Are deep squats bad for my knees? The prevailing wisdom on this topic would lead you to believe that squatting below parallel will cause injury to your knees by placing an unusual strain on your ligaments leaving the knee unstable and prone to injury. This theory was brought to light in the late 1950’s when Dr. Karl Klein was trying to understand why there happened to be a rise in the number of colligate football players sustaining serious knee injuries. He suspected it was due to the use of full ROM squats in university strength programs so he crafted a special instrument to analyze the knees of several of these football players who frequently performed deep squats.

In 1961, Dr. Klein released his findings, which recommended the squat be limited to a parallel depth. His reasoning stated that the use of deep squatting is detrimental to athletic development and “should be discouraged from the standpoint of its debilitative effect on the ligamental structures of the knee.” The following year, Dr. Klein’s findings were picked up by Sports Illustrated which became the catalyst to spread the fear of deep squatting. Next the American Medical Association weighed in on the topic cautioning against the use of deep squatting. It went so far as the Marine Corps even eliminated the squat-jumper exercise from its physical conditioning programs.

There has been a lot of pushback on this theory ever since its inception almost 60 years ago. Dr. Klein’s findings have failed clinical replication, even with the use of his special instrument. Fortunately, now in the present day we can use the advancement in exercise science and biomechanics research to settle this debate once and for all.

When we squat, our knee sustains two inversely related forces – shear and compressive – meaning that when the knee flexes during the squat, compressive forces increase while shear forces decrease. These shear forces are measured by how much our bones – femur and tibia – want to slide over one another in opposite directions. These forces challenge the small ligaments of ACL and PCL to hold our knees together and limited excessive forward and backward movement. In contrast, compressive force is determined by the amount of pressure the body is pushing on two parts. There are two areas that sustain this compressive force; 1) the meniscus as it absorbs the opposing stress between the tibia and the femur, and 2) the backside of the patella (kneecap) as pressure increases through the descent of a squat.

Science tells us that the ligaments inside our knees are under very little stress at the bottom of a squat due to the mechanics of this inverse relationship. Harmful shear forces are dramatically decreased due to an increase in compression and it seems that the deeper we squat the safer it is on the ligaments of the knee. The most well-known ligament, the ACL (Anterior Cruciate Ligament), is under little stress in the bottom of a squat.  In fact the stress to the ACL during a squat is actually highest during the first four inches of the squat decent (around 15-30° of knee flexion)* and continues to decrease the deeper the descent. The lesser known ligament, the PCL (Posterior Cruciate Ligament) sustains it’s max forces just above a parallel squat (around 90° of knee flexion).

It seems that Dr. Klein’s detrimental claims of the deep squat stretching out our ligaments, ultimately leaving them unstable is but a myth that just wont die. Science has since shown repeatedly that squatting deep may have a protective effect on our knees by increasing stability. In 1986, researchers compared knee stability among powerlifters, basketball players and runners. After a heavy squat workout, the powerlifters actually had more stability in their knees than did the basketball players did.  In 1989, another group of researchers were able to show that competitive weightlifters and powerlifters had knee ligaments that were less lax than those who never squatted. The prevailing research continues to show that the deep squat is a sage exercise to include in a healthy athlete’s training program.

-Adapted from The Squat Bible by Aaron Horschig