The primary mover during the seated or lying leg extension exercise is a group of four muscle heads on the front of the thigh, collectively referred to as the quadriceps. The largest muscle head is the rectus femoris, which sits in the center of the front thigh. The vastus intermedius lies under the rectus femoris. The vastus lateralis runs along the outside of the front thigh and the vastus medialis runs down the inside of the front thigh. All four muscles extend from the top of the thigh to the knee joint and all are involved in knee extension. During the standing leg extension, the transverse abdominis, which is the deepest layer of stomach muscle, and the obliques, the side abs, are also primary movers; they activate to help stabilize your body as you balance on one leg.