Muscle injury often occurs in the medial head of the gastrocnemius (MG), and stretching is used as a treatment. However, there aren’t any studies based on anatomical considerations and biomechanics. This study therefore investigated the macroscopic anatomical structure of the triceps surae muscle to design an effective and selective MG stretching method, prior to quantitatively verifying that method by ultrasonography. The macroscopic anatomy was analyzed in 16 Japanese cadavers (25 legs). Based on the anatomical findings and the arrangement of fascicles in the MG, it was concluded that ankle inversion might be beneficial for selective stretching of the tendon fiber bundles into which the MG inserts. A method in which the limb was initially positioned with the knee joint in extension and the ankle joint in plantar flexion was devised. Then, the ankle was dorsiflexed and inverted. The proposed method was compared with standard stretching and verified by ultrasonography in eight healthy adult males. This technique effectively and selectively stretched the MG, producing a significantly decreased pennation angle and increased muscle fiber length. This method may aid in the prevention of future injuries and could enhance the effect of therapy on the MG.