Purpose: The purpose of this study was to determine the muscle activity ratio according to the shoulder abduction angle by identifying the mean muscle activities and calculating the muscle ratios for use in developing strengthening methods. Methods: The participants were healthy adults in their 20s (n=19). The activity of the deltoid middle, upper trapezius, middle trapezius, and lower trapezius muscles was measured by 8-channel surface electromyography. Muscle activity was measured during 4 conditions of angle of shoulder abduction: $30^{\circ}$, $60^{\circ}$, $90^{\circ}$, and $120^{\circ}$. The data used in the analysis were the root mean square and % total muscle activity values. Results: The root mean square values for the deltoid middle, upper trapezius, middle trapezius, and lower trapezius muscles showed significant differences. No significant differences were detected in the % total muscle activity of the deltoid middle, upper trapezius, middle trapezius, or lower trapezius muscles. Conclusion: Future studies aimed at developing selective shoulder abductor muscle strengthening methods are likely to provide more effective results by using muscle activity ratios.