Does The Tailbone Tuck Provide Optimal Lumbar Stability
In Part 1 of this 3-part series, I introduced the common — but controversial — posterior pelvic tilt (PPT) cue. I explained how controversy stems from the opposing views about whether or not the PPT helps stabilize and protect the low spine from compression. I discussed its importance in poses where we want to contract rectus abdominis, potentially increase hip extension, and work on strengthening our hollow body position. I conveniently omitted discussing the PPT in poses where the lumbar spine is weight-bearing and thus at risk of compression. Today, we will unpack the PPT as it relates to spinal stability. Before we dive in, lets define spinal stability and examine how the muscles of the trunk accomplish this task. Please note that today’s post will get meaty into the scientific literature. So, if you are not interested in nerding out with me about all things spine you can skip ahead to the last section, Summarizing the Findings, and then stay tuned for Part 3 where we look at the PPT in asana.
Unpacking Spinal Stability
Very simply, spine stability describes the ability of the vertebral column to stay intact while forces or perturbations are applied to the spine. A stable spine is one that moves through space without buckling under the pressure. I came across a paper written by one of the leading experts in spine biomechanics, Stuart McGill and colleagues, where he uses the analogy of a fishing rod and guy wires to describe spinal stability1:
Suppose a fishing rod is place upright and vertical, with the butt on the ground. If the rod were to have a small load placed in its tip, perhaps a few newtons, it would soon bend and buckle. Take the same rod, and attach guy wires at different levels along its length and attach their other ends to the ground in a circular pattern. Now of critical importance — tighten each guy wire to the same tension. Repeat the exercise, loading the top of the rod and one will observe that the rod can now sustain huge compressive forces successfully […] The first role of the [trunk] muscles is to form the guy wires to prevent buckling […] and the critical role of the muscles to first ensure sufficient stability of the spine to that it is prepared to withstand loading, and sustain postures and movement.
As this analogy so aptly describes, spinal stability stems not from an inherent quality of the spinal column itself, but rather from the muscles in the trunk. Moreover, much to the chagrin of early lumbar stability researchers who dedicated their efforts to identifying a single trunk muscle that was most important for spinal stability, current creed understands spinal stability as the cumulative contribution of multiple trunk muscles performing in concert2. Indeed, one study set out to determine the relative contribution of individual trunk muscles to lumbar stability and found “no single muscle group contributed more than 30% to the overall stability of the lumbar spine”2. In other words, no muscle singlehandedly stabilizes the spine.
The PPT for Lumbar Stabilization
In the same paper where he analogized spine stability to a fishing rod and guy wires, McGill states that spinal stability is the result of “highly coordinated muscle activation patterns involving many muscles [with] recruitment patterns [that] must continually change, depending on the task”1. This idea is key to the discussion that follows. You will remember that I explained how the tail tuck cue is often given as a means to stabilize the low spine and thus prevent it from enduring undue compression. The difficulty with this argument lies in the nature of spine stability itself — there is no one single position where the spine is most stable. Rather, there are a large number of muscles that contribute to spine stability and their pattern of activation continually changes depending on what the body is doing.
Prescribing a PPT universally to stabilize the spine has two problems. First, while this pelvic position ensures greater contraction through rectus abdominis (RA), it is less effective in activating some of the deeper spinal support musculature such as the erector spinae (e.g., multifidus)3. Second and most disconcerting, McGill describes how the PPT preloads the spine and may contribute to a higher likelihood of low back injury. In fact, McGill has previously shown that the lumbar spine is most likely to endure compression injury during spinal flexion (i.e., when the natural lorditic curve is flattened such as during the PPT). For this reason, weight lifters performing a load bearing squat maintain their lumbar curves throughout the movement. That said, in yoga we do not load bear beyond a portion of our body weight so I am not jumping to the conclusion that spinal flexion is dangerous. Rather, I am simply coming to the conclusion that I don’t find supporting literature to suggest that spinal flexion or a PPT will stabilize the spine. This conclusion begs the question: what purpose does the natural lumbar curve serve in spinal stability? And why would eliminating that curve lead to more stability? McGill quotes, “I believe the correct rule of thumb is to preserve the normal low back curve […] it would appear to be unwise to universally recommend the pelvic tilt during exercise that loads the spine.”
Summarizing the Findings
I don’t have all the answers. In fact, this cue continues to make me pause in class and think about what intention I am aiming for. My objective in this series was to do just that — to get my readership and fellow practitioners to think about intention during movement. If I could finish with a message to each of the pro- and anti-tail tuckers….my message would be as follows. To the anti-tail tuckers: the PPT is not an inherently bad movement. It is a natural movement that we can use to contract rectus abdominis and contribute to hip extension. To the tailbone tucker extraordinaires: do not fear your lumbar curves! Flattening the lumbar curve does not protect against lumbar injury….in fact, ironically enough, it places the lumbar spine in its most vulnerable position (i.e., flexion). In the third and final part of this series, we will put theory aside and bring our discussion into practice as we look at the PPT in asana.
- Mcgill SM, Grenier S, Kavcic N, Cholewicki J. Coordination of muscle activity to assure stability of the lumbar spine. J Electromyogr Kinesiol. 2003;13:353-359. doi:10.1016/S1050-6411(03)00043-9.
- Cholewicki J, VanVliet JJ. Relative contribution of trunk muscles to the stability of the lumbar spine during isometric exertions. Clin Biomech (Bristol, Avon). 2002;17(2):99-105. http://www.ncbi.nlm.nih.gov/pubmed/11832259. Accessed July 30, 2018.
- Richardson C, Jull G, Toppenberg R, Comerford M. Techniques for active lumbar stabilisation for spinal protection: A pilot study. Aust J Physiother. 1992;38(2):105-112. doi:10.1016/S0004-9514(14)60555-9.