Electromyographical activity of different sections of deltoid and supraspinatus muscles during dynamic abduction of upper limb in various speeds and loading in healthy adolescent subjects

Background Understanding of shoulder muscles function in various dynamic tasks is effective to understand the underlying mechanism of the shoulder injuries in sport activities. The aim of this study was to identify the electrical activity of deltoid and supraspinatus muscles during the fast and slow shoulder abduction with and without an external load in healthy adolescent subjects.
Methods In 16 females (20-28 years old), using a surface EMG system (MA-300), the activity of deltoid (anterior, medial, and posterior portions), and supraspinatus muscles within 90º of shoulder abduction in slow (22.5 º /s) and fast (45 º /s) movements that were repeated with and without a dumbbell (equal to 5% of body mass) were registration. Repeated Measure ANOVA was used with SPSS-22 for statistics analysis having p=0.05.
Results In slow without load abduction, the intensity of the activity of anterior, medial, and posterior deltoid and supraspinatus Muscles were %18.22±1.92, %50.55±1.97, %50.60±17.50 and %39.49±3.78, microvolt respectively. In with and without load conditions, the posterior and middle deltoid muscles displayed significantly greater activity than the other muscles. Load factor significantly increased the muscle activity by1.19 times (P=0.0001). However, loading effect was greater in posterior deltoid than in other muscles. Speed factor displayed similar effect in all muscles.
Conclusion The cause of the higher activity of posterior deltoid than anterior deltoid could be specified by the mechanism of external rotation of the humerus during shoulder abduction. The strengthening of posterior deltoid should be noteworthy in rehabilitation shoulder exercises.