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Clinical Pearls: Rotator Cuff & Deltoid Force Couple

clinical pearls deltoid force couple rotator cuff
Rotator cuff and deltoid force couple

Reddy et al.1 compared subjects with subacromial impingement and subjects with normal shoulders with respect to muscle activity. Fifteen subjects in each group were studied by means of fine-wire electromyography. The impingement group demonstrated decreased mean muscle activity in comparison with the group of normal subjects.  The inferior force vector (from the infraspinatus and subscapularis) is less functional in subjects with impingement than is the superior compressive vector (from the supraspinatus) in the 30° to 60°arc. Humeral head depression is critical in the first portion of arm elevation and has been suggested to be insufficient in the subacromial impingement patient population.

Experimentally-induced fatigue of the rotator cuff has replicated superior humeral head migration or translation with arm elevation.2,3  This result has important implications for conservative treatment of shoulder impingement and underscores the importance of rehabilitation to maximize the endurance and strength of the rotator cuff musculature.

Let’s keep it simple: so if the humerus is migrating superiorly and impinging on soft tissues….we want it to move inferiorly right? 

The infraspinatus is usually a strong muscle which inserts very close to the axis of rotation, especially in abduction. The infraspinatus will be the main influencing fibers for the downward slide/translation of the humeral head. To start let’s focus on external rotation strengthening exercises with the arm at the side of the body and slow progress, over time, into higher and higher arm abduction positions while in scaption.4

References

  1. Reddy AS, Mohr KJ, Pink MM, Jobe FW. Electromyographic analysis of the deltoid and rotator cuff muscles in persons with subacromial impingement. J Shoulder Elbow Surg. 2000;9(6):519-523. doi:10.1067/mse.2000.109410
  2. Chen SK, Simonian PT, Wickiewicz TL, Otis JC, Warren RF. Radiographic evaluation of glenohumeral kinematics: a muscle fatigue model. J Shoulder Elbow Surg. 1999;8(1):49-52. doi:10.1016/s1058-2746(99)90055-1
  3. Teyhen DS, Miller JM, Middag TR, Kane EJ. Rotator cuff fatigue and glenohumeral kinematics in participants without shoulder dysfunction. J Athl Train. 2008;43(4):352-358. doi:10.4085/1062-6050-43.4.352
  4. Hughes PC, Green RA, Taylor NF. Isolation of infraspinatus in clinical test positions. J Sci Med Sport. 2014;17(3):256-260. doi:10.1016/j.jsams.2013.05.011

 

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