The quadriceps, or "quads", make up majority of the muscle of the anterior portion of the upper leg. The name quadriceps comes from the fact that there are 4 portions or "heads" that make up the quads.
The four heads of the quadriceps are:
- Rectus femoris (at right, in blue)
- Vastus Lateralis (at right, in yellow)
- Vastus Medialis (at right, in red)
- Vastus Intermedius (at right, in green)
The Rectus Femoris attaches on the ilium (hip) and covers most of the other 3 heads. The Vastus Lateralis, Medius, and Intermedius all originate on the upper part of the femur. All 4 heads insert, or attach, on the tibial tuberosity just below the knee.
All 4 heads of the quadriceps extend the lower leg at the knee, however, since the rectus femoris is the only head that attaches to the hip, it is the only head that works on flexing the upper leg at the hip.
- Knee Extension
- Hip Flexion (rectus femoris only)
Have you ever thought about why you have a knee cap, also known as the patella? It is a bit odd if you think about it. Your knee cap is, essentially, a free-floating disc of bone. The truth of the matter, however, is that the knee cap is a very important and beneficial tool to improve the effectiveness of the quadriceps.
The quadriceps muscles make up the patella tendon, that passes over the kneecap and attaches to the tibia, just below the knee. Since the tendon passes over the patella (kneecap), when the quadricep muscle pulls on the tibia to extend the leg at the knee, the patella increases the mechanical advantage of the knee.
If you think back to your basic physics lessons in school, the further the generating force (patella tendon) is from the centre of rotation (the knee itself), the less force is required to move the object. This is the same applied principle when using a lever arm. If you were to lift a heavy object, such as a piece of furniture using a lever, the longer the lever, the easier (less overall force) it would be to move said object.