Analysis of muscle activity by electrophysiological techniques is commonly used to analyze biomechanics. Although the simultaneous and intuitive understanding of both muscle activity and body motion is important in various fields, it is difficult to realize. This paper proposes a novel technique for visualizing physiological signals related to muscle activity by means of surface electromyography. We developed a wearable light-emitting interface that indicates lower-limb muscle activity or muscular tension on the surface of the body in real time by displaying the shape of the activated muscle. The developed interface allows users to perceive muscle activity in an intuitive manner by relating the level of the muscle activity to the brightness level of the glowing interface placed on the corresponding muscle. In order to verify the advantage of the proposed method, a cognitive experiment was conducted to evaluate the system performance. We also conducted an evaluation experiment using the developed interface in conjunction with an exoskeleton robot, in order to investigate the possible applications of the developed interface in the field of neurorehabilitation.