Feet-Hurt.Com

As we age, the one part of the body where we lose fat is the bottom of the foot. Normally the “fat pad” underneath the metatarsal bones cushion the foot. Once this padding is lost a person is then literally walking on the heads of the bones. Doing a lot of walking either barefoot or in shoes with flimsy soles creates a lot of pressure on the metatarsal heads and they begin to hurt thus causing metatarsalgia. A persons foot structure and amount of body weight that they are carrying can also contribute to the problem, as well as occupations that require a lot of standing or walking.

From this site:  http://www.foot-pain-explained.com/metatarsalgia.html