Flat foot. Coloured X-ray of the flat foot of a 25 year old woman. The decreased arching of the foot in this condition, also known as pes planus, means her sole lies flat on the ground. The bones are orange and flesh is green. The heel bone (lower left), metatarsals and tarsals are almost aligned in a straight line. Nearly all babies have flat feet, the arches only developing during childhood as the feet muscles and ligaments grow stronger. However in people with weak muscles and ligaments the arch can fail to develop. Flat feet can also develop in adults, for example people who gain weight rapidly. Treatment with arch supports and exercise is only needed if the feet are painful.