Abstract. [Purpose] This study compared the activations of the shoulder and trunk muscles during dominant-leg-extended knee push-up-plus (KPP) exercises on a mat, on an unstable surface, and with loading.[Subjects] Fourteen healthy subjects, all right-side dominant and with no history of injury or surgery to the shoulder or neck were the subjects.[Methods] The subjects performed dominant-legextended knee push-up-plus using three variations. Electromyography activities of the serratus anterior (SA), upper trapezius (UT), external oblique (EO), and internal oblique (IO) muscles were recorded.[Results] We observed significant differences in SA activity among the exercise conditions. Dominantleg-extended KPP on a stable surface produced the highest SA muscle activity. Dominant-leg-extended KPP using a wobble board produced the highest EO and IO muscle activity. [Conclusion] To apply the proper resistance to enhance selective SA muscle activity in KPP, the best technique is to raise the ipsilateral leg. Furthermore, KPP on an unstable surface can facilitate lumbar stabilization.