The muscles of the calf (the gastrocnemius, soleus and plantaris, a.k.a. the triceps surae) fuse to form the Achilles tendon which attaches to the calcaneus, or heel. Because the tendon is very dense, strong, and has constant tension on it, the blood circulation in the tendon is limited. This makes repair from a minor tearing injury difficult.
And these minor defects (tears) can lead to major tears over time due to heavy use such as running sports. Active Release Technique (A.R.T.) attempts to gently move the tissue to encourage fluid exchange and release the fibrotic (scar) tissue, which the body has laid down to heal the original tears. And addressing other gait, strength or range of motion imbalances helps to ensure that the injury will not return.