The plantaris is a muscle that arises from the lateral supracondylar ridge of the femur above the lateral head of the gastrocnemius muscle. The plantaris muscle then inserts into the medial side of the calcaneus close or attached to the Achilles tendon. The plantaris muscle passes between the soleus and the gastrocnemius muscles down to its insertion into the calcaneus. It lies within the superficial posterior compartment of the lower leg and is absent in about 10% of the population. It allows plantar flexion of the foot and it flexes the knee.
The plantaris is enervated by the tibial nerve. The muscle contains a high density of proprioceptive receptor and organs. This allows the plantaris muscle to provide proprioceptive feedback to the central nervous system regarding the position of the foot. The plantaris is a muscle that is mainly used by surgeons for tendon grafts, especially for Achilles tendon rupture to augment the repair. The plantaris muscle could be injured in association with an Achilles tendon rupture.
Occasionally, when performing a Thompson test to check for an Achilles tendon rupture, it may show an accurate result if the plantaris is intact. When a rupture of the plantaris occurs, the patient may feel a sharp stabbing pain in the back of the calf. The patient may also feel a pop similar to being struck from behind. The calf muscle will become swollen with significant bruising and the condition is often referred to as tennis leg. The injury occurs as a result of eccentric loading on the ankle with the knee extended. Treatment of this injury includes elevation, crutches, and pain control.