The study examined the muscle recruitment order of six shoulder muscles in young healthy overhead athletes when subjected to external rotation forces in a position of shoulder apprehension. Fifteen athletes underwent surface electromyography testing of their pectoralis major, long head of biceps brachii, serratus anterior, anterior deltoid, infraspinatus, and latissimus dorsi muscles in response to 0.5, 0.75, 1.0, and 1.25 kg weights. Across all weight conditions for both shoulders, the consistent muscle activation sequence was pectoralis major, biceps brachii, serratus anterior, infraspinatus, and latissimus dorsi. This recruitment order

1. Abstract
Koch K & Smith P
Title
Surface Electromyography of selected overhead athletes shoulder muscles
using maximum voluntary contraction from a relaxed position of appre-
hension in young healthy adults
Background
There is a growing body of evidence on shoulder instability but no norma-
tive dataset on muscle recruitment order in the most at risk group of young
healthy overhead athletes in the relaxed most vulnerable shoulder position
of apprehension.
Methods
15 young overhead athletes (right side dominant), with no previous shoul-
der instability, participated in a sEMG study viewing 6 shoulder girdle
muscles (Pectoralis major- clavicular attachment, long head of Biceps
brachii, Serratus anterior, Anterior deltoid, Infraspinatus and Latissimus
dorsi) order of activation with 4 different weights (0.5, 0.75, 1.0, 1.25 Kg)
creating an external rotation force on both the dominant and non dominant
shoulders. The subjects were initially relaxed and were told to react with
maximum voluntary internal rotation force when the disturbance was ap-
plied.
Analysis
Continuous data was converted to categorical mean rank order for com-
bined, dominant and non dominant shoulders at each weight. Pairwise
comparisons were conducted using the Friedman test.
Results
All 12 categories revealed significant results (P<0.05). Combined: 3/4
showed the same rank order of P B IAS L. Left: 2/4 showed the same rank
order of P B IAS L. Right: All showed the same rank order of P B IAS L.
In 10/12 pairwise comparisons, P and B showed to be highly significant
(P<0.01) with L. The remaining 2 were significant. In all comparisons: P
showed to be non significant with B, IAS showed to be non significant
with each other.
Conclusion
Across all results, a consistant sequence of muscle activation order was
seen which could be important in maintaining shoulder stability in young
healthy overhead athletes. This could be used in conjunction with clinical

2. investigations and potentially help prevent instability. Further investiga-
tions are needed in assessing reflexes and degree of muscle contraction on
shoulder muscle recruitment in a position of apprehension.