The accessory soleus muscle is an uncommon congenital anatomical variant with a prevalence ranging from 0.7 to 5.5%. Although intermittent and exertional symptoms caused by this supernumerary muscle have been well documented, acute injuries have not. We present a case of an isolated rupture of the accessory soleus tendon with myotendinous retraction, mimicking clinically a "tennis leg." A 29-year-old woman sustained a hyperdorsal flexion injury of the right ankle with a severe and sudden pain in the middle part of the calf. Radiographs were normal and the diagnosis of "tennis leg" was clinically suspected. Ultrasound demonstrated bilateral accessory soleus muscles. On the symptomatic side, there was a complete isolated rupture of the accessory soleus tendon with myotendinous retraction. These findings were confirmed by magnetic resonance imaging (MRI), which showed no other abnormality. To our knowledge, this acute and misleading presentation has not been reported previously.
Keywords: Accessory soleus muscle; Ankle; Magnetic resonance imaging; Tendon rupture; Tennis leg; Ultrasound.