When beginners play the piano, the activity of the forearm muscles tends to be greater than that of experts because beginners move their fingers with more force than necessary. Reducing forearm muscle activity is important for pianists to prevent fatigue and injury. However, it is difficult for beginners to learn how to do so. In this paper, we propose using electrical muscle stimulation (EMS) to teach beginners how to reduce this muscle activity while playing a tremolo. Since experts use wrist rotation efficiently when playing tremolos, we apply EMS not to muscles that are relevant to moving the fingers but to the supinator muscle and the pronator teres muscle, which are involved in wrist rotation. We conducted a user study with eight beginners to investigate how the forearm muscle activity changed through intervention with the EMS support system. After practicing with EMS, the activity level of the extensor pollicis longus muscle and extensor digitorum muscle on the forearm was significantly lower, and the participants felt less fatigue when playing tremolos. The results suggest that an EMS support system can reduce target muscle activity by applying EMS to other muscles to teach how to move limbs efficiently.