Hey hey readers! In this month's blog post, I go over what the so-called "quads" (i.e. quadriceps, quadricep muscles, quadriceps femoris group) are and what they do in the body. These large, powerful muscles are super important for movement of the knee and hip. The quads are actually comprised of four different muscles located on the front and side(ish) of the thigh, and these muscles are the primary antagonists to the hamstring muscles, which are located on the back of the thigh. The four quadricep muscles converge to ultimately connect to the patella (i.e. kneecap).
You might wonder why I am choosing to focus this blog post on the quads. Well, as a human movement professional (yoga teacher, personal trainer, corrective exercise specialist, and therapist), I have come to learn that understanding your own anatomy helps you to better embody your body. If you know what, and where, your muscles are in your body, you can more intelligently activate those muscles for optimal movement. Thus, I would like to share some detailed information about these tissues with the hope that this information provides more insight into your current daily movement practices.
What are the 4 Quadricep Muscles?
The four large quadricep muscles include - the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. All of these muscles extend the knee (aka the tibiofemoral joint), or in other words, they straighten the knee. This is in contrast to the hamstring muscles (which I will discuss in another blog post, so stay tuned), which cause flexion at the knee joint (i.e. bending the knee). The three vastus muscles (sometimes referred to as the vastus group) produce about 80% of the total extension torque at the knee, while the rectus femoris only produces about 20% of knee extension torque. This is partly because the rectus femoris also causes hip flexion, whereas contraction of the vastus muscles only affect movement at the knee.
All four quad muscles converge into a common insertion tendon (i.e. connective tissue that connects muscle to bone), known as the quadriceps tendon. The quadriceps tendon inserts into the patella (i.e. kneecap), and from here, it continues inferiorly (i.e. towards the feet) to form the patellar ligament, which then attaches to the tibial tuberosity (i.e. a bumpy area on the top of the tibia, the larger lower leg bone). Thus, all four muscles ultimately insert into the tibia (see image below).
Together, the quadriceps muscle, patella, and patellar tendon are referred to as the knee extensor mechanism, and these interconnected tissues are capable of producing and transferring very large forces across the leg. These structures are also subjected to large, repetitive forces over one's lifetime, thereby making these tissues vulnerable to injury at some point. The quadricep muscles are all innervated by the femoral nerve. Let's chat about each muscle in more detail.
Rectus Femoris. The rectus femoris is a cylindrical and superficial (i.e. closer to the skin) muscle that runs straight down the center of the front of the thigh. It originates on the anterior inferior iliac spine (i.e. front of hip bone, sometimes referred to AIIS), and it inserts into the tibial tuberosity via the patellar ligament. The rectus femoris is interesting in that it is the only quad muscle that actually crosses two joints - the hips and the knee.- thereby affecting movement at both of those joints (hip flexion, knee extension, respectively). In contrast, the other 3 quad muscles only cross, and affect movement, at the knee joint.
Vastus Lateralis. The vastus lateralis forms the lateral (or outer) aspect of the thigh. It originates on the greater trochanter, a bumpy protrusion of the femur (thigh bone), as well as the linea aspera, which is a longitudinal ridge on the shaft (or long part) of the femur. Similar to all the other quad muscles, it ultimately inserts into the tibial tuberosity via the patellar ligament. It extends the knee (like all the other quad muscles as well), and it also helps provide stability to the knee joint.
Vastus Medialis. The vastus medialis forms the medial (or inner) portion of the thigh. It originates on the linea aspera, similar to the vastus lateralis, and it inserts into the tibial tuberosity via the patellar ligament like the other quadricep muscles. It has the same function as the vastus lateralis - extend and stabilize the knee joint.
Vastus Intermedius. The vastus intermedius lies deep to (or beneath) the rectus femoris, just between the vastus lateralis and vastus intermedius. It originates from the femur shaft and, as described for the other quadricep muscles, it inserts into the tibial tuberosity via the patellar ligament. The vastus intermedius extends the knee.
What are Quads Used For?
Through the isometric, eccentric, and concentric activations, the quad muscles create extensor torque to perform multiple functions at the knee joint.
Through isometric contraction (i.e. contraction of the muscle without any change in length of the muscle), the quadriceps stabilizes and protects the knee. Through eccentric activation (i.e. contracting while lengthening), the quads control the rate of descent of the body's center of mass, such as when sitting, squatting, walking downstairs, or landing from a jump. Eccentric contraction also provides shock absorption to the knee, acting as a spring and dampening the impact of loading on the joint, such as when landing from a jump, descending from a high step, or when making the initial foot contact during running. Concentric activation (i.e. contracting while shortening) accelerates the tibia or femur toward knee extension (straightening the knee) when raising the body's center of mass, such as during running uphill, jumping, and standing from a seated position.
Summary
Thanks for taking this deep-ish dive with me into the world of the quadricep muscles. The quads consist of four muscles that affect movement primarily at the knee (all four muscles), but also at the hip (rectus femoris only). These muscles are large and powerful, and operate to create movements needed in walking, running, jumping, climbing, sitting, and standing. All four quad muscles converge into a strong tendon, known as the quadriceps tendon, which continues onward after attaching to the patella to ultimately connect to the tibia in the lower leg. Strong quadricep muscles help keep the knee protected from injury and allow for better, more pain-free movement in the body.
Having a more in depth knowledge base about your own anatomy allows you to better activate and sequence muscle movements, thereby creating more efficient movement in your daily tasks. Stay tuned for a future blog post where I will go over the opposing, or antagonistic, muscles to the quads - the hamstrings. Also, I am still working through gathering and reading research for my next big blog series, all about yoga for dementia. I hope to release installments for this series later in 2024.
As always, the information presented in this blog post is derived from my own study of human movement and anatomy. If you have questions about your quadricep muscles, please follow up with your physician, physical therapist, personal trainer, or yoga teacher. If you are interested in private yoga and/or personal training sessions with me, Jackie, email me at info@lotusyogisbyjackie.com for more information about my services. Also, please subscribe to my website so you can receive my monthly newsletters (scroll to the bottom of the page where you can submit your email address). This will help keep you "in-the-know" about my latest blog releases and other helpful yoga and wellness information.  Thanks for reading!
~Namaste, Jackie Allen, M.S., M.Ed., CCC-SLP, RYT-200, RCYT, NASM-CPT, NASM-CES, NASM-SFC, NASM-SFS
References:
Biel, A. (2014). Trail Guide to the Body. 5th Edition. Books of Discovery. Boulder, CO.
Long, R. (2005). The Key Muscles of Yoga. Bandha Publications.
Marieb, E.N. (2004). Human Anatomy and Physiology. 6th Edition. Pearson Education Inc. San Francisco, CA.
Neumann, D.A. (2017). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 3rd Edition. Elsevier, Inc. St. Louis, MO.