Achilles Tendinitis

Achilles Tendinitis, radiograph of insertional calcification of the achilles tendon
Radiograph of insertional calcification of the Achilles tendon. Image by Mikael Häggström, Wikimedia Commons, CC0.

Anatomy

Gastrocnemius

  • Origin: via two heads.
    • lateral head
      • arises from distal femoral epiphysis, on lateral surface of lateral condyle, by tendon that leaves smooth pit above popliteus
    • medial head
      • arises via tendon from smooth shallow pit on medial condyle at distal end of supracondylar ridge, & via fleshy fibres from popliteal surface of femur
    • medial & lateral heads form medial & lateral borders of distal half of popliteal fossa
    • They converge to lie side by side, with deep surface covered by aponeurosis which rubs on soleus aponeurosis
    • lateral head blends with soleal aponeurosis distally, but medial head is separate, & has tendon of plantaris interposed between it & soleus fascia
  • Insertion:
    • tendon of insertion rotates through 90°, (DAMP), & inserts into middle third of posterior surface of calcaneus, separated from it by bursa. spiraling of tendon helps in storing energy
  • Innervation

Plantaris

  • This vestigial muscle is seen in 93%
  • Origin
    • above gastrocnemius on lateral supracondylar ridge & its thin, twisted ribbon of tendon runs down between soleus & medial head of gastroc to insert into common tendon
  • Innervation

Soleus

  • Origin:
    • Arises from tibia (mostly) & fibula.
      • fibular origin
        • upper fourth, including head, then there is fibrous arch which spans tibial NV bundle (much like FDS) to reach tibia
      • tibial origin
        • middle third, down to hands breadth below lower end of soleal line
    • muscle is flat with deep & superficial aponeurosis; between aponeuroses short thick fibres run from anterosuperior down & posteriorly, & tendon of insertion arises from posterior layer
    • On anterior surface of deep aponeurosis is slender bipennate muscle belly attached to centre of aponeurosis. This belly lies on NV bundle, in groove between FDL medially & FHL laterally
    • muscle fibres of soleus contain deep venous plexus that can give rise to DVTs
  • Innervation:
    • two separate branches of tibial nerve (S1, S2)

Structure of Achilles Tendon

  • mostly type I collagen,
    • < 4% strain
      • no damage
    • 4-8% strain
      • microscopic damage
    • > 8% strain
      • macroscopic rupture occurs
  • Type III collagen is found post rupture, & is not as strong

Blood Supply of Achilles Tendon

  • controversial
  • density of blood vessels is lower in middle 1/3 of tendon
  • Blood flow decreases with increasing age

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.