The large four-part muscle group at the front of the thigh, worked heavily by squats and leg extensions, is called the quadriceps. Its full anatomical name is the quadriceps femoris, often shortened to “quads.”
The group contains four muscles: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. Together they form much of the front and sides of the thigh and are the primary muscles responsible for extending the knee. Rectus femoris also crosses the hip, so it helps flex the hip as well as straighten the knee.
The quadriceps connects to the patella through the quadriceps tendon. Force then continues from the kneecap through the patellar ligament to the tibial tuberosity on the shinbone. This arrangement lets the muscle group transmit power efficiently during standing, climbing, jumping, running, and rising from a squat.
A frequent mix-up is confusing the quadriceps with the hamstrings. The quadriceps lie mainly on the front of the thigh and extend the knee; the hamstrings are on the back and help flex the knee. Squats train several lower-body muscles, but leg extensions place especially direct emphasis on knee extension by the quadriceps.