This topic came up in Alwyn Cosgrove's recent article on T-nation titled, Cosgrove's Five Ah-Ha! Moments:The Education of a Misguided Trainer. This was one of the better articles I have read in a while on T-Nation. However, I respectfully disagree with the idea that the differences in overhead squat mechanics are a result of instability. This ah-ha moment for Alwyn came from being at a Gray Cook presentation. I think both Alwyn and Gray are exceptionally bright people, but I just don't see the logic in this one.
Here are the two photos posted in the article.
In the first photo, the subject has a very nice looking overhead squat. In the second photo, the guy squatting obviously has a movement issue of some sort. Now when taking the two subjects and placing them on their back to reduce the effects of gravity, there squats look much more similar to each other, unlike performing the movement on there feet. Lying on the back does take away the need for stability, but does not mean it is a stability issue.
But looking closely, I don't see a whole lot of similarity, primarily at the ankle, but also look closely at the degree of knee flexion. I also see an immobility issue in the shoulders and thoracic spine. Plus, looking at the lying on the floor photos, the first squatter cannot put his shoulders and arms through the floor to show his ability to extend the thoracic spine. Taking away gravity does make them look more similar and reduce the need for stability, but the question is why does the second guy need more stability? Because he doesn't have the mobility!
This may seem like a minor detail to some, but if a tiny bit of the foundation is off, the whole structure is going to be messed up. The ankle range of motion stops the second guy at a much higher depth than the first squatter because going any deeper will displace his center of mass too much that he would simply fall over. Also the inability to get full extension out of the thoracic spine and ROM hurt the depth as well.
Will adding stability and to where, I ask, increase his range of motion in this particular overhead squat?
Will the added stability allow this guy THE ABILITY TO DEFY THE LAWS OF PHYSICS?
I don't believe you can add stability without improving mobility first.
Looking at the photos, I would say that the issue is a mobility one and it is showing up at the ankle. However, the cause isn't necessarily an ankle issue, the immobility at the ankle could just be the SYMPTON of a broad array of issues. I don't know, but I do know that it isn't a stability issue.
Mike T. Nelson wrote some good stuff about mobility and stability in this recent blog post.
What is also interesting, as Mike stated in his blog, is that Gray Cook discussed the same thing in a recent Strengthcoach podcast. In the podcast he talked about how we need mobility in order to have stability, not the other way around.
Please feel free to discuss or disagree.