Scapular Muscle Recruitment Patterns: Trapezius Muscle Latency with and without Impingement Symptoms Ann M. Cools,*† PT, Erik E. Witvrouw,* PT, PhD, Geert A. Declercq,‡ MD, Lieven A. Danneels,* PT, PhD, and Dirk C. Cambier,* PT, PhD From the *Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, University Hospital, Ghent, and ‡private practice, Gravenwezel, Belgium Background: Altered muscle activity in the scapular muscles is commonly believed to be a factor contributing to shoulder impingement syndrome. However, one important measure of the muscular coordination in the scapular muscles, the timing of the temporal recruitment pattern, is undetermined. Purpose: To evaluate the timing of trapezius muscle activity in response to an unexpected arm movement in athletes with impingement and in normal control subjects. Study Design: Prospective cohort study. Methods: Muscle latency times were measured in all three parts of the trapezius muscle and in the middle deltoid muscle of 39 “overhand athletes” with shoulder impingement and compared with that of 30 overhand athletes with no impingement during a sudden downward falling movement of the arm. Results: There were significant differences in the relative muscle latency times between the impingement and the control group subjects. Those with impingement showed a delay in muscle activation of the middle and lower trapezius muscle. Conclusion: The results of this study indicate that overhand athletes with impingement symptoms show abnormal muscle recruitment timing in the trapezius muscle. The findings support the theory that impingement of the shoulder may be related to delayed onset of contraction in the middle and lower parts of the trapezius muscle. © 2003 American Orthopaedic Society for Sports Medicine Chronic shoulder pain is probably the most common upper extremity problem of recreational and competitive ath- letes involved in sports that require overhand arm mo- tions. 1, 35, 44 “Overhand athletes,” that is, athletes involved in racquet sports, volleyball players, throwers, and swim- mers, need full, unrestricted upper extremity function to perform optimally in their sport. 36 Nontraumatic shoulder pain in the overhand athlete is a diagnostic challenge. The causes of chronic shoulder pain are numerous, but it is often difficult to identify and to diagnose the exact problem. 6, 10, 37, 58 Research indicates that shoulder impingement is the most common cause of shoulder pain in overhand athletes. 2, 53 The term “shoul- der impingement” was first introduced by Neer 42 in 1972 and describes mechanical compression of the rotator cuff and subacromial bursa against the anterior undersurface of the acromion and coracoacromial ligament, especially during elevation of the arm. Primary impingement can be defined as impingement caused by outlet stenosis in the subacromial space in a stable shoulder. 8 According to var- ious authors, symptoms of impingement in the throwing athlete are often related to glenohumeral instabili- ty. 23, 24, 38 This secondary form of impingement can be defined as impingement secondary to instability of the shoulder. 8 Different anatomic structures can be impinged internally or externally, probably depending on the mo- tion and loading put on the shoulder during the pain- provoking activity. 8, 56 However, a possible instability in the shoulder is often “silent” and difficult to demonstrate by ordinary tests and has therefore been termed by some as “functional instability.” It is now thought that func- tional instability in the shoulder may lead to a vicious cycle involving microtrauma and secondary impingement and may eventually lead to chronic shoulder pain. 8 † Address correspondence and reprint requests to Ann M. Cools, PT, University Hospital Ghent, Department of Rehabilitation Sciences and Phys- iotherapy, De Pintelaan 185, 6K3, B9000 Ghent, Belgium. No author or related institution has received any financial benefit from research in this study. 0363-5465/103/3131-0542$02.00/0 THE AMERICAN JOURNAL OF SPORTS MEDICINE, Vol. 31, No. 4 © 2003 American Orthopaedic Society for Sports Medicine 542