The idea that our knees should stay behind the line of our toes while squatting was born. This simple coaching cue was broadly adopted in the fitness / coaching arena as it helped prevent clients from deep squatting by keeping the movement fairly shallow. It does not appear that "keep the knees behind the toes" was ever actually stated by Klein as a finding of his study. So why do so many PT's & coaches adopt this approach to squatting?
Fast forward to 2014, the Klein study is now 53 years old and it's probably a good idea to compare and contrast it against some contemporary research on the subject... So let's have a quick browse across at least the last ten years to see if this "knees behind the toes" thing still holds up.
Now... before I dig into contemporary research, I actually found this brief but very well constructed review of the book written by Klein & Allman - it contains several quotes taken directly from the book and I was surprised to learn that on p. 30 Klein states that: "The depth of the squat position should be controlled, with the thighs just breaking the parallel position (Figure 7)."
This is important to note - Klein himself said that the safe depth of a squat allows the thighs to be "just breaking" parallel to the floor... This would place the hips just below knee height at the bottom of the squat movement.
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| Thighs just breaking parallel - perfectly acceptable by Klein. |
This is big news because if you were to squat to a depth allowing the thighs to "just break" parallel as suggested by Klein - the hips will be slightly below the knees, and you will not be in a shallow squat position.
This position is illustrated by one of my students in the image to the right. The thighs are just breaking parallel, the hips are just below knee height. Perfectly acceptable based on the Klein study from 1961... and if you look at the knees in this position they appear slightly in front of the line of the toes without any excessive forward lean at the trunk. Looks OK to me.
In terms of contemporary research on the knee angle specifically, the ultimate study into knee position is this 2003 study which specifically tested the "knees behind the toes" approach and did a full bio-mechanical breakdown of how it changes/impacts the squat compared to unrestricted movement at the knees.
That 2003 study tells us that when the knees move in front of the toes, there is an increase in the forces/loading on the knee joint by about 28%. But the article also goes on to state that by restricting the knees to stay behind the toes "The restricted squat also increased forward lean, which was shown to increase lumbar shear forces". Interestingly, the increase in lumbar loading with the knees behind the toes was around 1000% higher than when the knees were allowed to pass in front of the toes.
Note the image B (from the 2003 study) shows the client has increased forward lean of the trunk when the knees are kept behind the line of the toes.
The NSCA has also adopted a fairly open approach to squatting technique. They cite this 2007 article which supports all the findings from the 2003 study, and clearly states that "Research indicates that the optimal squat technique is... unrestricted movement at the knees". So the biggest strength & conditioning association in the USA is happy for your knees to pass over the line of your toes during a squat movement.
Other articles which support the notion of deep squatting and unrestricted movement of the knee joint include this one on the biomechanics of deep squatting, you can also watch this video of a presentation free online which walks you through the squat in good detail (50 minutes) and you can actually download the powerpoints from that presentation here.
The bottom line: During 21 years of industry experience and my review of the literature on this topic, I can't find anything to support the idea that knee movement should be restricted in clients that have healthy / normal joint function.
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Links to references and further reading:
http://startingstrength.com/articles/squat_klein_suggs.pdf
http://www.uspla.org/sites/default/files/downloads/ncsa_articles/Optimizing_Squat_Technique.1.pdf
http://www.lookgreatnaked.com/articles/the_biomechanics_of_squat_depth.pdf
http://www.nsca.com/uploadedFiles/NSCA/Inactive_Content/Program_Books/PTC_2013_Program_Book/Schoenfeld%20PF.pdf
http://www.nsca.com/Pages/ContentRightNav.aspx?pageid=2147491558&terms=squat*
http://www.luciano.si/images/blog015_raziskava.pdf
http://www.luciano.si/blog/po-ep-po-krivem-obsojen-poln-neresnic-in-prepogosto-zapostavljen.php
