Another hot topic with the squat is knee health. I’m sure we’ve all thought it, come on now. Are all of those really deep and heavy squats good for the knee? I’ve been coaching others to squat for almost 5 years and I’ve luckily yet to have anyone’s kneecaps explode off of their bodies while squatting.

After what was far, far too much online researching with a drug like dependence on coffee, I’ve decided to put together my thoughts on the deep loaded squat and it’s effect on the knees.

So… Is deep squatting safe for the knee?

This is a very difficult question to answer and the best response is probably, it depends. Different parts of the squat stress the knee in different ways, everyone is an individual. Some forces increase over the course of the squat and some maximize at certain points. There’s also conflicting evidence in the medical literature (Why is it always so complicated???).

For example: Several authors state that compressive forces within the knee joint increase the deeper into a squat you descend and maximize at the very bottom of a squat(3). Other authors state that this research does not take into account several other factors and deeper squats actually decrease compressive forces on the knee(1).

In order to determine how dangerous squats are, we’ll have to first define the stresses on the knee joint during a squat. Understanding the stress on the knee will help shed some light on whether or not squats are actually dangerous. The 2 types of forces we’ll be talking about today are compressive and shear forces.

Compressive forces – The quadriceps muscle attaches to your knee cap (patella) via the quad tendon and the patella attaches to your tibia via the patellar tendon. As your knee bends the quadriceps and patella form a strap which will create compressive forces behind the patella during squatting.
Shear forces – Joints in our body have two major movements, a roll and a glide. As you bend the knee the joint spins in place (roll) but it also has some translatory motion (glide). These translatory forces (shear forces) are created during the squat, vary with the depth of the squat and stress different structures of the knee at different depths in the squat. These shear forces are in part, controlled by the musculature around the knee.

Both types of stress are important to the overall health of the knee and need to be taken into consideration when determining how safe deep squats are.

Aaron Swanson posted a great visual representation of what stresses are on the knee at different portions of the squat. As you can see, anterior shear forces are high at 0-60 degrees (stressing the ACL ligament) and posterior shear forces start around 50 degrees of knee bend and maximize at 90 degrees (Maximal stress to PCL ligament).

Compressive forces appear to peak much deeper in the squat, maximizing around the 90-130 degree mark based on this chart. This is in part due to surface area contact between the femur and patella (discussed below) and also due to the angle of pull changing from the quad as we descend further into the squat (as well as how hard the quad is contracting). Many studies that implicate deep squatting as maximizing compressive forces on the knee don’t take into account the “wrapping effect” of deep squats and soft tissue approximation of the hamstrings against the calves during the squat (1).

As we go through a squat, different surfaces of the femur and patella are stressed throughout the movement. Initially during a squat (0-10 degrees of knee flexion), there is no contact of the patella against the femur. As you get into 10-20 degrees of knee flexion you start to get some contact of the femur against patella. Initially the surface area of contact between the femur and patella is small. The boney surface area contact increases as we descend into the squat and maximizes at 90 degrees. After 90 degrees flexion the boney surface area decreases(3).

Now, if we have more force pressing into a smaller surface area, the stress on that one area increases. Imagine someone punching you lightly in the arm with their fist vs. punching you lightly in the arm with a knife. The knife obviously will cause more damage because it hits with a much smaller surface area while the fist has a larger surface area thus dissipating force. This is the theory behind increased patellar compressive forces as we descend into the squat.

More surface area = more force dissipation and less stress

Less surface area = less force dissipation and more stress

However, once we begin descending beyond 70 degrees of knee flexion in the squat, the quad tendon begins to contact the femur (Thus increasing surface area contact). This is known as the wrapping effect. Due to this effect some research (1) shows that compressive forces on the knee actually decrease after 90 degrees of knee flexion. (The same research implicates 90 degrees of knee flexion being the point of maximal compressive forces into the knee). Add into account the calf bumping into the hamstring at the bottom of the squat and this changes the compressive and shear forces again.

Keep in mind that not only are compressive and shear forces changing over the course of the squat, so is the stress on varying tissues in the knee. As alluded to earlier, different portions of the squat have different stresses on each ligament. Stress changes on different parts of the cartilage and meniscus of the knee as well during different parts of the squat. This will have major implications for individuals who are currently rehabbing from an injury and have pain.

Does deep squatting stress the knee more then a partial squat?

Theoretically, most individuals can handle more weight during partial squats (to say 90 degrees of knee flexion) compared to a maximally deep squat. Now, if compression and shear forces on the knee are greatest at 90 degrees of knee flexion then both squat variations are going to (or through) the most stressful portion of the lift. If the partial squatting athlete is utilizing more weight then the deep squat athlete, then the athlete using more weight will theoretically have more stress on the knee. In this case you could actually make the argument that partial squats are more stressful on the knee then deep squats (If weights used in the partial squat are higher then a deep squat)

Another major thing to keep in mind is that our bodies adapt to the stimulus that we place on it. What this means is that exposing your body to loaded deep squatting will promote change in your body. We’re all familiar with muscular growth and improvements in strength that occur with training the squat, but there are also changes within the joint itself, both with improvements in cartilage thickness and strength of tendons and ligaments (1)

So what does the research show about people who have a history of deep loaded squats and knee health. Most of the research in this realm is focused on olympic lifters, especially highly competitive and successful lifters. Olympic lifting athletes perform a great deal of deep squatting in their training. The research is not taking into account average Joe’s and Jane’s. Also keep in mind that successful athletes generally gravitate towards what they’re good at (olympic lifting) and wouldn’t be successful if they get chronic knee pain. Competitive olympic weightlifters also tend to be fairly young as well. Therefore you get a pretty large selection bias in this research. Anyway, the research in these lifters does look promising (1):

Weightlifters with an average of 17 years training experience (at a national and international level) show the same level of knee cartilage degeneration of an age matched general population
Weightlifters have been shown to have higher cartilage thickness then non-weightlifters. The thought here is that with squatting there are anabolic, biochemical and structural adaptations of the cartilage tissue, causing increased mechanical stress tolerance and hence protective effects against degenerative changes in both cartilage and meniscus. In rat models unloading cartilage causes atrophy and degeneration of the knee. Squats below 90 degrees stress cartilage of the odd facet preventing cartilage degeneration and atrophy of the odd facet (Which would otherwise not be stressed if deep squats were never performed)
Olympic lifters in general have a very low risk of injury compared to other sports (basketball, track and field, football and gymnastic) but the most commonly reported injuries occur to the knees and shoulders. (2)
In international level lifters 95% of knee injuries lasted less then 1 day
Deep squats loaded or not do not reach stress levels (shear force to ACL or PCL) to cause damage and do not cause knee ligament stability issues (Weightlifters generally have enhanced knee stability). The stress of deep squatting potentially causes adaptive positive changes in thickness and strength of ligaments and tensile strength of patellar and quad tendons.

Again, keep in mind that this research is in mostly elite lifters, but you can potentially infer that a long career of deep squatting with good technique at a competitive level may help protect the knees and potentially improve knee joint health. Of course there are several additional things to keep in mind before you go out there and start slamming deep loaded squats right away.

Bibliography:

Hartmann, H., Wirth, K., & Klusemann, M. (2013). Analysis of the Load on the Knee Joint and Vertebral Column with Changes in Squatting Depth and Weight Load. Sports Med.
Keogh, J. (2005, May 1). The Powerlifter’s Injuries. Pure Power Mag, 4-12.
Reinold, M. (2009, May 11). Solving the Patellofemoral Mystery. Retrieved January 10, 2016, from http://www.mikereinold.com/2009/05/solving-patellofemoral-mystery.html