The Achilles tendon is a composite tendon from the gastrocnemius and soleus muscles. The gastrocnemius crosses the knee joint, but the soleus does not. This is the basis of the Silfverskiold test to differentiate gastrocnemius and soleus tightness. It also potentially creates different stresses in the two parts of the tendon.
The tendon fibres have a spiral arrangement. In the upper part of the leg, the soleus fibres are anterior to the gastrocnemius fibres. Lower down, the tendon has rotated so that the soleus fibres lie medial to those of the gastrocnemius. A combination of the spiral and subtalar joint movements produce twisting of the tendon, which has been suggested as a factor in the production of tendonopathy and paratendonitis.
Recently there has been interest in the role of the plantaris tendon in Achilles tendon pain. The plantaris arises from the lateral femoral condyle and crosses obliquely between the gastrocnemius and soleus to insert in various ways to the heel (van Sterkenburg 2011). The differential movement of the tendons may create abrasion and adhesions in the plantaris tendon, which tend to be found in the painful mid-portion zone of the Achilles tendon (van Sterkenburg 2011). Lintz (2010) compared the material properties of Achilles and plantaris tendons and found plantaris to be stiffer and stronger, suggesting it could tether the Achilles.
Function and biomechanics
The Achilles tendon is the principal plantar flexor of the ankle, generating forces of six times body mass in late stance phase to control forward movement. Most forward progress is produced by "controlled falling" of the body momentum; the gastrosoleus muscles control this process by eccentric, followed by concentric, contraction in mid to late stance (Sutherland 1980)
James suggested that overpronation of the subtalar joint may increase the stresses in the medial tendon and predispose to tendonopathy. However they offered no data on foot shape in patients or controls. Angermann and Hovgaard (1999) found that 14% of their patients were overpronators and 27% had cavovarus feet. No study has looked at the distribution of foot shape in a large population with Achilles tendonopathy and compared with a control group. Indeed, there are problems of defining exactly what a cavus or overpronated foot are. Also, foot shape is of academic interest only, unless it is demonstrated that biomechanical treatment either before or after the development of tendonopathy prevents the condition or alters the natural history.