Why does rectus femoris become active in leg extension?

The rectus femoris becomes active in a leg extension because one of the actions of the rectus femoris is hip extension, as well as knee extension. The rectus femoris crosses both the hip and knee on the anterior side of the body. When the leg is extended it is one of the primary movers, whether it be straight leg extension, in which case it is the rectus femoris, illium and psoas muscles. In knee extension the rectus femoris works in conjunction with the other 3 quadriceps. However, in the case of a seated leg extension the rectus femois is less active, due to it already being shortened for the hip extension.