The plantaris muscle is a slender structure located in the lower leg. It often goes unnoticed due to its small size and minor role compared to the larger muscles surrounding it. Despite its inconspicuous nature, it is a consistent, though sometimes absent, component of the calf anatomy.
Anatomical Location
The plantaris muscle originates from the lower part of the lateral supracondylar line of the femur, just above the lateral head of the gastrocnemius muscle. It may also arise from the oblique popliteal ligament near the knee joint. From this origin, the muscle descends inferomedially through the posterior compartment of the thigh and leg.
The plantaris consists of a small muscle belly, which then tapers into a remarkably long, thin tendon. This tendon courses downward, running between the larger gastrocnemius and soleus muscles in the calf. Its path places it superficial to the soleus and deep to the gastrocnemius. The tendon ultimately inserts onto the posterior surface of the calcaneus, often just medial to the Achilles tendon or by merging with it.
Functional Role
The plantaris muscle contributes to two primary movements: plantarflexion of the foot at the ankle joint and flexion of the knee joint. Plantarflexion involves pointing the foot downward, similar to standing on tiptoes. While it assists in these actions, its mechanical contribution is insignificant compared to the much larger and more powerful gastrocnemius and soleus muscles, which form the bulk of the calf.
Beyond its minor mechanical role, the plantaris muscle is thought to play a proprioceptive function. It contains a high density of muscle spindles, which are sensory receptors that provide feedback to the nervous system about muscle length and tension. This suggests it provides important information regarding the position and movement of the foot and ankle, rather than acting as a strong mover itself. Its limited motor function allows its tendon to be harvested for reconstructive surgeries elsewhere in the body with minimal functional deficit.
Notable Characteristics
The plantaris muscle exhibits significant anatomical variability among individuals. It is not always present, with absence rates reported to range from approximately 7% to 20% of the population. This absence can be unilateral, affecting one leg, or bilateral, affecting both.
The plantaris is often considered a vestigial muscle in humans, implying it has largely lost its original function through evolution. In other mammals, such as primates and bears, the plantaris can be more developed and play a more substantial role in locomotion or grasping. The reduction in its size and function in humans is theorized to be related to the adoption of bipedalism. However, some recent research challenges this classification, suggesting it may have a sensorimotor role due to its proprioceptive properties.
Clinically, the plantaris muscle is notable for its involvement in a condition commonly known as “tennis leg.” This term refers to a tear or rupture of the plantaris muscle or its tendon, which frequently occurs during sudden movements, such as pushing off or lunging, often seen in sports. Symptoms typically include a sudden sharp pain in the back of the calf, sometimes accompanied by a popping sensation, swelling, and bruising. The pain and symptoms of a plantaris rupture can mimic those of more serious injuries, such as a tear of the larger gastrocnemius muscle or even an Achilles tendon rupture. Diagnostic imaging, such as ultrasound or MRI, is often used to accurately differentiate a plantaris injury from other calf conditions.