You are a coach, and an athlete asks, “Is deep squatting bad?” What is your reply? Why do you say, "Yes"? Why do you say, "No"?
You are an athlete, and you ask your coach, “Is deep squatting bad?" What was his or her reply? Why did he or she say, "Yes"? Why did he or she say, "No"?
This post seeks to clarify misunderstandings about optimal squat depth in terms of both safety and muscular involvement.
Origins of the Controversy
In 1961, researcher Dr. Karl Klein from the University of Texas published a study titled, “The deep squat exercise as utilized in weight training for athletes and its effects on the ligaments of the knee.” With a measuring tool that he created himself, Klein noted that Olympic Weightlifters, who frequently perform deep squats, displayed greater “laxity,” or looseness of the knee ligaments relative to the control group that did not frequently engage in such movement. From this observation, Klein concluded that the Olympic Weightlifting group had unstable knee ligaments as result of the deep squatting (1).
During the course of this study, Klein applied his own manual pressure to the subject’s knee from three different angles: inwards, outwards and forwards. Significantly, Klein questioned his subjects as to whether or not they performed deep squats before applying pressure, rather than after. This piece of information suggests the study may have been affected by a confirmation bias, meaning that Klein could have altered the applied pressure based on the subject’s squat history to achieve his own prejudiced findings (2).
Shortly after the study's release, the American Medical Association disseminated a statement cautioning against deep knee exercises, claiming, as per Klein's research, that such movement is harmful for the internal structures of the knee joint. A few states even outlawed deep squatting in secondary schools.
That was 1961.
What We Know Now
The research available to us today, in 2015, sheds new light on optimal squat depth. We have evidence that not only disproves Klein’s findings, but also shows that deep squatting is beneficial for joint and muscular health.
Roughly thirty years after Klein’s study, researchers compared knee stability in both Olympic Weightlifters and Powerlifters to the knee stability of 100 college students not involved with those two sports (3). This study employed a test that measured anterior cruciate ligament laxity (the anterior draw test) and revealed that this ligament was far tighter in the lifters than in the college students. The lifters also demonstrated more tightness than the students in the quadriceps active drawer test, which meant that they were less likely to suffer from posterior cruciate ligament tears. The lower the risk of tearing this ligament, the more stable the knee joint.
The deeper the squat, the higher the contact area between the muscles and bones which distributes the forces applied to the knee in a uniform, safe way. This is referred to as the “wrapping effect”(4). The wrapping effect minimizes shear force on the ligamentous structures of the knee, specifically to the anterior and posterior cruciate ligament. Shear force is defined as the lateral movement of one structure in one direction against another structure moving, laterally, in the opposite direction. The anterior cruciate ligament (ACL), serves to help the lower leg bone (the tibia) resist sliding forward and rotating inward relative to the upper thigh bone (the femur). The posterior cruciate ligament (PCL) serves to help the femur resist sliding forward of the tibia and to help the tibia resist slipping backwards from the femur.
Contrary to Klein’s findings, when only squatting to parallel or higher, shear forces upon the knee are at their most dangerous. During this movement, peak ACL shear forces occur at the top of the squat and peak PCL forces occur at 90 degrees of knee flexion (knee parallel to the hip), as highlighted by the image below (4,5,6,7).
Muscular Involvement and Articular Cartilage
When squatting below parallel, the effects of the shear forces upon the knee are mitigated by the wrapping effect. Additionally, the recruitment of the inner thigh muscle, the Vastus Medialis, is greatest in higher degrees of knee flexion. This muscle pulls the kneecap inward, counterbalancing the outward pull of the outside thigh muscle, the Vastus Lateralis, which is activated during the lower degrees of knee flexion. Therefore, if one were to squat only to parallel or above, the forces pulling the knee outwards would not be balanced by the forces pulling the knee inwards. Have you heard of Patellofemoral Pain Syndrome, or “Runner’s Knee?" This is created by an imbalance indicative of more time spent bending the knee slightly relative to the time spent bending the knee below parallel. Doing so means that the outward pull of the Vastus Lateralis goes largely uninhibited by the Vastus Medialis. For a person suffering from "Runner's Knee," incorporation of deep squatting into his or her workout regimen could correct this imbalance.
It is also important to consider how deep squatting affects articular cartilage and meniscal tissue. Articular cartilage runs the surface of a bone’s end, and it offers a smooth surface that allows the bones which make up a joint to move without friction. This cartilage can be found on the end of the tibia and femur where they face each other. The menisci are two half wedge shaped fibrocartilages that act primarily as shock absorbers between the lower and upper leg. Research has proven that these structures, like bones, ligaments, tendons and muscles, respond to the stresses placed upon them. In one study published in Arthritis and Rheumatism, it was discovered that nationally and internationally competitive weightlifters have no higher prevalence of degenerative cartilage than do men and women of the general population (8).
"If you don't use it, you lose it"
From a functional standpoint, the recommendations for half and quarter squats to avoid degeneration are counterproductive. These movements will leave some tissue and cartilage inadequately stressed and thereby weakened. The old saying, “If you don’t use it, you lose it,”is 100% fact. Julius Wolf, a late German anatomist and surgeon, stated that bone will adapt to the physical stresses or lack of physical stress placed upon it. This idea, known today as “Wolf’s Law,” applies equally to ligaments, tendons and muscle.
Well then, why might someone have pain(s) when deep squatting?
We can measure the ability of one’s squat strength and compare it to his or her other exercise capacities to determine the ideal frequency of squatting relative to other movements for that particular individual. This can also help determine the type of squatting that a person needs to preform more or less of. For example, we may find that someone experiencing pain is very strong in the deep squat relative to the deadlift, and therefore, needs to preform more deadlifts relative to squats.
Why is this so important? Muscle mass has been proven to be a better predictor of longevity in older adults than total body mass. This is because, in order to remain independent, we've got to be able to use the toilet or stand from a fall unassisted. In order to do these things, we need to develop and preserve deep squatting musculature (9).
For more information on strength ratios, and the philosophy of measuring the capacity of one exercise to another, I encourage you read my article:
1. Klein K. The deep squat exercise as utilized in weight training for athletes and its effects on the ligaments of the knee. JAPMR. 15(1):6 – 11. 1961.
2. Chandler T, Wilson G, and Stone M. The effect of the squat exercise on knee stability. Medicine and Science in Sports and Exercise. 21(3):299 – 303. 1989.
3. Bandi W. Die retropatellaren Kniegelenkscha ̈den. Pathomecha- nik und pathologische Anatomie, Klinik und Therapie [The retropatellar knee joint diseases. Pathomechanics and patho- logic anatomy, clinic and therapy]. Bern: Verlag Hans Huber; 1977
4. Kanamori A, Woo SL, Ma CB, Zeminski J, Rudy TW, Li G, and Livesay GA. The forces in the anterior cruciate ligament and knee kinematics during a simulated pivot shift test: A human cadaveric study using robotic technology. Arthroscopy. 16(6):633 – 639. 2000.
5. Li G, Rudy TW, Sakane M, Kanamori A, Ma CB, and Woo SL. The importance of quadriceps and hamstring muscleloading on knee kinematics and in-situ forces in the ACL. Journal of Biomechanics. 32(4):395 – 400. 1999.
6. Sakane M, Fox RJ, Woo SL, Livesay GA, Li G, and Fu FH. In situ forces in the anterior cruciate ligament and its bundles in response to anterior tibial loads. Journal of Orthopaedic Research. 15(2):285 – 293. 1997.
7. Markolf KL, Slauterbeck JL, Armstrong KL, Shapiro MM, and Finerman GA. Effects of combined knee loadings on posterior cruciate ligament force generation. Journal of Orthopaedic Research. 14(4):633 – 638. 1996.
8. Tiderius CJ, Svensson J, Leander P, et al. DGEMRIC (delayed gadolinium-enhanced MRI of cartilage) indicates adaptive capacity of human knee cartilage. Magn Reson Med. 2004;51: 286–90
9. Muscle Mass Index As a Predictor of Longevity in Older Adults Srikanthan, Preethi et al.