Every time you take a step, one of your heels has to support the whole weight of your body. As you move, the load is equal to 20 times your own body weight. The load is softened by a pillow of fat under the heel and a large sinew under the sole of the foot.

If an athlete does not warm up properly or a person with a sedentary job exercises heavily during the weekends, they might overload the muscles of the calf or the Achilles tendon. These are also fixed on the heel bone. When the muscles of the calf or Achilles tendon are overloaded there will be extra strain on the sinew and muscles in the soles of the foot. The overload can cause inflammation and even small cracks in the sinew.

Every time you sit down, sleep or otherwise rest your legs, the muscles of the sole of the foot will contract in an attempt to protect the damaged sinew. The pain in the heel will then no longer be felt. But when you get up again the pain will return and when you move again, the sinew will crack even more.

To compensate for the repeated damage to the sinew, the body will try and repair it in the same way that it would attempt to repair a broken bone, namely, by wrapping it up in bone. The result is a small bony projection on the heel bone called a calcaneal spur.

But it is not the spur itself that causes the pain. The spur is the result of a prolonged overload of the sinew at the sole of the foot.