Well known to anatomy specialists, the accessory soleus muscle was first demonstrated to be involved in painful syndromes in 1965 (Dunn). This supranumerary muscle situated in front of the calcaneum can be taken for a soft tissue tumor. The purpose of this work was to report a series of 21 patients with an accessory soleus muscle and to present the characteristic features, diagnostic methods, and treatment indications and modalities.

This series included 20 patients (one symptomatic bilateral case), fourteen men and six women, mean age 25 years. Seventeen patients practiced sports and ten had had a prior operation. All patients complained of exercise-related pain. The physical examination was normal with the exception of a tumefaction, which was soft at rest and hard at triceps contraction against resistance, lying laterally to the Achilles tendon. We studied plain x-rays, ultrasound studies, computed tomographies, and electromyograms and later MRI which became the reference method to demonstrate the details of normal muscle structure. Ten patients (one bilateral case) were not particularly bothered by the supernumerary muscle. Functional treatment was given and provided patient satisfaction. For the other ten patients, who wished to continue their physical activities, two underwent fasciotomy (including our first case where fasciotomy was undertaken because a tumor was suspected) and eight underwent resection of the supranumerary muscle.

The patients were followed for 6 to 19 years. Outcome was very good in all patients who were free of pain and had complete joint movement with symmetrical muscle force. Normal sports activities were resumed.

The accessory soleus muscle is found in 10% of individuals. It is often asymptomatic. The muscle inserts on the anterior aspect of the soleus muscle or on the posterior aspect of the tibia or the muscles of the deep posterior compartment. It lies anterior to the calcaneal tendon and terminates on the calcaneal tendon or the superior or medial aspect of the calcaneus via fleshy fibers or a distinct tendon. Frequent in primates, this anatomic variant is present during embryological development. Its persistence depends on phylogenetic evolution. Among other hypotheses (exercise-induced intermittent claudication, compression of the tibial nerve, excessive tension on the nerve innervating the accessory soleus muscle), this supranumerary muscle is generally considered to be the cause of a localized compartment syndrome. Pain experienced during exercise is the only symptom regularly reported by patients. A careful examination is required to rule out another local cause. Besides tumefaction lateral to the Achilles tendon, often found bilaterally, there is no other clinical sign. Plain x-rays, ultrasound and computed tomography simply demonstrate a "mass" in front of the Achilles tendon. MRI is the examination of choice enabling confirmation of the muscle nature of the mass and ruling out the possible diagnosis of tumor. Since there is no risk of aggravation, surgical treatment can be avoided if there is no complaint. If the patient is seriously impaired, surgery can be proposed. In our opinion, complete resection of the supernumerary muscle is the safest solution and should be preferred over simple fasciotomy.