Plantaris
| Plantaris | |
|---|---|
| Muscle Type | |
| Origin | Lateral supracondylar ridge of the femur and oblique popliteal ligament |
| Insertion | Calcaneus (medial to Achilles tendon) or into the Achilles tendon |
| Action | Weak plantarflexion of ankle and knee flexion |
| Synergists | |
| Antagonists | Tibialis Anterior, Extensor Digitorum Longus, Quadriceps Femoris |
| Spinal innervation | L5, S1, S2 |
| Peripheral Innervation | Tibial nerve |
| Vasculature | Popliteal artery (sural branches), superior muscular branches of posterior tibial artery |
Plantaris is a small, thin muscle in the posterior leg with a long tendon running along the calf; it weakly assists gastrocnemius in plantarflexing the ankle and flexing the knee, and is notable for its tendon often being harvested for grafts.
Muscle Type
Plantaris is a vestigial, spindle-shaped muscle of the superficial posterior compartment of the leg. It has a very short muscle belly (situated behind the knee) and an exceptionally long, thin tendon that descends along the medial side of the Achilles tendon. Because of this long tendon, plantaris is sometimes called “freshman’s nerve” (often mistaken for a nerve in dissection). Not everyone has a plantaris muscle – it may be absent in a small percentage of individuals.
Origin
Plantaris originates from the lateral supracondylar ridge of the femur, just above the lateral head of gastrocnemius, and from the oblique popliteal ligament of the knee. Its origin on the femur is adjacent to where the lateral gastrocnemius arises.
Insertion
The plantaris tendon courses distally, running between gastrocnemius and soleus. It typically inserts into the calcaneus medial to the Achilles tendon. In many cases it fuses with the calcaneal (Achilles) tendon or inserts on the calcaneal bone itself on its medial posterior aspect. The tendon can be very long, extending the entire length of the leg.
Action
Plantaris contributes weakly to ankle plantarflexion and knee flexion. However, its functional importance is minimal due to its small size. During movements like jumping or toe-off in gait, plantaris adds a negligible amount of force compared to gastrocnemius-soleus. It may have a proprioceptive role, given its high density of muscle spindles in some individuals.
Synergists
Plantaris acts as a synergist to gastrocnemius and soleus for plantarflexing the foot at the ankle, and to gastrocnemius for flexing the knee. It contracts alongside the triceps surae during calf raises or push-off, albeit with minimal force. Other plantarflexors (tibialis posterior, fibularis longus/brevis, flexor hallucis longus, flexor digitorum longus) are also synergists in plantarflexion. In knee flexion, it works with the hamstrings and gastrocnemius.
Antagonists
The dorsiflexor muscles of the anterior leg (tibialis anterior, extensor digitorum longus, extensor hallucis longus) oppose the plantarflexion action of plantaris. At the knee, the quadriceps femoris group (knee extensors) antagonize plantaris’ weak knee flexion action.
Spinal Innervation
Like the triceps surae, plantaris is innervated by spinal segments S1 and S2 (with contribution from L5) via the tibial nerve.
Peripheral Innervation
The tibial nerve (a branch of the sciatic nerve) innervates the plantaris muscle. A specific branch from the tibial nerve in the popliteal fossa region supplies plantaris, often the same branch that goes to the lateral head of gastrocnemius.
Vasculature
Plantaris is supplied by branches of the popliteal artery, particularly the sural arteries that also supply gastrocnemius. Additional blood supply may come from the superior muscular branches of the posterior tibial artery. Venous drainage follows into the popliteal vein via accompanying veins.
Clinical Application
Plantaris has a long tendon frequently used in reconstructive surgery as graft material (for tendon repairs in the hand or elsewhere) because harvesting it causes no major functional loss. Rupture of the plantaris tendon can occur and may cause a sudden sharp pain in the calf (sometimes called “tennis leg” along with a medial gastrocnemius tear). However, plantaris tendon rupture typically has a good prognosis with conservative treatment since other muscles compensate fully. Surgically, plantaris is expendable; surgeons sometimes remove it intentionally to relieve chronic pain (some cases of calf pain or tendonitis) or to use its tendon. On MRI, an accessory plantaris or variations in its course can be observed, but absence of plantaris is usually an incidental finding. Clinically, the presence or absence of plantaris does not significantly affect strength. Understanding of plantaris is often more important in anatomy (to avoid confusing its tendon with a nerve) and in surgery (as a graft or landmark).
