The quadriceps femoris or quads are a group of muscles located in the front of the thigh. They’re called that way because the group is comprised of four different muscles. They are the vastus lateralis, the vastus medialis, the vastus intermedius and the rectus femoris. The three vastus muscles originates on the femur bone and attaches to the patella, or kneecap.
The Rectus Femoris inserts into the hip bone then covers the three vastus muscles and also attaches to the kneecap.
One of the main functions of the quadriceps is to assist in extending the knee and it is best trained with heavy compound exercises such as squats or lunges, hex squats or sumo squats.
Just make sure you use good form because the quadriceps are prone to injuries including strains, tears and ruptures.
If you can’t do any of the just mentioned exercises or need to bring up a lagging muscle.
You can also add leg extensions as an isolation lift but it shouldn’t be a primary exercise for a quad development.