Gastrocnemius or twins

The gastrocnemius muscle is the most superficial of the muscles of the posterior region of the leg. It is formed by two muscular bellies called twins. The medial head originates above the medial epicondyle of the femur, from the popliteal plane, and from the posterior region of the knee joint capsule. The lateral lateral head originates above the lateral epicondyle of the femur, from the popliteal plane and from the articular capsule of the knee.

Its imposing tendon joins that of the soleus muscle forming the calcaneal (or Achilles) tendon which is inserted into the calcaneal tuberosity, in the middle third of the posterior aspect of the calcaneus.

With its action it extends the foot (the plantar flexes), rotates it internally, and also participates in the flexion of the leg (being, unlike the soleus, a biarticular muscle). With its contraction it elevates the heel during walking.

The twins can offer their maximum power at extended knee, while when the knee is flexed their effectiveness is minimal and the extension of the foot is deputed to the soleus muscle. For this reason the sitting calf mainly trains the soleus while the calf press trains both the twins and the soleus. The concept is evident when driving with a seat that is too advanced. If the knees are too bent the gastrocnemius becomes ineffective and it is more difficult to act on the pedal board.

It is innervated by the tibial nerve (L4-S1).

ORIGIN

Lateral twin: lateral epicondyle of the femur, popliteal plane and posterior portion of the knee joint capsule

Medial twin: medial epicondyle of the femur, popliteal plane and corresponding portion of the knee joint capsule