The intricacies of spinal stability can get complicated but the big picture is pretty simple. It can be summed up in a few sentences: a stable spine is required for strength and proper function of our limbs or whole body for that matter. The spine is stabilized by two mechanisms: intra-abdominal pressure (IAP) and muscle activity. Contraction of the diaphragm produces intra-abdominal pressure. Muscles including the transversus abdominis, pelvic floor and multifidus control that pressure and are responsible for spinal stability in the absence of it. Breath coordinates the system.
Just kidding. Jenny write a short blog? Never.
Let’s break this down further. Stability is “the precise control of excessive joint motion while allowing for the generation of necessary torques for desired multi-joint movement” (C. Frank 2013). In normal talk, it is a lack of movement at one joint allowing for enough force production to create movement at another joint. Spinal stability aka stiffness, enables limb motion and strength. It can be likened to a tree trunk and its branches. A large, stiff and stable tree trunk can support branches that can sustain a heavy load whereas a Gumby-like, thin tree trunk would give way to the weight of even the smallest branch.
This comparison of the spine to a tree trunk should give insight into why spinal stability is absolutely essential to the function and strength of all joints and limbs. If the spine isn’t stable, the body struggles to find stability elsewhere leading to compensation and overuse of small muscles trying to do a really big job that shouldn’t be their responsibility. It also lends to excessive movement in the spine which can cause degenerative disc disease, arthritis, fractures, spondylolesthesis, stenosis, nerve compression and the list goes on. This would be the equivalent of a flimsy trunk breaking under the load of a heavy branch.
So how do we achieve spinal stability? There are two components: intra-abdominal pressure and muscle activity. Both have been proven to be effective in stabilizing the lumbar spine and the preferred mechanism has been shown to be task, movement and posture dependent.
Let’s turn the spotlight on intra-abdominal pressure. I’m about to rattle off a whole bunch of contradictory research that will likely confuse most people so I’ll start with a few statements that are largely agreed upon. First, intra-abdominal pressure stabilizes the spine. Period. It’s a known fact. Next, contraction of the diaphragm which occurs on an inhale, increases intra-abdominal pressure. Another known fact. And lastly, contraction of the abdominals control and contribute to intra-abdominal pressure. Thats about where the clarity ends.
The mechanism by which intra-abdominal pressure stabilizes the spine is not clearly understood. The theory most supported in literature is that the increase in intra-abdominal pressure is accompanied by a co-contraction of the abdominals which provides spinal stiffness and stability. This implies that tension of the abdominals in a lengthened state (eccentric contraction) is essential to the mechanism. Within this body of research, some studies show that all abdominals are involved while the majority say it is mainly, if not solely, the transversus abdominis. Like I said, there isn’t a lot of agreement in the details.
Most research indicates that intra-abdominal pressure is the preferred strategy for activities that place a large extension moment on the spine such as lifting and jumping because the spine is able to stabilize without additional activation of the erector spinae. Intra-abdominal pressure may also be preferred with heavy loads but there is no consensus as to whether it unloads the spine or increases intra-discal pressure. Some theorize that the intra-abdominal pressure unloads the spine by directly pressing upwards on the rib cage via the diaphragm as well as indirectly by generating an extensor moment on the lumbar spine that decreases back extensor muscle activity. Others have shown that there is an increase in intra-discal pressure during a Valsalva maneuver with no reduction in erector spinae activity making the unloading effect of intra-abdominal pressure controversial.
At the end of the day, I think it’s safe to conclude that the unloading and stabilizing effect of intra-abdominal pressure and the muscles responsible for generating it are posture and task specific. But really big picture, we know intra-abdominal pressure stabilizes the spine and we know the diaphragm and the transversus abdominis are involved in producing it. I hope I didn’t lose too many of you with all of that.
Let’s move on to muscle activity and it’s role in stabilizing the spine. On an exhale, the diaphragm relaxes, reducing intra-abdominal pressure. A shortening contraction of the core muscles replaces the absence of intra-abdominal pressure with tension that increases spinal stiffness and creates stability of the spine. However, there isn’t a lot of agreement regarding which muscles do this.
Most studies refer to the transversus abdominis, multifidis and pelvic floor as the core musculature. Some say that transversus abdominis is solely responsible but other studies show that all muscles of the trunk increase spinal stiffness and can be involved. Research supporting the later demonstrates that muscle activation is position, task and movement specific making the most important stabilizer transient to the task. Regardless of which muscles are doing the work, coordination and proportional tension of the muscles are required to provide effective spinal stability.
Both intra-abdominal pressure and muscle activity have been shown to increase stiffness of the spine in isolation but in function, they always go together. I believe this fact, combined with everything being task, posture and movement specific, explains why the details of spinal stability cannot be agreed upon within research.
So how do I apply all of this to my patient care? No matter where the pain is, I always assess spinal stability. If impaired, it is prioritized in addition to addressing impairments directly at or surrounding the location of pain. This includes posture and breathing patterns since spinal stability is directly tied to both. When it comes to my progression of spinal stabilization exercises, I start with static postures in spinal neutral to re-coordinate the system through breath and conscious muscle recruitment. At first postures are gravity eliminated (aka lying down) and gradually progressed to positions against gravity (aka sitting, kneeling and standing). Once coordinated and able to stabilize against gravity, I progress to loading the system with weight and/or limb movement on a stable spine. This is when we are training the muscles of the core to work while both lengthening and shortening, keeping tension throughout the whole breath cycle. From there I progress patients into loaded multi-joint movements while maintaining neutral spine. The last step is training out of spinal neutral because let’s face it, life doesn’t happen in neutral so it’s important that we are strong in all positions of the spine. And then voila! You have achieved spinal stability! Sounds easy peasy right? Definitely not. Which is probably why I have a job. But it is totally doable. It just takes a lot of work and an understanding of your why which I have hopefully helped with a little bit today.