In this tutorial you will learn to add some more controls and a few mathematical expressions to make key framing easier. Many people had problems with the expression so I will type it below once again
body.tx = (L_Foot.tx + R_Foot.tx)/2
body.ty = (L_Foot.ty + R_Foot.ty)/2
body.tz = (L_Foot.tz + R_Foot.tz)/2
ALSO REMEMBER THAT THE EXPRESSION EDITOR IS CASE SENSITIVE