Plantar fasciitis is a common cause of heel pain. It occurs when you strain the tendon that runs along the arch of your foot, from your heel bone to your toes. Straining this tendon can cause it to become weak, swollen and inflamed. This can cause pain when you stand or walk and can occur in one foot or both feet. It is most common in people of middle age, or in people who are on their feet a lot- eg. Athletes.