Morphological Classification of the Sagittal Tendon Plate in the Soleus Muscle in Japanese Cadavers

SUMMARY: The human soleus muscle has attracted attention in the fields of sport science, rehabilitation medicine, etc. for improving exercise performance in training, preventing injuries, and rehabilitation. The sagittal tendon plate of the soleus muscle is particularly important in rehabilitation. Few studies, however, have evaluated the shape of the sagittal tendon plate in the human soleus muscle in detail and attempted to classify its variations based on its morphology. In this study, we conducted a detailed analysis of the morphology of the sagittal tendon plates in soleus muscle specimens from Japanese cadavers and constructed a morphology-based classification system and evaluated their frequencies of occurrence. First, the specimens were divided into those with a sagittal tendon plate that was visible on the anterior surface (pennate muscle group) and those without (non-pennate muscle group). Next, based on the “number” and “breadth” of the sagittal tendon plates, the pennate muscle group specimens were further classified into four classes: Class I (one tendon, thin), Class II (one tendon, slightly broad), Class III (one tendon, very broad), and Class IV (two tendons, thin). Subsequently, the specimens were further divided into three types based on the position where the sagittal tendon plate joined the Achilles tendon: median tendon type, lateral tendon type, and medial tendon type (a total 13 divisions). When 458 Japanese soleus muscle specimens were classified into these divisions, the occurrence frequencies of Class I-IV were 80.57 %, 4.59 %, 5.46 %, and 1.09 %, respectively. In Class I, the median tendon type was more frequent than the lateral and medial tendon types, accounting for 48.47 % overall. The classification types of the sagittal tendon plate and their respective occurrence frequencies shown in this study are expected to serve as fundamental data in implementing rehabilitation of soleus muscle.