Achilles Tendon Physiology

Introduction

Leg muscles are the most powerful muscle group in the body and the Achilles tendon is the thickest and strongest tendon in the body. Contracting the calf muscles pulls the Achilles tendon, which pushes the foot downward. This contraction enables: standing on the toes, walking, running, and jumping. Damage to an Achilles tendon can result in the inability to walk, run, or jump. Each Achilles tendon is subject to a person’s entire body weight with each step. Depending upon speed, stride, terrain and additional weight being carried or pushed, each Achilles tendon may be subject to up to 3-12 times a person’s body weight during a sprint or push off.

Anatomical Description of the Achilles Tendon

The Achilles tendon is fibrous tissue that connects the heel (calcaneus) to two calf muscles: the gastrocnemius and the soleus. The tiny plantaris muscle also connects to the Achilles tendon. Additional names for the Achilles tendon include: heel cord, heel tendon, Achilles heel and calcaneal tendon.

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Tendons are strong, tough bands of inelastic fibrous connective tissue

that connects muscle to bone. They are the body’s highest strength connective tissue. Sinew is a synonym for tendon. Tendons consist of elongated cells, minimal ground substance and collagen fibers. The collagen fibers are packed closely together, parallel to the direction of force. Each collagen fibril is arranged into fascicles which contain blood vessels and nerve fibers.

Tenocytes are specialized fibroblasts

that lie within the fascicles, they: appear as star shaped cells in cross sections and appear as rows parallel with the tendon fibers in longitudinal sections, synthesize both fibrillar and non-fibrillar components of the extracellular matrix, and are able to reabsorb collagen fibrils.

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The fascicles themselves are surrounded by epitenon, which is surrounded by the paratenon: the space between them is filled with a thin, lubricating film of fluid which allows gliding of the tendon during motion.

Unlike many other tendons, the Achilles tendon does not have a true tendon sheath

Instead, the Achilles tendon is surrounded by a paratenon composed of soft tissue. The outer layer of the paratenon is a portion of the deep fascia, the middle layer is called the mesotenon and the inner layer is continuous with the thin layer surrounding the tendon itself: the epitenon.

Compared with other parts of the body, Achilles tendons have a relatively poor blood supply. The Achilles tendon is supplied by two arteries. The posterior tibial artery supplies the proximal and distal sections of the Achilles tendon and the peroneal artery supplies the midsection of the Achilles tendon.

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Blood is supplied to the Achilles tendon in two ways

Blood is supplied to the proximal portion of the Achilles tendon by the muscles connected to the Achilles tendon. Blood is supplied to the distal portion of the Achilles tendon by the tendon-bone interface. In both cases, the supply is largely via the mesotenon portion of the paratenon.

Blood vascularity is weakest at the Achilles-heel connection and blood supply is weakest at a point approximately 3/4″ – 2 3/8″ (2-6 cm) above the Achilles tendon – heel bone connection.

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