Who Rules -- The glutes or the quads? Well, it is complicated.

We have often talked about how important it is to be able to achieve terminal hip extension for an athlete, and arguably for everyone. This means one must have strength of the glutes into that terminal range so one can actually achieve the range of motion and access it functionally. If one does not, then extension movements may occur in the lumbar spine via some anterior pelvic tilt. However, one must not dismiss that upright posture needs sufficient quadriceps strength as well -- meaning, hip extension and knee extension get us to an upright posture and make locomotion possible. If we make the hip flexors or quadriceps tight, due to weakness of the lower abdominals or glutes,  we get anterior pelvic posturing and less hip extension (these are admittedly very rough principles, we all know it is far more complex that this).  What I am saying is that there is an interaction amongst groups of muscles, functional patterns of engagement, recruitment and whatnot. 

One must clearly realize how much knee and hip motions are coupled and work with and off of eachother.  If we bend over in a squatting type motion, we are in hip flexion and knee flexion. When we stand, hip and knee extension. These guys play off of eachother.  One must consider these issues when movements are more advanced and loading and loading rates are magnified, such as in squatting type lifting.  

A few weeks ago Bret Contreras in conjunction with Strength and Conditioning Research put out an article by Yamashita , yes, a 1988 article.  "EMG activities in mono- and bi-articular thigh muscles in combined hip and knee extension."  What this article looked at was what happened during isolated hip extension and isolated knee extension, and more importantly, what happened to the forces when both joints loaded simultaneously, paired in generating extension at the hip and knee, as in a squat. 

This article suggested that when hip and knee extension forces are generated in conjunction, the knee extensors are more activated than if the same force was generated in isolation. What this seemed to suggest is that during the extension phase of a squat, it is easy for the quad thigh muscles (rectus femoris, vastus medialis in this study) to to try and rule the movement, from an activation perspective -- the hip extensors (g. max and semimembranosus) take second seat.  We have talked many times about the dangers of this principle when we frequently say "the glutes should be in charge of the hip, not the quads, when the quads try to apply dominant control of the hip motion, trouble may ensue." Admittedly, this may not be entirely true and it is very loosely stated, but the principle has some sound value when it is approached from how we intend it to be heard, that many athletes do not have sufficient glute strength, hip extension range of motion, and poor control of pelvic neutral. So, they dump into the quads because as we see here in this study, they are very appropriately positioned to help synergistically drive the positioning for, and activity of, hip extension motor pattern production. Is this why we see small buttocks and large quadriceps in distance runners, and the opposite in sprinters ?  We think so, but we need to dive deeper into the research to prove or disprove it, but the principles seem to make sense.
This is why I like to initially drive my glute and hip extension work with my clients in a more knee flexed position, such as supine bridges.  I cannot say it better than Bret Contreras did when he reviewed this article,  

"So exercises that involve less knee extension (glute bridges, hip thrusts, deadlifts, pull throughs and back extensions) will tend to produce much greater hip muscle activation than those that involve more knee extension (squats, lunges, and leg presses), although there are always other factors involved of course!".  

If you are not following Bret's and Strength & Conditioning Research's work, you are missing out, They are thorough and insightful, they do their homework, learn from them.
We clearly need to dive into some newer research on this topic, we will see if we can squeeze out the time. 

- Dr. Shawn Allen, the other "gait guy"

Here is an embedded code for the beautiful slide that accompanied Strength and Conditioning Research's summary of the study. If you cannot find it above in this post, goto their Facebook page and scroll to Sept 22nd, 2016. You will find it beautifully laid out there.  Beautiful job S&CR!

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Yamashita  1988. Eur J Appl Physiol Occup Physiol. 1988;58(3):274-7. EMG activities in mono- and bi-articular thigh muscles in combined hip and knee extension.