According to AZ Central, seated leg extensions work the four muscle heads of the quadriceps, which are in the upper thighs. Standing leg extensions also work the quadriceps and other abdominal muscles. AZ Central explains that standing leg extensions work the transverse abdominal, or stomach muscle, and the obliques at the same time. Obliques are side abdominal muscles that enable individuals to balance on one leg.
The quadriceps include the rectus femoris, vastus intermedius, vastus lateralis and vastus medialis, according to AZ Central. The rectus femoris is the largest of the quadriceps, and it sits front and center in the thigh; the vastus intermedius is located beneath the rectus femoris; the vastus lateralis runs along the outside front of the thigh; and the vastus medialis runs along the inside front of the thigh.
Proper technique is critical when performing leg extensions, AZ Central explains. The weight should be placed slightly above the ankle. The leg should not be extended all the way because full extension places pressure on the knee. When the leg is lowered, the motion must stop when the knee is at a ninety-degree angle to prevent patellar compression, which can cause serious knee injury. Leg extensions are typically performed on a leg-extension machine.