Knee health is a frequently discussed topic in the fitness community. There’s a lot of popular advice that gets thrown around about things you should avoid because they’re bad for your knees – if you followed all of them, you’d have to give up running, sitting, standing, most sports, cheap sneakers, expensive sneakers, sandals, high heels, lifting with wraps, lifting without wraps, cold weather, gluten, meat, dairy, and sugar. Most of all, you’d definitely have to avoid both deep squats and partial squats.

The great “don’t go deeper than 90 degrees” versus “ass to grass” squatting debate has been raging for years. It dates back to 1961, when Karl Klein performed some studies at the University of Texas which supposedly found that weightlifters who regularly performed deep squats had increased laxity in their knee ligaments compared to those who did not. He concluded that deep squats were bad for the knees and should be discouraged. This was then picked up by the American Medical Association and became official advice. Despite the poor design of the study, the fact that the results haven’t been replicated, and the lack of research relating Klein’s results to injury risk, the take-home message stuck and even to this day there are many health and fitness professionals who will confidently pronounce that deep squats are bad for you.

Squat Biomechanics

One of the common explanations given as to why deep squatting is – supposedly – bad for the knees is that “there’s more pressure on the knee joint” once it is flexed past ninety degrees under load. Is this actually the case? It turns out to be more complicated than that. There are two main forces acting on the knee joint during a squat – compression forces and shear forces. These forces peak at different points in the movement, and they stress different structures in the knee joint during different portions of the movement. Much has been written about the biomechanics of the squat – not all of it in agreement – but there’s a nice summary analysis of the various different forces here.

In a nutshell, the evidence that squatting past ninety degrees puts more stress on your knees than squatting to ninety degrees is pretty shaky. In fact, some authors have argued that stopping a squat at ninety degrees actually produces higher forces on the knee joint than continuing through the full range of motion. To add weight to that argument (pun intended), they point out that most athletes are capable of using higher loads when they are performing partial squats compared to full depth squats – meaning that the forces encountered by the knee joint are likely to be even greater.

Adaptation

Biomechanics is not the whole story, though. One of the most misleading ideas that many athletes, coaches, and therapists have is that the body functions like a machine. In some respects, this may be a useful simplification, but it also pushes us towards assumptions that are at best misleading, and sometimes completely incorrect.

Unlike a machine, the human body has evolved to adapt specifically to the demands that we put on it. Muscles become stronger; neurons function more efficiently; connective tissues become denser and more resilient. There’s even some evidence that the cartilage in the meniscus of the knee, rather than being “worn away” by use, may actually become healthier as a result of weight-bearing exercise.

Your Mileage May Vary

All of this comes with an important caveat: everyone is different. Remember where we talked about the different stresses on the knee during different portions of the movement? For some people, especially those with pre-existing knee problems, it’s entirely possible that some kinds of squats aggravate their knees while others do not.

Differences in range of motion at both the hip and the ankle will also change the kinematics of the movement. People with hip dysplasia, for example, might need to alter how they squat to find the movement that works for them, and this may change their optimal squat depth. You should avoid coaches who believe that there is only one correct way to squat that applies to everyone.

Bear in mind, too, that the body takes time to adapt to a new movement. If you are used to performing your squats to ninety degrees, and then suddenly try to deep squat with your usual weight, you can expect to develop problems! I find that people who report having knee problems with deep squatting very often fall into this category. If you want to change how you squat, then drop the weight down and re-condition your body to the new movement gradually.

The Verdict – FALSE (with caveats)

There’s no reason to think that deep squats are necessarily bad for you. For most people, deep squats are not a problem providing they are performed correctly and with a suitable progression. However, for some people, whether because of physical differences (such as a lack of mobility at the hips, for example) or because of a pre-existing knee injury, deep squats may be better avoided either for a period or in some cases, altogether. If you’re not sure whether the way you are squatting is right for you, or you’re not confident that you’re doing it correctly, then I’d strongly recommend booking a few sessions with a qualified and experienced strength coach. Investing in learning good technique almost always pays off in the long run.