Chirp Pulse Microwave Computed Tomography (CP-MCT) has been developed to visualize activities of forearm muscles during exercise. The change in the gray levels between two images obtained before and after loading exercises showed position dependence. This was produced by physiological- or biochemical- changes in forearm muscles caused by grasping a rubber ball in the bolus tank that was filled with saline solution. However, nobody knows the truth how this change was produced. In order to infer the mechanism of the gray level change caused by the exercise, the forearm exercise was investigated also by using electromyography and Time-Resolved Diffuse Optical Tomography (TR-DOT). Measurements by CP-MCT and TR-DOT were not performed simultaneously, but both images obtained under the same experimental condition showed similar changes in each tomogram. They were characterized by activation of inner muscles needed for continuing the exercise.