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Introduction Strains of the soleus are widely found both in amateur and professional athletes. For their accurate regional diagnoses, understanding the anatomy of the spatial relationship between muscular fibers and tendinous structures is important because their interfaces are susceptible sites to muscle strains. Therefore, this study evaluated the precise architecture of the soleus. Materials and Methods We evaluated the precise anatomical architecture of the soleus in 87 formaldehyde‐fixed soleus muscles. To calculate mean relative physiological cross‐sectional area of each muscular fiber compartment, we measured the fiber length, volume, and pennation angle in isolated compartments. Results The posterior soleus surface was covered by a broad aponeurotic posterior insertion tendon (PIT), which continued inferiorly to the insertion tendon. The anterior surface had three aponeurotic origin tendons, lateral origin tendon (LOT), medial origin tendon (MOT), and tendinous arch, which were arranged along the soleus margins. The anterior bipennate muscle portion (ABP), surrounded by the three origin structures, terminated as the sagittal insertion tendon (SIT), which continued inferiorly to PIT. The posterior main muscle portion behind LOT and MOT was separated into lateral and medial portions by the SIT. The soleus thus possessed a broad musculotendinous junction. Furthermore, ABP exhibited wide structural diversity in shape and size: in extreme cases, it was duplicated or absent. Conclusion Systematic anatomical descriptions of the soleus will be useful for accurate regional diagnosis of its strains with magnetic resonance imaging and ultrasonography.

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