BeneFIT PT’s Anatomy Series: The Knee (part 2)

We are continuing BeneFIT  Physical Therapy’s Anatomy  Blog Series this month with the Knee Joint. Each month we will feature a different part of the body.  BeneFIT PT’s staff is highly trained in knowing all about human anatomy so that we can effectively help our patients with their specific areas of concern, rehabilitation and strengthening.

Last post we began our discussion of the knee joint by highlighting the bones, ligaments, and tendons of the knee.  Throw in the bit about the meniscus and last week’s post was pretty long and informative.  Which is why we broke our knee anatomy post into 2 parts.  With that much information highlighting all the muscles that surround the knee felt like an overload.  So without further adieu let’s dive into what makes our knee joint move!


The knee is composed of 3 basic muscle groups that provide the majority of its stability and function.  They can be split up based on their location on our legs.  The first are the quadriceps which are principally responsible for extension at your knee, basically these are the muscles we use to straighten our leg.  The second group are the hamstring muscles which are responsible for flexion of the knee, we use these muscles when we want to bend our knee to bring our heel towards our butt.  Finally, the third is the gastrocnemius muscle.  This guy is located on the back of your calf.  It helps you point your toes (this is called plantar flexion).  Even though these guys look like they just sit on your lower leg, they actually start above the knee joint on the femur and help stabilize the knee.  Let’s dive in to a little more detail of each.


Quadriceps Femoris (quads) – The quads are a large group of four muscles on the front of the thigh that help straighten (extend) the leg.  Despite being 4 individual muscles they are all connected by sheaths of fascia, which is connective tissue that keeps them together, to act as one giant muscle.  All four muscles come together to form the quadriceps tendon located just above the kneecap. The quad tendon attaches to the top of the kneecap and then extends over the kneecap.  From there it continues from the bottom of the kneecap to a bump on the top of your tibia.  This portion of the tendon from the bottom of the kneecap to the tibia is called the patellar ligament (or patellar tendon). When looking down at your leg the four quad muscles are:

  • Knee anatomy, quadricep muscles

    The four muscles of the Quadriceps: rectus femoris, vests intermedium, vests lateralis, and vests medialis

    Vastus lateralis – positioned on the outer (lateral) side of the front (anterior) of the thigh.

  • Vastus intermedius – positioned on the front (anterior) and central part of the thigh. It lies underneath the rectus femoris (see fig.3).

  • Vastus medialis – positioned on the inner (medial) side of the front (anterior) thigh. It aides in stabilizing the patella (knee cap).

  • Rectus femoris – positioned on the front (anterior) thigh. It is also minimally assists in flexing the hip (bends the hip).


Hamstrings – Located on the back side (posterior) of the thigh, the hamstrings primary function is to bend (flex) the knee, but it will also help in extending the hip.  This is due to the fact that some of the hamstring muscles start on the pelvis.  Where as the quadriceps muscles come together to create one tendon to cross the knee the hamstrings actually cross the knee on either side of the knee.  Two of the hamstring muscles wrap around towards the front of the tibia and attach at a location called the pes anserine.  The third swings around the other side of the knee and attaches to the top of the fibula.  The hamstrings are comprised of the following 3 muscles:

  • Knee anatomy, muscles around the knee.

    The three muscles of the hamstring. Semimembranosis, semitendinosis, and biceps femoris.

    Semimembranosus – begins on our ischial tuberosity (sitting bones) along the femur and attaches to the inner (medial) tibia at the pes anserine.  This is the most medial (inner) of the three hamstring muscles.

  • Semitendinosus – this muscle also starts at the ITs (ischial tuberosity) then runs along the femur to the inner (medial) tibia at the pes anserine.  This is the middle of the three hamstring muscles.

  • Biceps femoris – this muscle actually as two parts to it.  The longer part is aptly called the long head while the shorter part is called the short head.  The long head begins on the pelvis runs along the femur and then attaches to the head of the fibula.  The short head starts on the back of the femur, runs down it’s length, and then also swings laterally to the head of the fibula.

Lower leg muscles

Gastrocnemius (gastroc) – The gastrocnemius makes up the main bulk of your calf muscles. There are two heads, one along the backside (posterior) outer (lateral) leg and one along the inside (medial) leg. They both start on the back underside of the femur at the femoral condyle (the very end of the femur) and attach to the heel bone (calcaneus) by way of the achilles tendon. This muscle plantarflexes the ankle (pointing your toes towards the ground like pushing a car gas pedal) allowing you to stand on your tip toes as well as helping to bend (flexes) the knee.

Plantaris – The plantaris muscle is a very small muscle that runs almost parallel to the gastrocnemius muscle.  It starts along the bottom of the femur by the gastrocnemius and runs along the same path to attach by the heel.  However, the muscle is so small that current research suspects it does little to any assistance in plantar flexing the ankle or bending the knee.  In fact, current research indicates that our body uses it as more of a sensory muscle to help us figure out where our knee is in space while we move.  That ability is called proprioception.

Popliteus – The popliteus muscle is the forgotten muscle of the knee.  Most medical professionals don’t mention it, most research barely talks about it, some people aren’t even sure what it truly does!  It starts on the lateral border of the femur wraps behind the knee and ends along the medial border of the tibia.  Our current research indicates that this muscle rotates the knee slightly to help lock it in place when we straighten it.  The current thinking is that it helps in posterior-lateral stability.

lower leg anatomy

The image on the left highlights the location of both heads of the Gastrocnemius. The image on the right has removed the gastrocnemius so you can see both the plantaris and popliteus muscles. ©2013 Pearson Education Inc.


Bringing it All Together

As you can see the knee joint is more complicated then we typically give it credit for.  The knee has a number of different common injuries you might have to deal with based on the combination of bone, muscle, ligament, meniscus, or nerves involved in the injury.  Touching base with your Physical Therapist can speed up the process in figuring out what exactly is causing your pain…even without an x-ray!

If you’re having any issue with your knee currently and want to get it checked out just contact either of our offices.  Our Doctors of Physical Therapy can see you without a prescription, covered by your insurance, often times within 24 hours from your call!