Shoulder external rotation fatigue and scapular muscle activation and kinematics in overhead athletes.

Joshi M, Thigpen CA, Bunn K, Karas SG, Padua DA. J Athl Train. 2011;46(4):349-57.

Unquestionably, understanding the anatomic and physiologic functions of the shoulder during repeated overhead athletic activity is one of the most challenging and intellectually demanding tasks for any sports medicine clinician to master. Due to the shoulder complex having a mobile base of support it is truly imperative for the clinician to know the roles of the various shoulder girdle muscles and what effects repeated overhead activity and fatigue will ultimately have on shoulder performance. In this article Joshi et al investigated the effect of a glenohumeral external rotation (GH ER) fatigue protocol on scapular muscle activation and kinematics during a D2 upper extremity PNF pattern. The investigators hypothesized that GH ER at 90° of abduction would result in infraspinatus fatigue along with a combined increase of upper trapezius, lower trapezius and serratus anterior muscle activity and destabilization of the scapula. Utilizing both EMG and 3D motion analysis 25 healthy male (n=15) and female (n=10) collegiate overhead athletes (6 baseball, 4 tennis, 12 volleyball, 3 swimmers) were recruited for this study. Inclusion criteria consisted of a minimum >30 mins of repeated overhead activity 3xs/week, no previous injury history of the dominant arm, free from shoulder pain and not currently taking medication for shoulder pain. After completing MVIC for the aforementioned muscles, the subjects performed a seated weighted (25% of peak infraspinatus MVIC) exercise mimicking the D2 PNF pattern with muscle activity being observed in both the ascending and descending phases. While connected to a 3D motion analysis set-up and EMG, the subjects performed a pre-fatigue protocol consisting of 5 sets of a D2 PNF pattern, keeping pace with a metronome in order ensure a consistent pace. After completing the pre-fatigue protocol, the subjects then executed weighted, prone, repeated GH ER at 90° of abduction while utilizing a metronome. Bout 1 was ceased after the subject stopped keeping pace with the metronome or was physically unable to continue the GH ER exercise. The subject performed 4 more bouts of the fatigue protocol until the number of reps performed was < 50% of the number of reps in the first set. Fatigue was defined as a 25% decrease of their pre-fatigue level ER force. The subjects then performed the same D2 PNF movement pattern after the fatigue protocol. The results demonstrated alterations to both scapular muscle activation and kinematics. Specifically, there was a 4% decrease in lower trap activation, a 4% increase in infraspinatus activity during the descending phase and a 3° increase in scapular upward rotation from pre to post-fatigue, and no effect on scapular ER or posterior tilting.

The authors believe the resultant changes in low trap and infraspinatus activation, as well as, scapular upward rotation can partially be attributed to not accounting for not manually stabilizing the scapula during the GH ER fatigue protocol. GH abduction at 90 has been shown to elicit larger lower trap activation than infraspinatus activation so that the scapula can be stabilized along the thoracic wall. By not manually stabilizing the scapula, it’s possible that the lower trap fatigued early in the process, and the infraspinatus had to increase activation to stabilize the GH joint in order to make up for the scapulothoracic stability that had been lost. That said, it’s possible that introduced fatigue and loss of scapular stability is setting up the infraspinatus for overuse injuries and potentially failure. The increase in upward scapular rotation was a surprising result due to the decrease in lower trap activation. What are your thoughts of this study? What’s the likelihood that your overhead athletes have a lower trap activation issue resulting from fatigue? Do you introduce a fatigue component during your overhead evaluation in order to examine scapular kinematics? Is it something that you do during every shoulder evaluation, and if so, what tests are you using? What injury prevention efforts are you making to minimize the number of shoulder injuries incurred by the overhead athletes that you work with?

Written by: Mark Rice
Reviewed by: Stephen Thomas