Anatomy and supply
The triceps surae muscle is a three-headed muscle at the dorsal lower leg. It lies within the superficial posterior compartment. Its heads and body determine the surface anatomy of the calf. Its insertion, the Achilles tendon, is easily visible and palpable at the heel. The triceps surae muscle is innervated by the tibial nerve. It is made up by:
- Soleus muscle (right): originates at the upper fibula, tibia and the tendinous arch of soleus which stretches between the tibia and fibula.
- Gastrocnemius muscle (left): has its origins at the medial epicondyle (medial head) and lateral epicondyle (lateral head) of the femur. It overlaps the soleus muscle almost completely.
At the distal third of the lower leg both the soleus and gastrocnemius muscles merge into a common tendon, the Achilles tendon (tendo calcaneus), which inserts at the posterior calcaneus. The neurovascular bundle coming from the hollow of the knee (posterior tibial artery and vein and tibial nerve) runs under the tendinous arch of soleus after which it courses between the superficial and deep flexor muscles.
The triceps surae muscle crosses multiple joints. Its most important function is the plantar flexion in the upper ankle joint enabling the lifting of the heel against gravity when walking or jumping. The maximal jumping power is achieved by adequate stretching of the muscle, i.e. stretching of the knee joint. The plantar flexion fixes the lower leg when standing and thus prevents the upper body from falling forward. Furthermore the muscle is the strongest supinator of the lower ankle joint. The gastrocnemius muscle contributes to a small extend to the bending of the knee.
Clinically the triceps surae muscle is the reference muscle for the nerve root S1. It can be compressed due to a herniated disk or vertebral fracture. Classic symptoms are irritations and pain along the buttock and posterior lower leg and a weak or absent Achilles tendon reflex. Additionally the triceps surae muscle is often functionally restricted. The affected patients present with a so-called talipes calcaneus caused by the overbalance of the anterior muscles of the lower leg. Both walking on toes as well as rolling the foot from the heel becomes practically impossible.
The Achilles tendon is the strongest tendon of the human body. Its load bearing capacity amounts up to one ton which is why its rupture goes along with a loud “whipping” sound. It is usually associated with previous damages due to chronic false strain. The microtraumas disturb the blood supply of the tendon leading to a decrease of its strength. The region about 3 to 5 centimeters proximal to the tendon insertion is particularly vulnerable as it is relatively poorly supplied already. In adolescents the Achilles tendon rupture often comes along with an osseous fracture of the calcaneus bone.