"Anyone who says that full squats are 'bad for the knees' has, with that statement, demonstrated conclusively that they are not entitled to an opinion about the matter" - Mark Rippetoe
One of the most repeated bits of "everybody knows" information about strength training, the fact that squatting below parallel is damaging to the knee joint, can be officially removed from the exerciser's body-of-knowledge.
When Dr. Karl Klein of the University of Texas published a study on supposed detrimental effects of squatting deeply in 1961 (1), it became a virtual law among athletic coaches to forbid their players from performing this exercise. The American Medical Association then made it their official position that squatting deeply has negative impact on the ligaments of the knee joint, thus bringing the issue into general medical recommendations.
In the decades since Dr. Klein's study, no other researcher has been able to replicate his results, and many experts have gone on record with both experience and anatomical data on their side indicating that the findings make no sense from a biomechanical standpoint. Finally, Bill Starr, NFL strength and conditioning coach, competitive powerlifter and Olympic lifter, author of The Strongest Shall Survive, as well as being one of Klein's test subjects has spoken at length at the basic flaws in the 1961 study which render the conclusions invalid.
Last month, the final nail was driven into the coffin of the "deep squats are bad" myth with a study published in the journal Sports Medicine. Researchers Hagen Hartmann, et. al. of the Department of Human Movement Science and Athletic Training, Institute of Sports Sciences, Goethe University in Frankfurt released their findings that, rather than causing destructive degeneration of the knee, deep squatting in fact was a major element in stabilizing the joint (2).
The researchers found "the highest retropatellar compressive forces and stresses can be seen at 90°," indicating that the angle at about a half-squat is the least stable point in the movement. This is consistent with the writings of strength training experts such as Mark Rippetoe, who point out that, since the hamstring tendons actually insert toward the front of the lower leg, they act to stabilize the knee only when the thigh is past the point where it is parallel to the ground.
The study continues, "With increasing flexion, the wrapping effect contributes to an enhanced load distribution and enhanced force transfer with lower retropatellar compressive forces...With the same load configuration as in the deep squat, half and quarter squat training with comparatively supra-maximal loads will favour degenerative changes in the knee joints and spinal joints in the long term". Put simply, squats terminating above parallel actually pose a greater danger to the knee joint than full squats.
This study is a welcome publication to many fitness enthusiasts. Sam Scott is a competitive CrossFitter and member of the Rocky Mountain Scottish Athletes, as well as a fitness trainer who is completing his degree in Health and Exercise Science at Colorado State University. The results of Dr. Hartmann's study come as no surprise to him. "This single study [Klein, 1961] had almost destroyed the reputation of the full squat - one of the best exercises for any sort of athletic performance," Scott went on to say "Most people who actually work with athletes know that squatting deeply is not only not dangerous, but better on the knees than partial squats. I hope this study will get rid of some of the fear surrounding full squats".
The publication concludes with one concern that is valid regarding squats of any type: get qualified training to learn to perform the movement correctly, and use progressive training loads to build strength over time.
The barbell back squat, to full depth, can and should be used by anyone who is physically capable of it, and who is interested in dramatically improving their overall athletic performance, their health, or their physique. Now the proof has been published.
(1) 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: 6–11, 1961.
(2) Sports Med. 2013 Jul 3. Analysis of the Load on the Knee Joint and Vertebral Column with Changes in Squatting Depth and Weight Load.; Hartmann H, Wirth K, Klusemann M.