Fatigue of the Rotator Cuff Causes Altered Shoulder Mechanics and Impingement
Another interesting article from the most recent issue of Journal of Athletic Training on rotator cuff fatigue and glenohumeral kinematics. In this study, the authors used dynamic fluroscopic video to assess superior humeral head migration in 20 asymptomatic subjects before and after fatigue of the rotator cuff. The assessment of migration during a dynamic activity is a fairly novel approach as previous studies that have attempted to quantify superior humeral head translation have used static imaging such as radiographs and MRI.
During the study the authors used the prone horizontal abduction (with thumbs up) exercise to fatigue the cuff until there was a documented decrease of at least 40% strength. This was an important part of the study, as a reader I want to be sure that fatigue had occurred and a 40% drop is significant for me. I must say, though, that I wish the authors had performed a more specific rotator cuff exercise, such as simple external rotation. The prone exercise has shown EMG activity of the cuff but also the deltoid and scapula musculature.
Results of the study show that humeral head migration increased by an average of 0.79mm during elevation in a fatigued state. While this seems small, keep in mind that the average subacromial space is between 2mm and 14mm in healthy subjects (can be about 50% smaller in pathological patients), thus reducing subacromial space by up to 40%!
Clinical Implications
RTC fatigue can lead to shoulder impingement. An interesting component of the article that the authors did not bring up in their discussion was that the humeral head actually migrated inferior prior to fatigue as the RTC acted to maintain the humeral head within the glenoid fossa as the deltoid provided a superior orientated force vector. After the fatigue, not only did the humeral head not migrate inferior, it moved superiorly. This has obvious implications for subacromial impingement. Both strength and endurance of the cuff should be addressed in rehabilitation, makes sense why a deconditioned person so much more likely to develop shoulder impingement.The rotator cuff should never be worked to failure. The fatigue protocol used by the authors lasted for less than 90 seconds. That is all it took for the RTC to loose 54% of it’s strength. Now think about that last time you had a patient come to you with “impingement” that reported playing tennis or golf for the first time in months, or even more simply, painting a room in their house. With only 90 seconds of activity the cuff fatigue enough to decrease your subacromial space by up to 40%. What should your treatment be for that patient? Minimize the initial inflammation, work on cuff strength & endurance but by all means, DON’T work the cuff to failure.
The rotator cuff should NEVER be worked to failure!
I never understood this training technique. If you work the cuff to failure, then what is going to dynamically stabilize the humeral head later that day when you are reaching overhead?