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.