Sever's disease occurs just before the closing of the heel bone growth plate. At this time in the child's development, the Achilles tendon is pulling on the heel bone with a significant amount of force; this causes increasing tension on the heel bone. The result is inflammation where the Achilles tendon attaches to the heel.
Sever's disease usually occurs when a child starts a new sport. Symptoms usually resolve with rest. To prevent the problem from recurring perform stretches that relax the Achilles tendon.