Background: Understanding of shoulder muscles function in various dynamic tasks is effective to understand theunderlying mechanism of the shoulder injuries in sport activities. The aim of this study was to identify the electricalactivity of deltoid and supraspinatus muscles during the fast and slow shoulder abduction with and without anexternal 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) wereregistration. 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 andsupraspinatus 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 activitythan 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 allmuscles. Conclusion: The cause of the higher activity of posterior deltoid than anterior deltoid could be specified by themechanism of external rotation of the humerus during shoulder abduction. The strengthening of posterior deltoidshould be noteworthy in rehabilitation shoulder exercises.