Background and Aim: Sports such as baseball, volleyball, and racquet sports demand skills that place substantial load upon the athlete's shoulder when his or her arm is in an overhead or abducted position. Athletes who participate in these sports are consequently at higher risk for overuse injuries of the shoulder. Infraspinatus syndrome is defined as a condition of frequently painless atrophy of the infraspinatus muscle caused by suprascapular nerve neuropathy. The syndrome typically causes symptoms that mimic those of rotator cuff tendinopathy, and the diagnosis is often overlooked until the condition fails to respond to a traditional rotator cuff treatment program.

In this study muscle timing and activities of shoulder rotator cuff compared between normal athletics and volleyball players with infraspinatus syndrome.

Materials and Methods: Thirty voluntary male professional volleyball players (20 healthy and 10 patient) in the range of 20 -30 years contributed in this study. Patients group in base of electroneurography had injury of suprascapular nerve at right side. Subjects lied prone on table and their shoulder were 90 degree of abduction and medial rotation with 90 degree of flexion in elbow with vertically position of forearm at the lateral side of table. Electromyographic activities of maximum voluntary contraction of infraspinatus, middle trapezius and teres minor . Muscles were recorded three times with monitoring of dynamometer for normalization of dynamic activities. Activities (RMS) of above muscles were recorded during three dynamic motions similar to spark skill and onsets of their activities determined by external synchronized apparatus.

Results: There was significant difference between normalized RMS of infraspinatus,. and middle trapezius of patients in dynamic motion (p<0.05). Whereas healthy subjects showed significant difference between infraspinatus. and two other muscles (p<0.05). Comparison of normalized RMS indicated different in all three muscles between two groups (p<0.05).  

The sequence of onset three muscle activities were different between two groups significantly (p<0.05). This sequence in patients was middle trapezius, teres minor and infraspinatus whereas in healthy subjects was middle trapezius, infraspinatus and teres minor. Duration of above muscle activities were different in two groups (P<0.05). In patients infraspinatus were active less than other two muscles.       

Conclusion: Professional volleyball players are candidate of suprascapular nerve and infraspinatus muscle weakness. Synergic and stabilizer muscles after this condition with more activities contribute and caused reduce of accuracy of motion and early muscle fatigue.