Achilles tendonitis is basically an overuse injury and pain may be experienced at the insertion into the heel bone or in the mid-portion of the tendon about 4 cm above the heel bone. The Achilles tendon is the thickest and strongest tendon in the body. It is found at the back of the lower leg and connects the calf muscle (made up of the Gastrocnemius and Soleus muscles) to the heel bone (calcaneus).