Are ‘Ass to Grass’ squats bad for your knees?
The squat is a corner stone exercise in almost every strength and conditioning program. However a myth still lingers (deep within the physio world) that deep squats are dangerous to the knees and should never be performed. There is a range of opinions when it comes to optimum depth of the squat. Some die hard lifts believe in the mantra ‘Ass to Grass’ squat … ‘this is the way.’ While others believe quarter squats are sufficient for functional training such as Dr Joel Seedman.
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Where did the myth that squats are bad from your knees originate?
In 1950s Dr. Karl Klein who started to look at the increased number of college footballs suffering serious knee injuries. He surmised that these injuries were due to full range of motion deep squat that the athletes where performing during training. He analysed the knees of athletes who regularly performed deep squat with a rudimentary device.
In the early 1960, he published his research and presented his finding proclaiming that deep squats stretched out the ligaments of the knee (1). He recommended that squats should only be performed to parallel. Any squats that were performed deeper than parallel would compromise the stability of the knee and exposed the athlete to an increased risk of serious injury.
In 1962 Sports Illustrated published Klein’s theory, this ignited the fear of deep squatting. The American Medical Association (AMA) released a position statement warning against the use of deep squats (2). The Marine Corps removed the “squat jump” exercise from their physical conditioning programs (2). Deep squat were even banned during PE Lessons in New York school (2).
Some coaches disputed Dr.Klein finding, Dr. John Pulskamp wrote, “full squats are not bad for the knees and they should certainly not be omitted out of fear of knee injury” (5). The damage was already done! by the end of the 1960s S&C coaches across America were afraid of performing full depth squats for fear of injury athletes. They stopped coaching full depth squats and removed them from the training programs totally (1).
Thankfully over the past 60 years biomechanics and exercise science research has progress. We now have a better understanding of the forces and stressed placed on the structures of the knee during the squat.
Biomechanics
During the squat the knee undergoes 2 forms of stress compression and shearing.
Compressive force occurs when a physical force presses inward on an object, causing it to become compacted. Within the knee two areas are exposed to compression forces. The meniscus absorbs the opposing stress between the tibia and the femur. The second is between the articulating surface of the patella and the femur. As the knee flexes during the squat, the patella comes in contact with the femur. The deeper the squat, the more boney congruence occurs between the patella and the femur dissipating the forces.
Shear forces are unaligned forces pushing one part of a body in one specific direction, and another part of the body in the opposite direction. In the knee this is displayed by the femur and tibia gliding over each other in the opposite directions. High shearing forces can potentially damage the ligaments inside the knee, namely the Anterior Cruciate Ligament (ACL) and Posture cruciate ligament (PCL). These ligaments are some of the primary structures that hold our knee together and limit excessive anterior & posterior travel.
When we look at these forces (shear and compression) we see that they are typically inversely related. This means when the knee flexes during the squat, compressive forces increase while shear forces decrease (6). This effect is reversed during open chain exercises with high load such as leg extension machine which are renowned for causing knee pain.
Ligament Safety
Many medical professionals bodies have warned against the use of deep squats due to the perceived excessive strain placed on the ligaments. BUT, these warnings have minimal basis evidence research back up these claims Current research now tells us that the ACL which resist the motions of anterior tibial translation and internal tibial rotation and PLC that resist forces pushing the tibia posteriorly relative to the femur. Are actually placed under minimal stress in the deepest part of a squat. ACL injuries are especially common in sports that require high velocity direction changes Such as skiing, rugby, football, basketball, lacrosse, American football etc.
During the squat the stress to the ACL is actually highest during the initial 12 cm of the descent, between the first 15-30° (7). As depth increases the forces placed on the ACL significantly decrease. The highest forces ever measured on the ACL during a squat has only been found to be around 25% of the force needed to tear the ligament (8).
The PCL undergoes maximal forces around 90° of knee flexion just above a parallel squat position (10). Similar to the ACL, the PLC is never placed under excessive stress. The highest recorded forces on this ligament have been only 50% of the estimated strength in a young athlete’s PCL (10).
Research has shown that the deeper you squat the safer it is on the ligaments of your knee. Shear forces are substantially reduced due to an increase in compression. Also, the hamstrings and quadriceps contract at the same time (Lombards paradox) to stabilise the knee. As we squat the hamstrings work with the quadriceps to counteract and limit excessive movement deep inside the knee (6).
Knee Stability
Dr. Klein original findings have never been replicated by researchers using copies of Klein’s original testing equipments. Research has now disapproved Klein’s findings and discovered that athletes who squat below parallel have no difference in the laxity of their knee ligaments than those who only squatted to parallel (3).
Deep squats may have a protective effect on the knees by increasing the stability. Research in 1986 looked at the knee stability between powerlifters, basketball players, and runners. Post heavy squat workout the powerlifters actually had more stable knees than the basketball players (who just practiced for over an hour) and runners (who just ran 10km) (9). Another group of researchers in 1989 demonstrated that competitive weightlifters and powerlifters had knee ligaments that were less lax than those who never squatted (4). Research continues to show that squatting below parallel is a safe exercise to include in a healthy athlete’s training program.
When can deep squatting be harmful?
In theory most of the damage that the knees would sustain from deep squats would be due to excessive compression forces. Some authorities claim that because deep squats raise compression forces at the knee there is potential to cause patella osteochondral defect or meniscus damage. While in theory an increase in compression would lead to a greater susceptibility for injury there has been no correlation established by research.
If compression forces damaged the knee, we would expect to see excessive arthritis in the knees of weightlifters and powerlifters. However, this has not been highlighted. There is little evidence of cartilage wear in the knees as a result of long-term weight training. Elite athletes who sustain loads up to 6x bodyweight to the knee in the bottom of a deep squat have relatively healthy knees compared to the average person (11)
Ass to Grass Considerations
You should consider a few things when determining optimal squat depth. Initially the athlete should be able to perform a bodyweight squat to full depth with good form (no posterior pelvic tilt, no valgus knee stress etc) NO IF OR BUTS!
The depth of the squat should be based on the requirements of the athlete’s sport. A deep squat is not necessary for a football player. They can still acquire optimum strength and power from a parallel depth squat. However a weightlifter requires a full depth squat to allow them to lift the maximum amount of weight in competition.
The athlete’s training history and injuries also needs to be considered when determining optimal squat depth. If an athlete has knee pain and is injured, deep squats may not be the optimal choice. The depth of the squat must be limited to a pain free range of motion.
The depth of the squat should also be limited if the athlete cannot perform a squat with good technique. Poor technique only increases the risk of injury.
An athlete can only lift so much weight with a compromised technique before the structures become stressed and unlimitedly lead to injuries. Squatting to full depth with poor technique is a surefire way ion getting injured.
Summary
In the last 60 years we have discovered that deep squatting ‘ass to grass’ is actually not as dangerous as Dr. Klein made it out to be. Research again and again has failed to support the theory that deep squats are bad for the knees in healthy athletes.
For athletes without injuries and using good technique, performing the squat to full depth should not cause injury to the knees as long as heavy loads are not used excessively. Optimal training programs should employ a pendulum wave protocol with a light, medium and heavy intensity cycles in order to lessen any harmful effects of constant heavy loading. Now that the mainstream myth is busted and you have a deeper understanding of which depth of squat is optimal for you… Get your ass under the bar!!!!
References
Todd T. Karl klein and the squat. Historical Opinion. NSCA Journal. June-July 1984: 26-67.
Underwood J. The knee is not for bending. Sports Illustrated. 16: 50, 1962.
Pulskamp, JR. Ask the doctor. Strength and Health. 1964, May. p. 82.
