Thighs are high on the workout priority list for many women looking to improve their physique. Knee extension exercises -- straightening your knee from a bent position -- are often included in weight-training programs because they tone your quads -- the large muscles running along the front of your thighs. These exercises work four muscles at once, saving time in your busy schedule.
The rectus femoris muscle is the most superficial muscle in the front of your thigh. It begins above your hip, attached to the front of your pelvis by a tendon. This muscle runs down the middle of the front of your thigh. At the bottom, it attaches to the top of your kneecap. The rectus femoris also contributes to a thick ligament -- the patellar ligament -- that attaches the quad muscles to the front of your lower leg bone. This ligament pulls your knee straight as your quad muscles contract during the knee extension exercise.
Three additional muscles help straighten your knees during the knee extension exercise. These muscles start on the thighbone, up near your hip joint. The vastus lateralis muscle starts at the outer edge at the top of your thighbone and runs along the outside edge of the front of your thigh. This muscle attaches to the outside of the top of your kneecap and joins the patellar tendon at the front of your lower leg.
The top of the vastus intermedius muscle attaches to the upper two-thirds of the front of the thighbone. It runs along the middle of the front of your thigh and attaches to the top of your kneecap. This muscle is also connected to the patellar ligament.
Vastus Medialis (Oblique)
The vastus medialis muscle -- also called vastus medialis oblique -- runs from the top of your thighbone, down along the inner edge of the front of your thigh. It attaches to the top of your kneecap, toward the inside. Connective tissue at the bottom of this muscle joins the patellar ligament.
Typically, the seat of a knee extension exercise machine is close to upright, targeting all four quad muscles. However, you can make your rectus femoris work a little harder by leaning the seat backward. This is the only quad muscle that crosses your hip joint. In an upright position, some of the tension is taken off this muscle at your hip. By leaning the seat backward, you stretch the rectus femoris. This adds tension to the muscle, increasing resistance as you straighten your knee during the extension exercise.
- Eastern Illinois University: The Knee Joint
- Journal of Applied Physiology: Activation Varies Among the Knee Extensor Muscles During a Submaximal Fatiguing Contraction in the Seated and Supine Postures
- The Open Rehabilitation Journal: EMG Activation of the Vastus Medialis Oblique and Vastus Lateralis During Four Rehabilitative Exercises
- Hemera Technologies/AbleStock.com/Getty Images