Quad is the shorthand word used to describe the large quadriceps muscle found on the front of the thigh. You can easily feel this muscle working by sitting in a chair and extending your leg straight out in front of you. Being that the root word "quad" means four, there are four main parts of the muscle.
Rectus Femoris
The rectus femoris is found near the center of the thigh. It begins at the iliac bone of the pelvis and runs down to the circular patella bone, which is also known as the knee cap. The main function of the rectus femoris is to extend the knee and flex the hip. Knee extension takes place when you move your lower leg from a bent to straight position. Hip flexion takes place when you move your thigh toward your stomach. A squat exercise involves both of these motions. To do a squat, stand with your feet about shoulder-width apart and rest your arms at your sides. Slowly descend toward the floor by bending your knees. Once your thighs are parallel to the floor, stand back up and repeat.
Vastus Lateralis
The vastus lateralis is the segment of the quads that is on the side of the thigh. The root word "lateral" means toward the lateral side of the body. This muscle starts at the greater trochanter, which is the large rounded portion of the femur, and it ends at the patella. The main function of the vastus lateralis is to extend the knee. In weight training, a seated leg extension will work this muscle. To perform this exercise, sit on the leg extension machine with your feet hooked under the padded lever arm. Alternate lifting and lowering the arm by bending your knees.
Vastus Intermedius
The vastus intermedius runs from the femur to the patella and it lies directly under the rectus femoris in the center of the thigh. In similar fashion to the vastus lateralis and rectus femoris, it functions to extend the knee, so exercises like squats, leg extensions and leg presses will work this segment. The vastus intermedius and lateralis do not cause hip flexion. That is specific to the rectus femoris.
Vastus Medialis
The vastus medialis is the part of the quad nearest to the center line of the body. Any time you see the word "medial" in anatomy, it means toward the midline of the body. An adjunct to this muscle is the vastus medialis oblique, or VMO for short. This muscle runs down the inside of the thigh, crosses the knee and ends at the tibia bone of the shin. In training, you can place emphasis on this muscle by doing a split leg squat or lunge. To do this exercise, place your legs in a staggered stance, slowly lower yourself down by bending your knees and rising back up.
