Unusual extended fibular origin of the human soleus muscle: possible morpho-physiologic significance based on comparative anatomy.

A bilateral anomalous extended origin of the soleus muscle was observed in a 73-year-old female cadaver. It arose from the head, neck, and proximal two-thirds of the medial crest of the fibula and ran through the posterior intermuscular septum of the leg, from the lateral border of the fibula. The soleus muscle formed the vault of a muscular tunnel, overcoming the deep flexor muscles of the leg that was about 15 cm in length and directed inferiorly and laterally. The extended fibular origin delimited a blind recess lateral to the muscular tunnel and parallel to the posterior surface of the fibula. This recess measured 6.5 cm in length and extended 3.5 cm above the inferior opening of the muscular tunnel; the superior portion of the flexor hallucis longus was housed within it between the portions of the extended origin from the medial crest of the fibula and posterior intermuscular septum. The neurovascular bundle of the posterior leg coursed in the muscular tunnel. The tibial origin and calcaneal insertion of the soleus muscle were normal. Phylogenetic studies of the muscles of the lower limbs in mammals indicate that the fibular origin of soleus is more constant than the tibial origin and, in primates, the fibular origin is the only one observed in most monkeys. The case reported might be considered a conspicuous enlargement of the fibular origin observed in primates. This large fibular origin of the soleus muscle may prove to be a difficulty during surgery when accessing the proximal two-thirds of the fibula for ligation of the peroneal artery.