For those of you using the single leg squat, I am about to show you a much better way to train it that I learned from Mike Boyle a while back. Fist, take a second to look at the anatomy of the single leg squat as pictured below and in the last post. Pay attention to the non-working leg. What you will see is the hip of the non-working leg is having to isometrically contract for the entire duration of the set. The Psoas (hip flexor) originates and attaches at the transverse processes of L1-L5. So what you will commonly get from people with past or present back pain is a complaint of low back pain during or after the set. This is one of the reasons Stuart Mcgill chooses the "bowler squat" for his single leg squat training.
Here is how we train our single leg squat (notice the hip no longer has to flex as it is able to now stay relaxed):
Another thing to point out is that the athlete in the video is squatting to a depth of 14" with an additional 60 lbs load. Let's also add that he was unable to do a single leg body weight squat on his right leg to a 22" box when he first started :-) Hard work = PROGRESS!!