Achilles Tendonitis: Inflammation in the Achilles tendon, connecting the calf muscle for the heel bone, typically because of overuse. This brings about pain and stiffness on the back from the heel, Specifically throughout exercise. The plantar fascia will be the thick connective tissue that supports the arch of the foot. https://www.theorthopaedicandpainpractice.com/why-does-the-back-of-my-heel-hurt/