This study aimed to investigate the behavior of the aponeurosis and external tendon of the medial gastrocnemius muscle (MG) in dynamic actions. Seven males performed maximal and submaximal concentric (CON) and eccentric (ECC) plantar flexion exercises preceded by an isometric action (Pre-iso) on an isokinetic dynamometer at a pre-set velocity of 5°/s. In addition, the subjects performed static ramp actions with the ankle positioned at dorsi-and plantar flexed positions. During the exercises, the length changes of the aponeurosis and external tendon of MG were determined using ultrasound apparatuses. In CON, the length of external tendon significantly shortened from the level of Pre-iso in all test conditions. However, the change of aponeurosis was significant only in the maximal exercises. In ECC, the external tendon was elongated in all test conditions, while the aponeurosis was not elongated even in the maximal effort. In static action, the lengths of both aponerurosis and external tendon increased with increasing force levels. The present results indicate that the behavior of aponeurosis of MG depends on the type of action and that the aponeurosis of MG plays a simple role of muscle force transmission in eccentric actions, while external tendon functions to store elastic energy.