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Muscle flexor digitorum accessorius is the muscle of the second layer of the sole. It arises by two heads and gets inserted into the tendon of flexor digitorum longus muscle. A variant of this muscle has often been incriminated in causation of tarsal tunnel syndrome. The action and phylogenetic importance of this muscle remains a matter of disagreement among different researchers. The present study was planned to study the morphology of this muscle in human.
The second layer of the sole was dissected in forty-seven cadavers. Origin, insertion and any variations of this muscle were observed. Extension of origin of medial head of this muscle into the tarsal tunnel was also observed.
The muscle was found to be very variable. The medial head was more variable, ranging from the complete absence to the bulky fleshy origin. It had fleshy origin from medial surface of calcaneus that extended deep to medio-malleolo calcaneal axis, i.e. into the tarsal tunnel in 80% of the cases. The lateral head was relatively constant and its origin was aponeurotic in all the cases. Insertion of this muscle was very variable, the commonest being insertion on the deep surface of flexor digitorum longus tendon. An accessory muscle-flexor digitorum accessorius longus-was observed in two cases.
The bulky medial head appears to be assuming the function of primary toe flexor in the plantigrade man. The origin of medial head extends into the tarsal tunnel in majority. A bulky medial head should be considered as a possible cause of tarsal tunnel syndrome.