When an accessory soleus muscle is present, it consists of a soft-tissue
mass bulging medially between the distal part of the tibia and the Achilles
tendon. It usually inserts with a separate tendon on the calcaneus
anteromedial to the Achilles insertion, and may be a cause of pain on
exercise. One may suspect a soft-tissue tumor, such as lipoma, hemangioma,
and even sarcoma, but the anomalous muscle has a typical appearance on
plain radiographs, and the appearance on computed tomography is diagnostic.
If the patient is asymptomatic, no therapy is required, but if pain or
other discomfort is provoked by exercise, exploration with fasciotomy or
excision of the accessory muscle is recommended, as was done in six of our
eleven patients who were seen between 1968 and 1985.