Muscle Fatigue and Shoulder Impingement

Shoulder impingement is a common cause of pain with overhead exercises. The mechanism of shoulder impingement was thought to be a structural issue between the humerus and the acromion process, but that has recently been challenged. This post will discuss the up-to-date mechanism of shoulder impingement and how it relates to treatment. 

There is much debate on the actual cause of shoulder impingement. Traditionally, shoulder impingement was attributed to irritation of the tissues between the humerus and the acromion process. The subacromial bursa and the supraspinatus tendon would get compressed between the two bones as the shoulder moved overhead leading to shoulder pain. However these mechanics in the shoulder have been found in non-painful shoulders as well making their relationship to shoulder pain questionable. 

Photo Credit:  NIAMS

Photo Credit: NIAMS

Instead of shoulder impingement being caused by altered mechanics between the acromion process and the humerus, it has been suggested that shoulder impingement is actually a problem of the rotator cuff tendon. The supraspinatus tendon swells which decreases the space between the humerus and acromion process leading to shoulder pain. Swelling in the supraspinatus tendon can occur either from prolonged rest, such as when sleeping, or with overloading. 


The altered swelling in the supraspinatus tendon has been shown in research by Karen McCreesh in the British Medical Journal Open Sports & Exercise Medicine. Her research found that loading the supraspinatus in those with shoulder pain caused an increase in muscle thickness 24 hours after exercise. Similarly, the article found that the distance between the acromion process and humerus was reduced for that same time period after exercise. 

This was in comparison to the response in non-painful shoulders which actually had a decrease in supraspinatus tendon thickness after exercise. The non-painful shoulders also had a temporary decrease in the distance between the acromion and humerus but returned to normal after 6 hours. 

There are a couple of reasons why this research article is important for those with shoulder impingement syndrome. 

  1. Avoiding loading the shoulder muscles to fatigue early in rehabilitation
    Exercises are needed to strengthen the supraspinatus tendon. However, loading the tendon to fatigue can lead to more swelling and discomfort so it is important to find the balance of loading enough but not too much. 

  2. Allowing sufficient recovery for the tendon between exercise
    The supraspinatus tendon takes longer to return to baseline in a painful shoulder than in a non-painful shoulder. Allowing 24 hours between rehabilitation exercises provides sufficient time for the supraspinatus to return to baseline before loading it again. 

  3. Impingement syndrome might not be a structural cause
    The emphasis on impingement syndrome was commonly on the humerus and the acromion process. So if there was a bone spur in the space between those two bones, surgery was a likely recommendation. But if the cause of impingement syndrome is related to the supraspinatus tendon, surgery is likely not necessary for recovery. 

In conclusion, the cause of shoulder impingement syndrome seems to be trending towards a tendon related issue and not a bone mechanics issue. The recommendation is to treat syndrome impingement syndrome through a conservative approach including soft tissue therapy and exercise. When creating a rehabilitation program, it appears that avoiding loading the shoulder muscles to fatigue can help to reduce irritation of the shoulder and lead to better outcomes. 


McCreesh KM, Purtill H, Donnelly AE, et al Increased supraspinatus tendon thickness following fatigue loading in rotator cuff tendinopathy: potential implications for exercise therapy BMJ Open Sport & Exercise Medicine 2017;3:e000279. doi:10.1136/bmjsem-2017-000279