The plantaris tendon is a long, slender structure in the back of the lower leg. It is one of the three calf muscles, known as the triceps surae, but is significantly smaller than the gastrocnemius and soleus muscles. While overlooked due to its minimal contribution to movement, it can be the source of sharp pain when injured. Understanding its anatomy is the first step in recognizing injury symptoms and the path to recovery.
Anatomy and Function of the Plantaris
The plantaris muscle is a small, spindle-shaped muscle that originates from the lower part of the femur, just above the knee joint. Its long, thin tendon travels down the leg, positioned between the gastrocnemius and soleus muscles. The tendon continues along the inner side of the Achilles tendon, attaching to the heel bone, known as the calcaneus. Its long, nerve-like appearance has led to it being nicknamed the “freshman’s nerve” in anatomy labs.
Functionally, the plantaris is a weak assistant to the larger calf muscles. It provides minor force for plantar flexion of the ankle (pointing your foot downwards) and also aids in flexing the knee. Its contribution is so minimal that it is regarded as a vestigial structure, meaning it has lost most of its ancestral function. It is congenitally absent in 7% to 20% of the population, and its absence does not impact normal leg function or mobility.
Common Injuries and Symptoms
The most frequent injury to the plantaris is a rupture, which often occurs during athletic activities. This injury is commonly referred to as “tennis leg” because it is seen in sports that demand sudden acceleration or lunging, such as tennis or basketball. The mechanism of injury involves a forceful push-off from the foot while the knee is extended. This action can cause the muscle or its tendon to tear, usually where the muscle belly meets the tendon.
The primary symptom of a plantaris rupture is a sudden, sharp pain in the back of the calf. Athletes often describe the sensation as feeling like they were struck in the leg. The rupture may be accompanied by an audible “pop” or snapping sound, and the pain can cause an individual to stop their activity immediately.
Following the initial rupture, other symptoms begin to develop. Swelling and tenderness are common at the site of the injury, located where the calf muscle transitions toward the Achilles tendon. Bruising may appear over the next day or two, sometimes tracking down the inner part of the calf. The calf muscle may also feel tight or exhibit cramping sensations.
Diagnosis and Treatment Options
A suspected plantaris tendon injury requires a medical evaluation to confirm the diagnosis and rule out other conditions. A physician will conduct a physical examination, feeling for tenderness in the calf. An important part of this assessment is to differentiate the injury from a more serious rupture of the Achilles tendon, as a plantaris rupture is a much less severe injury.
To obtain a definitive diagnosis, imaging tests are often used. While X-rays are not useful for this type of soft tissue injury, an ultrasound is an effective tool for visualizing the plantaris muscle and tendon and confirming a tear. An MRI may be used to get a more detailed picture of the injury and rule out associated damage to other structures or conditions like deep vein thrombosis.
Treatment for a plantaris tendon rupture is almost always conservative and does not require surgery. The immediate approach follows the RICE protocol: Rest, Ice, Compression, and Elevation to manage pain and swelling. Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) can also be used to reduce pain and inflammation.
Once the initial pain and swelling subside, physical therapy is often recommended. A therapist guides the patient through exercises designed to restore the leg’s range of motion, strength, and flexibility. Recovery is straightforward, and the prognosis for a full return to daily activities and sports is excellent.