Q: I have been training for the Boston marathon and have now developed pain on the underside of my foot by my heel. Could this be plantar fasciitis?A: The deep plantar fascia is a thick layer of fibrous tissue on the bottom of the foot under the arch. It is attached to the skin by strong fibers, and it also attaches to the calcaneus (the heel bone) on one end and splits into five strands to attach to the bones of each toe at the other end.