The Function of the Knee

Outside of the lower back, the knee is one of the most common areas of the body we experience injuries and chronic pain. For professional athletes, more games are missed and money lost on knee injuries than almost any other type of injury. For recreational movers like you and I, it is just as common for knee issues to prevent us from staying on top of our fitness and physical activities. Understanding why we as humans commonly experience back pain makes sense: the lower back is essentially the crossroads of the body; tons of muscles, tissues, and kinetic chains feed through the lumbo-pelvic complex. It stands to reason that the low back has a large potential for things to go awry. But when it comes to the knee, why do so many of us experience symptoms and pain?

Anatomically, the knee is unfortunately predisposed to acute, sports-related injuries. The way the knee joint is set up structurally, I contend that a large part of what makes it so susceptible to injury is that it does not have many ways of compensating. Biomechanically, the knee is often at the mercy of how the foot and/or hip joints move and function. Core stability originates from the trunk and pelvis, ensuring optimal movement and loading through the hip joint, while closed-chain interaction with the ground starts with our feet, ensuring optimal movement and load through the ankles. The knee joint is situated right in the middle of the action as far as lower extremity movements go, and thus needs the capacity to dissipate, transmit, and tolerate large forces generated from the core and feet to keep everything communicating and functioning optimally.

Common acute knee injuries:

  • MCL/LCL/ACL/PCL sprains/tears
  • Meniscus-related issues
  • Hamstring strains

Common chronic knee symptoms:

  • Patellar tendonitis
  • Quad tendonitis
  • Popliteal pain
  • Patellar tracking issues

No matter the issue we might be dealing with in our knees, there are a few key concepts to understand in our attempts to better facilitate our treatments and training strategies. Understanding the 3 major roles the knee joint plays in movement gives a ton of insight into how to keep our knees healthy. They are:

  • screw-home mechanism and rotational stability
  • patellar tracking in relation to quad balance and ITB activity
  • transmission of GRF through the fibula/biceps femoris

Without further ado, let’s dive in!

The Screw-Home Mechanism

As I mentioned previously, we often associate the knee with flexion and extension, movements that predominantly occur in the sagittal (front to back movements) plane. What often goes underappreciated is how much rotation occurs at the knee, and what role that rotation plays in the function of our lower-extremity kinetic chains.

This is a good visual demonstration of how the knee locks and unlocks in rotation as it goes through flexion-extension moments:

When the knee is fully extended (straight), the femur rotates internally relative to the tibia, and the tibia rotates externally relative to the femur.

When flexion is initiated at the knee, the femur rotates externally relative to the tibia, and the tibia rotates internally relative to the femur.

In gait, the external rotation of the tibia is coupled with supination of the foot and external rotation of the hip. The inverse is also true, whereby the internal rotation of the tibia is coupled with pronation of the foot and internal rotation of the hip. If any of these joint motions are not fully accessible, it could potentially lead to excessive motion at the other joints, increasing mechanical stress and creating symptoms. In order to fully understand and resolve our knee symptoms, we need to make sure that each of these coupled movements at the foot, knee, and hip are integrated well. Below are a couple examples of how these ideas might manifest as knee pain:

An immobile navicular bone in the foot is preventing full pronation in the foot. A lack of adequate pronation then leads to increased subtalar inversion, which shifts the talus position medially. The talus shifting medially then neurologically inhibits all the muscles in the lower extremity moving laterally. Decreased muscle strength of the glute medius, TFL, and other abductors then destabilizes the hip leading to an aggressive knee valgus, and the medial knee becomes painful. Our symptoms are at the knee, but in order to restore them we need to retrace the origins of the dysfunction and improve mobility and joint-loading capacity at the navicular bone.

OR

An unstable SI joint stuck in anterior torsion. This decreases the hip joint’s available external rotation. A lack of hip external rotation then becomes compensated for by increased external rotation of the tibia. Excessive external rotation of the tibia then leads to an imbalance between the lateral and medial hamstrings over time. The lateral hamstrings have more mechanical stress on them, and over time we start to get pain in the posterior lateral knee.

There are numerous potential ways in which any one person can compensate around dysfunction. The above are some hypothetical examples. The primary point is that rotation does in fact occur at the knee, and it is greatly affected by our both feet and our hips. In order to optimize our knee function, it is imperative that we consider rotational stability and how things are functioning above and below at the hip and foot respectively. Ensuring optimal stability at the pelvis and movement through the hip, as well as stability of the foot and movement in the ankle will allow the knee to stay within its optimal parameters for movement, reducing the likelihood of symptoms.

Patellar Tracking

The patella being one of the primary structures involved in knee mechanics, it is important to have an understanding of the role the patella plays in movement, as well as what muscles are involved in those movements.

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The patella is a sesamoid bone. We have just a few sesamoid bones in the entire body, the others are located in the flexor hallucis brevis muscle on the bottom of the foot. What characterizes a sesamoid bone is that it is embedded within the tendon. Sesamoid bones essentially act like pulleys, allowing for increased mechanical advantage and improve a tendon’s ability to transmit forces.

Our patella is the largest sesamoid bone in the body, and it is primarily acted upon by some very large and powerful muscles, the quadriceps. Optimized knee mechanics require balanced patellar tracking, and balanced patellar tracking is ultimately determined by balance in the quadriceps. We have 4 quad muscles (hence, “quads”), and each function just a bit differently. Take a look at them below.

 

 

Going clockwise from upper left:   Vastus Lateralis (outer quad), Vastus Medialis (inner quad), Vastus Intermedius (middle quad), and Rectus Femoris (two-joint quad, crossing both the knee and the hip).

 


Vastus Lateralis (Outer Quad)Vastus Medialis (Inner Quad)Vastus Intermedius (Middle Quad)Rectus Femoris (notice how it crosses the knee AND hip joints)

Common sense would tell us that if the lateral quad muscle fibers pull with too much tension relative to the medial quad fibers, the patella will track more laterally; the opposite is also true. We definitely want to make sure that our vastus lateralis and vastus medialis are working in harmony in order to keep our patellar tracking optimized.

However, what I find to be far more of a common problem in patellar tracking is the rectus femoris. The rectus femoris is the muscle you feel pulling tight when you the all-too common kneeling hip flexor stretch. It is the most superficial quad muscle, and has the most direct impact on patellar function due to uniqueness among the quads. As you might notice on the picture above, the rectus femoris crosses both the knee joint through the quad tendon, patella, and patellar tendon attaching on the tibia, but it also crosses the hip joint attaching on the ASIS. Because the muscle crosses two joints, the rectus femoris is both a hip flexor and a knee extensor. This makes it more susceptible to unwanted compensation.

Patellar tendonitis is perhaps the most common knee condition I treat regularly. It is sometimes also referred to as “Jumper’s Knee.” It is often characterized by a sharp pain occurring just below the patella during activities like squatting, walking up or down stairs, lunging, or other activities involving knee bending. The rectus femoris muscle is usually our prime suspect when it comes to patellar pain. Taking into account that the muscle crosses two joints, it is often important to appreciate the rectus femoris’ role on the hip, not just the knee. As I’ve mentioned several times already, the knee is often at the mercy of what is occuring at the hip and the feet. This is another piece to that puzzle. Appreciating the functional relationship between the hip flexors and knee extensors is the key to maintaining normal patellar tracking and patellar tendon health.

Shock Absorption & Dissipation of Ground Reaction Forces

When we’re talking about movements and functions of the knee, our conversations are usually focused on the previous two ideas we’ve covered. Far less understood is the role the knee plays in shock absorption and transfer of kinetic energy via ground reaction forces. What does all this mean? Keep reading to find out.

The knee actually has 4 bones associated with it. Most of us know of the femur (leg bone), tibia (shin bone), and patella (knee cap), but what we often fail to appreciate is the role the fibula (outer lower leg bone) plays in knee mechanics.

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The fibula has two primary roles within the knee joint. First, the distal end helps to form the talocrural (true ankle) joint, while the proximal end provides additional stability for the knee joint and helps to transmit force from the lateral foot through the lateral hamstrings and into the pelvis. We are mostly focused on the proximal portion and how it helps to transmit force through the lower extremity.

As part of the deep longitudinal subsystem, the tibiofibular joint is incredibly important for providing the knee with additional stability for activities like running and jumping that require a lot of force to be transferring through multiple joints in sequence. In gait, as we heel strike, the energy upon contacting the ground is sent through the heel, then the ankle, then the fibula and knee joint, through the hamstrings, and into the pelvis. If we lose our ability to respond to load in the tibio-fibular joint, then we are likely to experience knee symptoms, chronic hamstring tightness/irritation, and dysfunctional compensation further upstream or downstream in the hips or ankles.

Take a quick glance at the feathery connective tissue between the fibula and tibia. Notice the oblique direction of the interosseous membrane. This tissue plays a significant role in shock absorption in the lower leg.

Wrapping Up

The knee is a more complex joint than most of us give it credit for. Symptoms in the knee are incredibly common, and we need to have better ways of understanding, assessing, and correcting dysfunctions in the knee.

Often times we must consider the function of the joints above and below the knee, as well as how the muscles acting on the knee are functioning. We must appreciate that the knee joint has several important roles, and that each must be working optimally to minimize our risk of injury. The knee plays an important role by providing:

  • Rotational stability in athletic movements
  • Optimized tracking of our patella
  • Absorption and dissipation of ground reaction forces in athletic movement

Make sure that you are assessing the knee through several different lenses, understanding there are more functions in the knee than just flexion and extension. As we work our way up through the body, we are going to take a closer look at the hips and what the most important factors are when it comes to hip function and movement.