Are Squats Bad For Your Knees?



Whether it is while I am speaking with patients or perusing social media, I often come across the belief that squatting is dangerous and harmful to the knees.


I would like to challenge those beliefs.


Squatting is one of the most fundamental movements we perform. Every time you sit in a chair or onto the toilet you are squatting.


Ok, but those are light movements and the knee bend is controlled, right?


They are, but that does not mean heavy loads or deep knee bends are inherently dangerous. Don’t believe me? Let’s take a deeper dive into the research.


Is compression dangerous?

The first argument against squatting is the compression is dangerous. If you perform loaded squats repeatedly, the cartilage will wear down leading to a future of setting off airport metal detectors. This has some merit as the more you load a squat, the greater the compressive forces at the knee joint rise.


But we aren’t giving the body enough credit.


Our body is remarkably resilient. It adapts to stressors and makes modifications accordingly. We have built-in fail-safes to handle rigorous stress, such as ligaments, tendons, and muscles. Cartilage is an impressive tissue with great shock absorbing properties, but the other structures can disperse some of the compression and protect against shear forces.


As we expose our body to stress, it adapts. The keys are how much stress the body is exposed to and how fast the stress is applied. If you gradually increase the intensity of exercise — take the squat for example — you gradually build muscle, neuromuscular control (strength and power), tendon and bone thickness, and cartilage resilience.


Yes, cartilage responds favorably to compression. The combination of repeated compression, shear, and torsion may be an issue, however.


This is where we have to blend load with the depth of the squat.


How deep should you squat?

This is a challenging question to answer as it depends on many factors. If you are a powerlifter, for example, you must squat deep (crease of the hip past the knees) for the squat to count in a competition. If you are a weightlifter or CrossFitter, you need to be comfortable catching a clean or snatch in a deep squat position. If you are trying to build muscle and improve your health, deep squats may be unnecessary — but not useless.


Let’s start with the impact of deep squatting on knee health.


Injury prevalence amongst individuals who perform deep squats is low. At the international level, weight lifters (snatch and clean and jerk) often train 10 times per week. Despite the high load of deep squatting, knee injuries are uncommon, regardless of age.


A study assessing 13–16-year-old weightlifters found injury rates to be 0.0017 per 100 training hours, lower than more common sports such as basketball (0.30), track and field (0.57), football (0.10), and gymnastics (0.044).


Now let’s look at the reasons why deep squatting may not be dangerous.


Compressive forces behind the patella (knee cap) and compressive stress in the knee joint are greatest at 90 degrees of knee flexion (a half squat as the bottom half of the leg is angled forward — squat while looking in a mirror and you will see parallel is closer to 110–120 degrees of knee flexion).


There is a caveat.


At 90 degrees, the contact between the quadriceps tendon and the knee joint are substantial, helping distribute load and force. Another consideration is the amount of weight you can successfully squat when bending to 90 degrees. The greater the depth of the squat, the harder it is to lift heavy weight (more on the why behind this later).


Is it more harmful to load up additional weight and reduce the squat depth or keep the weight lighter and squat deep?


Once again, it depends on your body’s adaption across your training history.

When performing deep squats — greater than 120 degrees of knee flexion — the contact area of the bones within the knee joint diminishes. You can rely more on the force transfer and protective effects of surrounding ligaments and muscles, provided the progression has been gradual enough for those tissues to build resilience.


The compressive forces are greatest between 80–100 degrees of knee flexion. The forces reach their dynamic peak when transitioning from the lowering phase to the standing up phase of the squat. Another factor to consider is speed. The faster you descend and transition to standing, the greater the compressive forces, reaching up to 24 times your body weight. That number sounds scary, but it is only half of the equation.


How much stress can the body tolerate?


This is the primary issue with text neck graphics. Just because learning your head forward creates more strain does not mean it is dangerous for your neck. Many activities increase stress to the body — running, jumping, going down stairs, picking up a child — but the body isn’t harmed.


When assessing knee joint cartilage in weightlifters, the results tend to be positive. Compared to controls, people who frequently exercise and load the knees have less degeneration. The cartilage, tendons, and muscles build increased mechanical stress tolerance over time.


Even in the spine, frequent squatting at high loads may be beneficial. Recent research found less degeneration in the spine when comparing weightlifters to matched sedentary controls. This is an incomplete picture, however, as we would want to see studies comparing active people who heavily load the spine and active people who do not.


This does not mean degeneration cannot occur, but it will not be the result of compression. Overtraining or being sedentary, eating poorly, rarely sleeping, and maintaining high-stress levels are likely more harmful and likely to cause knee degeneration than consistent deep squatting. But you can still control many squat variables as you build up your tolerance and resilience.


Understanding the factors that influence compression allows us to make appropriate modifications to squats. Box squats can reduce the increase in compression during the transition phase by providing a braking mechanism. Keep weight lighter at first and aim for a depth close to parallel. Use slow and control speeds, such as 3–4 seconds during the lowering phase and 1–2 seconds to rise to the top.


These are the big picture items. What about the details of squatting mechanics?

Let’s dive a little deeper into form.

Do the knees need to stay behind the toes?

When I started treating in the clinic, I often told patients to keep their toes behind their knees during the squat. One of my favorite drills was squatting facing the wall. The goal was to perform full depth squat with the toes within 4 inches of the wall — bonus points if they completed the exercise holding their hands above their head.


I don’t do that anymore.


The forward progression of our knees is largely determined by our height and leg bone length (tibia specifically). Squatting should not look the exact same person to person. There can be some fundamental similarities, but individual variance must be taken into account — not just across people but within them.


Our bodies are not symmetrical. One foot is often bigger than the other. The hip sockets can differ in orientation and depth, altering the amount of mobility available. We also develop different amounts of muscle side to side based on dominance and daily activities (occupation or sports). Alignment does not matter for health (neither does posture for that matter).


There will be some similarities in squatting mechanics across people, but we need to allow for a differing amount of hip and knee bend based on the body type. Some of the fundamental beliefs — such as knees over toes — need to be challenged as well.


If you try to force the knees to stay behind the toes, there will be a greater mobility demand at the hips and spine — both will flex more. If you are worried about shear forces at the knee, you are simply transferring them to the hip and back. Keep in mind, those shear forces are not inherently dangerous, even for the back. Have I mentioned the body adapts?


Relative increases in stress do not equate to relative increases in injury risk.


Similar to the compressive force studies, shear forces at the knee are greatest at 90 degrees of flexion. The deeper you squat, the more protection you receive from muscles and tendons.


But what about ligaments? Are you at risk of tearing your anterior cruciate ligament (ACL) if your knees fly past your toes?


The ACL can tolerate 1730–2160 N (depending on the study) and the posterior cruciate ligament (PCL) can tolerate up to 4000 N. Studies show the squat only reaches up to 41.56% of the ACL threshold and 51.56% of the PCL threshold. Furthermore, ligament and tendon thickness and tensile strength increase with consistent loading through squatting and weightlifting.


The more you look at the research, the more you come to realize the half squat may be the most strenuous of all.


Strenuous, not dangerous.


We need to flip the narrative. Instead of taking a black and white view and avoiding the squat, we should instead seek how can we modify the squat and training parameters to maximize performance and benefit.


Squat depth influences squat performance

So far I have focused on the safety of deep squats, but what about their training value. Even if the squats are safe, are they beneficial?


It depends on what you are trying to achieve.


If you want to compete in powerlifting, weightlifting, or CrossFit, the deep squat is a necessity. If you play a sport that often puts you in a deep squat position — football, volleyball (digs), wrestling — then deep squats may improve your strength and power at extreme ranges of motion. For everyone else, you need to ask why you want to squat deep.


If the goal is to build muscle or strength, deep squatting may work against you. Here are the ranges of motion to maximize the activation of specific muscles:

  • Hamstring: 10–70 degrees of knee flexion (half squat)

  • Quadriceps: 80+ degrees of knee flexion (parallel squat)

  • Erector Spinae (if squats are loaded): 80+ degrees of knee flexion (parallel squat)

  • Glutes: 90+ degrees of knee flexion (full squat)

Our muscles have optimal ranges to maximize force development. This is known as the length-tension relationship. Our muscles, tendons, and joints act as a pulleys system, using lever arms to assist with force production. There is a sweet spot for generating the most amount of force possible.


Try doing a squat right now. Which part of the squat is the easiest? Part of it has to do with body position —how much you rely on contracting a muscle versus relying on bony anatomy to support you against gravity — and partly it relies on the length-tension relationship.


The position of the squat will influence how much you challenge different muscle groups. As the squat deepens past full depth, the length-tension relationships become poorer and the squat becomes more difficult.


Squatting as far as you can go (e.g. past about 120 degrees of knee flexion) doesn’t yield any added benefits for strength or muscle development. It does, however, train you to be stronger in that extreme range of motion.


That is why strength training with full-range squats is beneficial for certain sports. It can help you build resilience and mobility as well. Research shows strength training is an effective method for becoming more flexible.


I often squat to full depth and advocate for my patients to do so, but it isn’t to maximize overall strength. Instead, I do it to challenge my body and build resilience. Make sure you build up slowly. Don’t jump into max-outs with your butt touching your heels if you have never performed weighted full-depth squats.


Takeaways

Squatting is one of the best exercises you can perform. When adding resistance by placing a bar on your back or holding a weight, it becomes a total body exercise. It is also one of the fundamental movements of life.


I often come across advice on the internet or hear patients relay advice from healthcare providers to avoid certain activities. “Don’t run or squat because they are bad for your knees” or “lifting heavy weights damages your back” or “make sure your form is perfect before adding resistance.”


These recommendations are not founded in research. They are fear-mongering and cause more harm than benefit.


We need to advocate for more movement, not less. Our bodies are remarkable. They are resilient and adaptive (I don’t think I can repeat this enough). If you build up your tolerance to a stimulus over time, you can squat, run, and lift with a variety of techniques, loads, and intensities.


If you want to squat deep, go for it, provided you gradually ramp up the resistance. If you haven’t squatted in a while, take a couple of days off after the session to recover. Assess your diet, sleep, and stress to get a better picture of your recovery capacity. Let your body guide you.


If you want more guidance, reach out to a trusted, credible source or send me a message. Any time you seek health information, make sure the information is backed by a credible source and research.


Don’t let people scare you into avoiding exercise.