What is the Achilles Tendon?

achilles tendonitis

Figure 1: Normal Achilles – Tendon dark in color and crescent shaped.

The Achilles tendon is the strongest and largest tendon in the body. It is a tendinous structure (attaches muscle to bone) that forms from a combination of the gastrocnemius and soleus muscles located in the calf.

The tendon attaches to the heel bone (calcaneus) and causes the foot to push off (plantar flex) when the calf muscles tighten. The tendon is necessary for normal walking, running, and jumping. Athletic and traumatic injuries to the Achilles tendon are common and can be disabling.

What is the Injury to the Achilles Tendonitis?

Tendinitis is inflammation of a tendon. Thus, Achilles tendinitis is an inflammation of the Achilles tendon. The inflammation may be localized to the end of the tendon closest to the heel or may spread upward to affect even the muscles of the calf. Swelling often occurs and pain is felt upon contraction of the calf muscles. In severe cases, pain may be felt even at rest.

Generally, Achilles tendinitis begins as a dull pain at the back of the lower leg just above the heel when pushing off the ground with the foot. Unless the activity is stopped, the condition rapidly gets worse until any activity requiring a push-off from the ground by the foot becomes quite painful and nearly impossible. If it is left untreated, it can develop into one of two more serious conditions — Achilles tendinosis and Insertional Calcific Tendinitis.

These are degenerative change in the tendon and are very resistant to treatment. A normal Achilles tendon in solid in color and crescent shaped, Figure 1. Abnormal changes inside Achilles tendon called Achilles tendinosis, Figure 2. Abnormal tendon usually thick and not solid black in color. Will usually see white speckles in the tendon.

thick achilles tendon

Figure 2: Achilles Tendinosis – Thickening of the Achilles tendon. Tendon round and thick.

How does the injury occur? 

The majority of Achilles tendon injuries are due to overuse injuries. Other factors that lead to Achilles tendonitis are improper shoe selection, inadequate stretching prior to engaging in athletics, a short Achilles tendon, direct trauma (injury) to the tendon, training errors and heel bone deformity.

There is significant evidence that people with feet that role in excessively (over-pronate) are at greater risk for developing Achilles tendinitis. The increased pronation puts additional stress on the tendon, therefore, placing it at greater risk for injury.


The latest studies on Achilles tendonitis recommend a treatment plan that incorporates the following three components:

  • Treatment of the inflammation
  • Strengthening of the muscles that make up the Achilles tendon using eccentric exercise. These are a very specific type of exercise that has been shown in multiple studies to be a critical component of recovering from Achilles tendonitis
  • Biomechanical control (the use of orthotics and proper shoes).

Orthotics for Achilles Tendonitis – What the Research Tells Us

A 2014 study looked at the effect of using foot orthotics on the Achilles tendon.  The researchers found that running with foot orthotics resulted in a significant decrease in Achilles tendon load compared to running without orthotics. This study indicates that  foot orthoses may act to reduce the incidence of chronic Achilles tendon pathologies in runners by reducing stress on the Achilles tendon1. Orthotics seem to reduce load on the Achilles tendon by reducing excessive pronation. 

How does over-pronation cause Achilles Tendinitis?

During normal gait, the thigh bone (femur) and the major leg bone (tibia) rotate in unison as your foot goes through a normal rolling-in (pronation) and rolling-out (supination).

However, when a person over-pronates, the tibia continues to rotate internally when the femur is rotating externally. The resulting counter rotation of the femur and the tibia causes a shearing force to occur in the Achilles tendon.

This occurs because the Achilles tendon is made of two muscles—one of which (the Gastrocnemius) is attached to the femur while the other (the Soleus) is attached to the tibia. This shearing force twists the tendon at its weakest area, namely the Achilles tendon itself, and causes the inflammation.

Because the over-pronation puts such a great stress on the Achilles tendon, custom functional orthotics designed to control over-pronation are an important part of the treatment plan for this problem.

Self Treatment of Achilles Tendonitis?

Achilles tendonitis should never be self-treated because of the potential for permanent damage to the tendon. While you are waiting to see your doctor, however, some patients have found relief from symptoms with the use of Silipos Achilles Heel Guard during the day and a Night Splint at night.

A topical pain reliever like Orthogel Cold Therapy can provide temporary relief of pain. These items, including a couple of different types of night splints are available on the Achilles Tendonitis page at www.FootAnkleStore.com.

Achilles tendonitis only gets worse with time. If you are experiencing symptoms Call today for an evaluation in our convenient Seattle office.


1Effects of foot orthoses on Achilles tendon load in recreational runners   J. Sinclair, J. Isherwood, P.J. Taylor. Clinical Biomechanics; August 2014.

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