Prof. John told me that his feet has a fractured bone. Therefore sometimes when he walk, he will feel an acute pain between the ankle & the balls of his feet.
He relate the incident back to few years ago when he was in Tokyo. He had to give lecturers in Tokyo, therefore he had to wear formal attire with leather shoes. And so happen he walked almost 1 hour back & forth to find a shop. When he reached back to his apartment in Tokyo, his feet started to complain, calling out in pain. He ignored the pain.
However, nothing much happened until recently when the pain at the same position kept coming back to him. When he went to doctor & took X-ray, he found out that there is a small piece of bone fractured at his feet, therefore causing the pain.
Now, he wear sports shoes most of the time, always cautioning people of walking too long on cement with formal shoes.