The trunk: mobility, stability, or balance?

backbendflip.jpg

What do you consider your core? Is it different than the trunk? And what, exactly, is its role during specific movements, like walking?

From an anatomical point of you, the trunk is the structure that consists of the neck, thorax, abdomen, and pelvis. The spine, structural component of the trunk, serves as a connecting point for the head and the pelvis. The ribs, another structural component of the trunk, protect the vital organs necessary for life and connect to the thoracic portion of the spine.

The architecture of the spine with its four curves, is designed to withstand axial stress. Axial stress is a stress that changes the length of the body. The four curves allow you to remain upright even though compressive forces (like gravity) are constantly exerting a downward pull.

The muscles of the spine keep the spine upright during day to day life by continually making small adjustments to hold the body seemingly still. They are also involved in breathing and voluntary movement.

The spinal muscles are often divided into two systems for discussion purposes: global and local. The global system is the one that enables voluntary movement. If you want to roll yourself up into a little ball? That’s the global system. Move into a deep back bend? Also the global system.

The local system, on the other hand, maintains stability, for lack of a better word. If you were to be hooked up to an EMG all day, the muscles that comprise the local system would show low level activity, always. That’s because these muscles keep you upright. Without them, you would be like the Wicked Witch at the end of the Wizard of Oz—a big puddle.

If you were to place pressure mats that measure force underneath your feet during quiet standing, you would see that the pressure doesn’t remain constant. This is because of a concept called postural sway. The body is kind of like a pendulum, with micro adjustments happening at the ankles and hips that cause a ripple effect up the body. In fact, when rigidity sets in, like in Parkinson’s, at least one research study shows an absence of postural sway.

Postural sway, then, is predicated upon both stability and mobility of the spine (as well as other factors, which I won’t discuss today). In fact, some studies show increased postural sway during dynamic tasks in subjects with chronic neck pain. This makes sense when you consider the suboccipital muscles are sensory rich, sending lots of information to the central nervous system about the position of the head; the cervical receptors also have reflexive connections to the visual and vestibular systems. If the spine has a lot of extra movement (i.e., doesn’t have good stability), the neck may feel like it’s doing far more work to keep the head stable than it would if the spine were quieter (i.e., had more strength).

This is also consistent to what I have experienced when working with clients with non-specific neck pain. Once they gain a little bit of strength, the neck pain dissipates.

Three of the muscles of the local system, the transversus abdominis, the multifidus, and the psoas have been accused of being “off,” or not firing properly, or having a delayed activation time, or being chronically tight. I do not work in a laboratory setting, nor am I an anatomist, but if local stabilizers keep you upright, it would stand to reason these individual muscles aren’t really the sole contributors to low back pain/pelvis positions/lumbar lordosis. Instead, what those of us who work with people in a movement setting can say is a lot of muscles keep us upright. Our experience of feeling unstable/tight/discomfort is multifaceted. Learning how to control movement in a variety of positions will help all of the muscles get stronger, even the ones that make up the local system because load causes changes in the ability to withstand force all the way down to the skeletal level.

The muscles of the global system include the pectoralis major, latissimus dorsi, rectus abdominis, external oblique, rectus femoris, and gluteus maximis. These muscles cover large areas, are capable of producing high amounts of torque, and are the first line of defense against extrinsic load. Remember, extrinsic load can come in a variety of forms, so someone like Matthew Fraser, the Crossfit Games winner the last three years, has a very different external physique than Alex Hannold, the free climber who soloed El Capitan. Both, however, have visibly strong lats and abs, just different sizes to meet the demands of the external loads regularly placed upon them. They both also have high amounts of mobility to get into the positions their respective sports demand.

If you are a movement practitioner, why should you care about the different core muscles and how things work together? Understanding that the spine is designed to be both stable and mobile is key for helping people of all ages and abilities feel better in their bodies. Let’s look at this a little bit closer.

One way to quickly assess the mobility of the spine is to observe a person’s ability to receive the floor. People who struggle with feeling sensation when they are lying down and need cushions and blankets to create a barrier between them and the floor are a) not used to the feeling of a hard surface, kind of like the princess and the pea or b) not distributing pressure well across the points of the back that are in contact with the floor. In either situation, spending a little time down on the ground and mobilizing the muscles that move the trunk will probably make the person feel less sensation when he or she lies down. It will also decrease the amount of stiffness the person likely feels and almost certainly demonstrates during movement.

Additionally, just the act of getting up and down from the floor requires the spine to bend and be mobile. Helping people explore different ways to get up and down from the ground can create more mobility and adaptability of the trunk and torso.

Here’s what interesting about the muscles of the global system (at least to me, and it could very well be argued that what I find interesting veers towards unusual). They are the very muscles that are frequently considered the culprits for “tightness” when it comes to things like decreased shoulder flexion and a tendency towards an anterior pelvic tilt. Maybe the problem is less that specific muscles are tight and more that the trunk isn’t able to move in a variety of ways. “Aren’t those the same thing?” you might be wondering?

Not necessarily.

The ability to rotate the spine during walking allows the arms to swing. You don’t have to force an arm swing, they just do it because the trunk is rotating. If the arms don’t swing, there is a very high chance the thoracic spine doesn’t rotate, and when muscles aren’t used for basic, everyday motions, range of motion decreases. Again, another example of how the torso and shoulder girdle are connected.

The isolated view that’s often taken when it comes to something like improving shoulder mobility typically ignores whether the spine can flex, bend, extend, and rotate. So maybe when addressing shoulder mobility it makes sense to work on spine mobility as well. Maybe.

So what about stability? How does that work?

Well, it’s two fold. As we’ve already discussed, basic strength is helpful for gaining a sense of stability.

When I say sense, that’s really what stability is. Research shows posture isn’t indicative of pain or dysfunction, so you can’t say looking a certain way is problematic, and I have met people over the years who are active, have posture many movement professionals would consider less than optimal, but don’t have any pain (and these are people in their 70s and early 80s). Whatever they are doing is enough to create a sense of security. Stability means you feel strong enough to support your structure and external load and that you have a sense of control in your spine, so if you are knocked off balance, you can catch yourself. Basic strength creates stability.

The other way to help people feel more stable is to train…balance. That’s right, the best way to improve postural control is through balance training. In fact, a systematic review and meta-analysis showed balance exercise training improves postural control in older adults, while strength and multi-component exercise interventions don’t.

Let me guess, you threw the BOSU away and the stability balls are sitting in an exercise closet somewhere because of the 2008 article in the Journal of Strength and Conditioning Research that showed using unstable surface training did not increase muscle activation in “highly resistance-trained individuals.” There are a number of ways to train balance. Unstable surface training isn’t necessary. You can use single leg variations, eyes closed conditions, split stance positions, heel to toe variations, obstacle course variations, ball throwing options and more to challenge balance. Dynamic balance training feels and looks a lot like play, which makes it enjoyable for both the practitioner and the student. And if you sort of liked using unstable surface training, but you are embarrassed to use any of it because you are afraid your co-worker will judge you for your lack of an evidence based approach, pull the unstable surfaces back out and let your general population clients walk on, . Their balance will improve and they will have fun doing it—just know that standing on a stability and doing squats probably won’t improve anyone’s golf game.

Unless you are working with a very specific population, most people can benefit from a movement or exercise program that incorporates a little bit of mobility, a little bit of strength, and a little bit of balance. If someone is more strong than mobile, work on a little more mobility, and if a person is mobile, but lacks balance, work on more dynamic balance. If you address a person’s needs while maintaining his strengths and keep it at least a little bit fun, the entire process is a lot more enjoyable. Plus, you get the added benefit of the global and local systems working in a balanced way.

Like what you’re reading? Sign-up for the newsletter to be notified of new content.