We are taught in our educational institutions and by many medical professionals never to let the knees cross the toe plane during weight bearing exercise. But, what happens when we walk up or down a staircase. Doesn’t the knee cross over the toe plane with each step? Or what about squatting? Watching my two kids, they are constantly squatting a#$ to the grass while playing or to pick things up. If this is a perfectly normal movement pattern for children, than at what age does this movement become bad for the knees, if at all? Many in the health and wellness community insist the deep/full squat is bad for the knees as it causes excessive shearing stress on the knee capsule. So which is it?
A study out of 2002 provides valuable insight into squat depth and muscle activation. In this groundbreaking study, the researchers found glute activation during full squat to be greater than twice that of partial squats (35.4% compared to 16.9%), hamstring activation to be similar, while quadriceps activation dominated during the partial squats only (1). Understanding the importance of this can be a critical element in protecting oneself from injury, while achieving desired training outcomes. This would probably explain the low rate of knee injury incidence competitive Olympic weightlifters, as they squat deep and often in both competitive and auxiliary lifts.
Let’s examine the fundamental biomechanics of the squat for a moment. Basically we have hip flexion/extensions, knee flexion/extension, plantar/dorsiflexion and spinal pressurization/stability.
What muscle groups are we trying to work in the squat? The quads yes. But, also the glutes, hamstrings, adductors, deep and superficial musculature about the hip and core and spinal stabilizers and extensors. Half/parallel squats, as seen in the above study provide mostly quad dominance, while deep squats provide complete activation of the rest of the musculature about the legs, hips, core, and low back.
Thinking about the fact that muscles exert their force by pulling on bones we can gain a better understanding of how proper technique deep squatting can actually protect the knee. With greater activation/recruitment from a larger number of muscle groups, the knees are better protected during both the eccentric and concentric phases of the motion. For example, in the correct deep squat position (knees pulled out to the sides, slight lean in the torso, lordotic/neutral posture), the adductors on the inside of the thigh and hamstrings on both sides of the knee stabilize the knee to negate the anterior forces of the quadriceps. This combined with greater VMO activation at the bottom 15 degrees of motion and the knee can be kept strong and stable throughout the movement.
So where did this half squat phenomenon come from anyway? It appears to have originated back in 1961 with a study done by Keneth Klein (1961). In his study Klein pre-qualified 128 competitive weightlifters and compared their knee ligament stability to that of 360 college students with little or no training experience. Bias could have become an issue in this study as the subjects were pre-qualified into two groups, weightlifters and students.
The ligament stability test was performed with manual force application, rather than a measurable/quantifiable force. Apparently the subjects would be pre-qualified (lifter or not) then the testers would determine how much force to apply to their knee structure. The combination of pre-qualification and non-quantifiable manual pressure application, could potentially lead to biased and/or flawed results (4).
The story goes that once the study was made public, the news spread like wildfire, and soon deep squats became public enemy number one in the physical therapy and medical side of physical fitness. Bodybuilders, Olympic Weightlifters, and Powerlifters seemed to trust their gut instincts and stick with what worked for them, and has since been proven to work now.
Other studies have followed the Klein effort, but with dramatically different results. For instance, 10 years after Klein’s study, Meyers et al reproduced the study using the same mediolateral collateral ligament testing instrument to measure collateral ligament stability with completely different findings. In this 8 week study, sixty-nine subjects were randomly assigned different groups involving variations of the deep and parallel squat. The researchers found no significant differences in collateral ligament instability and knee joint flexibility within any of the treatment groups (5). Contradictory to Klein’s oft-referred study, the study by Meyers et al found that neither deep squat nor parallel squat were detrimental to knee stability.
Other studies have had similar findings (2,6). In fact a study from the American Journal of Sports Medicine performed in 1986 compared the knee health of powerlifters to that of college basketball players and 10K recreational runners. The researchers found in increase in posterior/anterior knee laxity in the runner/basketball player group, not the weight training group (7).
Contradictory to popular belief, research has shown that properly executed deep squats (for those with good lower extremity health) do not increase knee laxity/instability, but can actually increase muscle recruitment, which can lead to increases in performance on the field. So, if deep squats don’t cause knee instability than what could some of the possible culprits be? In 1989 a group of researchers tested the knee laxity of 20 recreational long distance runners before and after running. The researchers found an increase in ligamentous laxity post exercise (3).
Stay tuned for Deconstructing The Squat Part II: Breakdown of squat techniques
- Caterisano A, Moss R, Pellinger T, Woodruff K, Lewis V, Booth W, Khadra T. The effect of back squat depth on the EMG activity of 4 superficial hip and thigh muscles. Journal of Strength and conditioning Research. 16(3); Pp 428-432. 2002.
- Chandler T, Wilson G, Stone M. The effect of the squat exercise on knee stability. Med Sci Sports Exerc. 21(3). Pp 299-303. 1989.
- Johannsen H, Lind T, Jakobsen B, Kroner K. Exercise-induced knee joint laxity in distance runners. British Journal of Sports Medicine. 23(3); Pp 165-168. 1989
- Klein K. The deep squat exercise as utilized in weight training for athletes and its effects on the ligaments of the knee. J Assoc Phys Ment Rehabil. 15; Pp 6-11. 1961.
- Meyers E. Effect of selected exercise variables on ligament stability and flexibility of the knee. Research Quarterly. 42(4); Pp 411-422. 1971.
- Panariello R, Backus S, Parker J. The effect of the squat exercise on anterior-posterior knee translation in professional football players. American Journal of Sports Medicine. 22(6); Pp 768-773. 1994.
- Steiner M, Grana W, Chilag K, Schelberg-Karnes E. The effect of exercise on anterior-posterior knee laxity. American Journal of Sports Medicine. 14(1); Pp 24-29. 1986.