The plantaris (french : muscle plantaire) - named from its occasional attachment to the fascia covering the sole of the foot (=planta)- is a fusiform, somewhat flattened muscle with a very long ribbon- shaped tendon.

Origin

• The lower two inches of the outer bifurcation of the linea aspera together with the posterior surface of the femur immediately below that ridge ;
• and the adjacent part of the posterior ligament of the knee joint.

Insertion

The inner side of the lower portion of the posterior surface of the calcaneum ; sometimes, however, it blends with the inner border of the tendo Achillis, and sometimes it is continued into the inner division of the plantar fascia.

Structure

Arising fleshy, the fibres of this small muscle have a somewhat penniform arrangement and converge upon the thin tendon, which appears first on the inner side of the muscle, and soon becoming free runs downwards and slightly inwards across the calf between the gastrocnemius and the soleus. In the lower third of the leg it lies along the inner border of the tendo Achillis, with which it is sometimes blended.

Nerve-supply

From the internal popliteal branch of the great sciatic nerve by small filament which enters the deep aspect of the muscle near the upper part of its inner border.

Action

This vestigial muscle is a feeble extensor of the ankle and flexor of le knee joint. By its attachment to the posterior ligament of the knee joint it will tend to draw backwards that ligament during flexion of the knee, and so prevent its being caught between the articular surfaces.

Relations

Behind lie the fascia of the popliteal space, the biceps, the gastro- tiemius, and the external popliteal nerve ; in front are the popliteal vessels and internal popliteal nerve, the popliteus muscle and the soleus.

Variations

In addition to the above-mentioned variations in the point of insertion, this muscle may sometimes be double at its origin, and it is not infrequently deficient.