Acromiohumeral Distance and 3-Dimensional Scapular Position Change After Overhead Muscle Fatigue
Maenhout A1, Dhooge F, Van Herzeele M, Palmans T, Cools A. J Athl Train. 2015 Jan 16. [Epub ahead of print]
Take Home Message: After overhead shoulder fatigue, healthy individuals have an increase in acromiohumeral distance and altered scapular position, consistent with a protective, impingement-sparing situation.
Muscle fatigue due to overhead sports has been hypothesized to contribute to shoulder impingement (and subsequent rotator cuff pathology) by altering the position of the scapula and narrowing the subacromial space. However, conflicting evidence exists in the literature and as a result, no consensus has been established. This may be due to the fact that most investigators use different fatiguing protocols that do not resemble overhead sports activity and only indirectly measure the subacromial space. Therefore, the objective of this study was to assess the effect of a fatigue protocol that resembled overhead sports activity on scapular position and acromiohumeral distance, which is indicative of the amount of subacromial space. For this study, a total of 29 healthy recreational overhead athletes (14 men, 15 women, ~22 years of age) were tested at 3 elevation positions (0⁰, 45⁰, and 60⁰ of abduction) before and after a shoulder muscle fatiguing protocol. The fatigue protocol consisted of repeated shoulder internal and external rotation with resistance while the shoulder was at 90 degrees of abduction. The authors measured acromiohumeral distance (using ultrasound) and scapular position (using an electromagnetic motion-tracking system). The researchers demonstrated an increased acromiohumeral distance following fatigue when the upper extremity was actively positioned at 45⁰ or 60⁰ of abduction. The scapular position also changed after fatigue to a more externally and upwardly rotated position at 45⁰ or 60⁰ of abduction and more posteriorly tilted at all angles.
Results from this study suggest that muscle fatigue following overhead activity increases the acromiohumeral distance and leads to a more externally and upwardly rotated and posteriorly tilted scapula. These changes are surprising and suggest that overhead sports fatigue does not narrow the acromiohumeral distance but actually enlarges it, indicative of an impingement-sparing situation. It is important to note that acromiohumeral distance measurements were only performed at low angles (below 90⁰ of abduction) while the fatiguing protocol was performed at 90⁰ of abduction. It would be interesting to further investigate changes in acromiohumeral distance at larger angles. Another consideration is that the population chosen for this activity were young, healthy individuals with no history of shoulder pathology. This specific population seems to be able to compensate effectively following muscle fatigue while it is possible that older or injured populations may not. In addition, the muscle fatiguing protocol only looked at one bout of fatigue but it would be interesting to see how the results would change if fatigue was repeated. The researchers also indicate that their protocol may have specifically targeted the glenohumeral muscles, allowing sufficient compensation by the scapulothoracic muscles. It is possible that greater fatigue of the scapulothoracic muscles may not spare the shoulder from impingement. This study necessitates further clinical studies to investigate the role of impingement in the development of rotator cuff pathology in overhead athletes. In the meantime, this study serves as a nice reminder that we should carefully evaluate fatiguing protocols and patient populations used in each study before running to conclusions on this important topic. To determine if one of our athletes is at risk it may be ideal to evaluate him/her before and after practice or a game to see how that specific individual is responding to their activities.
Questions for Discussion: If acromiohumeral distance and scapular position are not to blame, what other factors do you think contribute to rotator cuff pathology in this population? Do you think acromiohumeral distance and scapular position would be compromised in a different population?
Written by: Katie Reuther
Reviewed by: Jeffrey Driban