The “butt wink” at the bottom of the squat has received lots of attention over the years. Rightfully so, as it has been shown by multiple sources that spinal flexion under load can damage the spinal disc. However, I think a major cause of butt-winking is consistently overlooked.
Many people (including the sedentary population) adopt faulty trunk-stabilizing strategies. In fact, it is epidemic in the weightlifting world from CrossFit to powerlifting. The most common strategy for trunk-stabilizing involves extending the lumbar spine and tilting the pelvis forward to create stability (think arching the low back). See Figure 1&2 -vs- 3&4 Instead of using the core to properly create intra-abdominal pressure, the athlete uses hyperactivity of the spinal erectors and hip flexors to compress the passive tissues of the spine for stability. In the short term, this is a very effective strategy to facilitate stability but over time can lead to injury to the joints and disc of the spine. It is effective but not at all efficient. This has been described multiple times through the years as, “open scissors” (Kolar DNS) or “Lower Crossed Syndrome,” (Janda) and most recently “extension compression stabilizing strategy” or ECSS (Richard Ulm). I have seen Dr. Ulm present multiple times over the years and he has given me the best understanding of this concept as it applies to the lifting community.
As Humans we have approx 120 degrees of hip flexion, and most all of it is needed to squat parallel or below. When an athlete adopts the ECSS they use up a portion of the 120 degrees of hip flexion needed to squat. We tend to only see the effects at the lumbar spine and pelvis of ECSS (i.e. lumbar hyperextension and anterior pelvic tilt.) However, the ANTERIOR PELVIC TILT IS CLOSED CHAIN HIP FLEXION. In an attempt to stabilize the trunk, precious hip flexion is used up. If the lifter tilts the pelvis forward 20-30 degrees they now only have a maximum of 90-100 degrees left available to squat. This is not enough hip flexion to achieve the desired depth. As they descend into a squat, using this ECSS method to stabilize, the hip runs out of flexion and abuts the pelvis. At this point the only way to achieve desired depth is to flex the lumbar spine at the bottom, giving us the characteristic “butt wink”. This is often the result of poor co-contraction of the abdominal wall leading to the characteristics of ECSS.
(Shown above) A perfect example patient with L4/L5 disc herniation with L5 nerve root involvement. Squatting increased pain, especially peripheral leg pain. In the video on the left we can see increased paraspinal tone with large spinal flexion at the bottom of the squat. On the right neutral spine with improved posterior-lateral intra- abdominal pressure and absence of lumbar flexion at bottom of squat.