teres minor, and subscapularis—that stabilize the shoulder and allow for a wide range of motion. The supraspinatus tendon, in particular, is prone to impingement because it passes directly under the acromion. Between the acromion and the rotator cuff is the subacromial bursa, a small, fluid-filled sac that acts as a cushion, reducing friction between the bones and tendons.

In a healthy shoulder, there’s just enough room for the tendons and bursa to slide freely. But in cases of subacromial impingement, this space is reduced, leading to irritation of the rotator cuff tendons and inflammation of the bursa. If the impingement persists, the constant friction can even cause tears in the rotator cuff, which adds another layer of complexity to the injury.

Symptoms of Subacromial Impingement: How to Recognize It

Subacromial impingement often presents with specific symptoms that can make it easier to distinguish from other shoulder conditions, like frozen shoulder. People with subacromial impingement usually experience pain when lifting their arm overhead, reaching behind their back, or lying on the affected side. The pain tends to be sharp or aching and is often localized to the top or front of the shoulder.

One of the classic signs of subacromial impingement is a painful arc between 60 and 120 degrees of shoulder abduction, meaning that lifting the arm in this range triggers the most pain. Other common symptoms include tenderness in the front of the shoulder, weakness in the arm, and a feeling of “catching” or “clicking” when moving the arm in certain directions.

Another sign to watch for is night pain. Many people with shoulder impingement find that their symptoms worsen when lying on the affected side, making it difficult to get a good night’s sleep. This nighttime discomfort is a significant quality-of-life issue for many clients, as lack of sleep can lead to fatigue, irritability, and even slower healing.

Subacromial Impingement vs. Frozen Shoulder: Key Differences

Subacromial impingement and frozen shoulder can seem similar because they both cause pain and stiffness in the shoulder. However, the two conditions are distinct, especially when it comes to how they develop and respond to treatment. Frozen shoulder, or adhesive capsulitis, is marked by a gradual loss of range of motion due to thickening and tightening of the shoulder capsule. The shoulder “freezes,” making it difficult to move in any direction, even passively.

With subacromial impingement, however, the shoulder doesn’t necessarily lose its range of motion entirely. Instead, it’s the overhead movements or specific ranges that become painful. While a person with frozen shoulder might struggle to lift their arm at all, someone with impingement can often still move their arm within certain limits, although pain may arise in specific positions.

Another difference lies in the underlying causes. Frozen shoulder is often idiopathic (meaning the cause is unknown) or related to systemic conditions like diabetes, while subacromial impingement is more likely related to repetitive use, muscle imbalances, or structural factors like bone spurs. This difference in etiology is critical because it influences the best treatment approach for each condition.

Causes of Subacromial Impingement

Several factors contribute to the development of subacromial impingement. These can range from structural issues within the shoulder to functional problems related to movement patterns, posture, and muscle balance.

One of the most common causes of subacromial impingement is repetitive overhead activity. Sports like swimming, baseball, and tennis, as well as occupations involving repetitive lifting, put significant strain on the shoulder, increasing the risk of impingement. Over time, these repeated movements can lead to inflammation, narrowing the subacromial space and placing more pressure on the rotator cuff tendons and bursa.

Poor posture is another major contributor. Rounded shoulders or a forward head posture can tilt the scapula in a way that reduces the subacromial space, making it easier for impingement to occur. In these cases, even everyday activities can trigger shoulder pain.

Additionally, muscle imbalances in the shoulder girdle can play a role. Weakness in the rotator cuff or the muscles surrounding the scapula can result in poor shoulder mechanics, forcing the rotator cuff tendons to work harder and creating an environment where impingement is more likely. Structural abnormalities, such as bone spurs on the acromion or a naturally curved acromion, can further narrow the subacromial space, contributing to impingement.

Diagnosing Subacromial Impingement: What to Expect

Diagnosis of subacromial impingement typically involves a combination of physical examination, patient history, and imaging studies. During a physical exam, a healthcare provider may perform specific tests to assess shoulder function and identify pain patterns. One common test is the Neer Impingement Test, where the arm is raised in a way that compresses the rotator cuff under the acromion. If this movement produces pain, it may suggest impingement. The Hawkins-Kennedy Test, which involves internally rotating the shoulder while the arm is lifted, can also provoke pain in cases of impingement.

In some cases, imaging studies like X-rays or MRIs are used to get a clearer view of the shoulder structure. X-rays can reveal bone spurs or other structural issues, while MRIs provide a detailed look at the soft tissues, such as the rotator cuff tendons and bursa. These images help confirm the diagnosis and assess whether additional injuries, like rotator cuff tears, are present.

Treating Subacromial Impingement: Conservative Options First

For many people, subacromial impingement can be effectively managed with conservative treatments, focusing on pain relief, restoring mobility, and improving shoulder mechanics. Physical therapy is often the cornerstone of treatment, as it helps address the underlying causes and reduces the risk of recurrence.

In physical therapy, clients work on strengthening the rotator cuff and scapular stabilizers, which help support the shoulder joint and relieve pressure on the impinged structures. Stretching exercises for the pectoral muscles, upper trapezius, and posterior shoulder can also improve posture, opening up the subacromial space.

In addition to physical therapy, modalities like ice and anti-inflammatory medications can help reduce pain and inflammation. Ice can be especially useful after activities that aggravate the shoulder, as it helps reduce swelling in the affected area. Over-the-counter NSAIDs, like ibuprofen, can provide temporary pain relief, although they should not be relied on long-term.

For cases that don’t respond to conservative treatments, a cortisone injection may be an option. A cortisone shot delivers powerful anti-inflammatory medication directly to the impinged area, reducing inflammation and providing relief for several weeks or even months. However, repeated cortisone injections aren’t typically recommended, as they can weaken the tendons over time.

When Surgery is Necessary

In cases where conservative treatments fail to provide relief, surgery may be considered. The most common surgical procedure for subacromial impingement is subacromial decompression, also known as acromioplasty. During this procedure, a surgeon removes part of the acromion or any bone spurs that may be reducing the subacromial space. This “clearing” of the space helps alleviate pressure on the rotator cuff tendons and bursa, allowing them to glide more freely.

While surgery is generally effective, it’s typically viewed as a last resort, given the risks associated with any invasive procedure. The recovery period can be lengthy, requiring a commitment to post-surgical rehabilitation to restore strength and range of motion.

 

Preventing Subacromial Impingement and Promoting Shoulder Health

One of the best ways to prevent subacromial impingement is to practice good posture and maintain balanced shoulder strength. Regularly strengthening the rotator cuff and the muscles surrounding the shoulder blade is essential, especially for individuals involved in overhead activities. Exercises like scapular retractions, external rotations, and face pulls help keep the shoulder stable and reduce the likelihood of impingement.

Maintaining flexibility in the chest, upper back, and shoulders can also help prevent postural imbalances. Incorporating stretches into a daily routine, particularly if you spend long hours sitting or working at a desk, can go a long way in maintaining shoulder health.

For athletes, proper technique and adequate rest are key. Working with a coach or trainer to ensure correct form can reduce the risk of repetitive stress injuries. Allowing time for recovery between intense training sessions is equally important, as it gives the shoulder a chance to rest and recover.

Final Thoughts on Subacromial Impingement

Subacromial impingement can be painful and frustrating, but with the right approach, it’s a manageable condition. Recognizing the signs, seeking an accurate diagnosis, and addressing the root causes can make a significant difference in both short-term relief and long-term shoulder health. Whether you’re an athlete or just looking to stay active, understanding the mechanics behind subacromial impingement is the first step in overcoming it.

By incorporating preventive measures, engaging in strength and flexibility exercises, and being mindful of proper movement patterns, you can reduce the chances of developing shoulder impingement. And if it does occur, working with a qualified healthcare provider or therapist can help you get back to the activities you love, free from pain.

Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider for any questions regarding your health or a medical condition.

References

• Neer, C. S. (1972). "Anterior Acromioplasty for the Chronic Impingement Syndrome in the Shoulder: A Preliminary Report." Journal of Bone and Joint Surgery.
• Michener, L. A., et al. (2003). "Anatomic and biomechanical mechanisms of subacromial impingement syndrome." Clinical Biomechanics.
• Kuhn, J. E. (2009). "Exercise in the treatment of rotator cuff impingement: A systematic review and a synthesized evidence-based rehabilitation protocol." Journal of Shoulder and Elbow Surgery.