 This is because the trajectory of the center of pressure (COP) and the angle of the ground reaction forces is determined by the shape and stiffness of the foot and needs to match the subject's build in order to produce a normal gait pattern.Andrysek (2010) found 16 different types of feet, with greatly varying results concerning durability and biomechanics.The main problem found in current feet is durability, endurance ranging from 16–32 months These results are for adults and will probably be worse for children due to higher activity levels and scale effects.