My husband and daughter work as civil engineers. Early in their college education they studied “statics”. Statics is the analysis of loads acting on physical systems that are in static equilibrium with their environment, that don’t experience acceleration.
Later in college, they also studied something called “dynamics”, which is the branch of mechanics dealing with the movement of objects and the forces that drive that movement.
But even things that civil engineers consider static, like the foundation of a tall building, will be affected by outside forces, making the case for them to also be analyzed as potentially dynamic.
Objects that are static have potential energy. An apple hanging from the branch of a tree has potential energy because that static energy would be transformed to kinetic energy (energy that an object possesses by virtue of being in motion) if it falls to the ground.
Although the human body might be considered static when it’s still, like sitting in meditation, I view the body as a fairly dynamic system of tension and compressive forces – loads or forces are always acting on our body, whether we are sitting, sleeping, running, or downward facing dogging.
All physical systems, whether a building or a human body, need a plan for stability. Stability is a fundamental concept for analyzing any physical system and creating a plan of action that allows that system to carry out its functions in the best way possible.
I just read a scientific paper using the parable of the six blind men and the elephant to illustrate the ambiguity surrounding the concept of “stability” in human spinal biomechanics. In other words, if you get six biomechanists together to consider spinal movement (the elephant in the parable) you are going to get six potentially different definitions of stability.
If stability can be an ambiguous concept for scientists studying human movement mechanics then you can only imagine how unclear the concept becomes when used by/spoken about by yoga instructors.
Just like the six blind men all trying to define the elephant by where they are touching it, it’s probable that six different yoga teachers will tell you six different things about how stability applies to the body and its relationship to yoga asana practice. Most of the time it’s about pulling your belly “in and up” or “against the spine”, which I have a strong opinion about and is going to have to be an entirely different blog post.
Sometimes what any movement teacher says is backed by evidence-based science, sometimes in personal experience or anecdotal hearsay, and sometimes a bit of both.
And, stability is always context dependent, depending on the task and the system. I think this is why there is ambiguity about the definition of stability. Again, we bump right up against “one size does not fit all”, which is something so important for movement/yoga teachers to understand and be able to work with.
As a yoga practitioner, yoga and movement teacher, and athletic gal, the definition of stability is important to me. It’s a crucial and necessary aspect of my body’s ability to support me while I sit in meditation, move in and out of a yoga posture, backpack 50+ miles carrying a weighted pack, or deadlift a heavy barbell. I’m going to repeat myself:
The need for stability is always context dependent.
The foundation for spinal stability begins with the concept of potential energy – that thing I talked about earlier in this post.
Part of this explanation is math. Ugh.
There’s this whole scientific formula to explain elastic potential energy in the human body. It’s got numbers, the letter “k”, which stands for “stiffness” (stiffness is a good thing in the musculoskeletal system for stability purposes), and at least one multiplication sign.
But the bottom line is this… objects, including your spine, have potential energy when they are static, like that apple on the tree I spoke about earlier. Unlike an apple hanging from a tree, the human body has all these elements of elastic bodies (think muscles, fascia, tendon, ligaments) that have potential energy because of their ability to be dynamic, or deform under load, storing potential energy, and then recovering that potential energy when the load is removed.
For instance:
- When you are standing still in Tadasana, the physical system of the spine has potential energy. If you are pushed from behind, your spine (because of cueing from the motor system) will move into a new position to help adapt to falling or avoiding that fall, which is potential energy transformed to kinetic energy.
- When you transition into and out of a yoga posture, you are using elastic potential energy. Your tissues deform under the load (of moving against gravity, weight bearing through the legs or arms, or side bending your trunk) when moving dynamically. For instance, when you lift up from supine into Cobra pose, you are loading the tissues of your back body spine into a more shortened or contracted state and the tissues of your front body spine into a more lengthened state. Both are a deformation of elastic tissues under load. When you return back down to the ground you return to those tissues having potential energy.
Keeping yourself from falling while being “perturbed” out of tadasana or moving into and out of cobra requires “stiffness”. Remember, part of the scientific formula for potential energy has the letter “k” in it to represent stiffness. In terms of the human body, the greater ability for the elastic tissues to respond with stiffness, the more stable your structure, especially when you are moving it.
Stiffness, however, does not mean brittle or that you have tight hamstrings or pectoralis muscles. It’s not about “tight”, as in “I feel stiff” after a long car ride or because I can’t touch my toes. It’s scientific terminology linked to stability.
Although our joints need stiffness for sufficient stability, too much stiffness can cause problems. In order to bear larger loads, more stiffness is required, but in general, only a modest amount of stiffness is required to prevent what we call “displacement” of your joints.
Much of this has to do with what we call “motor control”. In the mid-range of joint motion, which we may be encouraged to push past in some yoga asana practices, our nervous system responds appropriately with motor cues that allocate just enough muscle activation to deal with stabilizing a joint.
We might call this normal passive stiffness – in other words, your body has the wisdom to employ stop gap measures near the end of joint range of motion that create a mechanical stop in order to ensure joint stability. It is reflexive (and there’s a voluntary component, too) if there isn’t dysfunction in the motor system.
When we are injured, have repeatedly pushed ourselves much past neutral in a yoga asana practice, or enter and exit a position where optimal movement patterns are no longer supported, our motor system can’t hear the “alert” and respond with passive stiffness that allows for stability. New movement patterns begin to predominate because ultimately, your nervous system/body is trying to keep you moving through your life. This can result in the loss of normal passive stiffness or aberrant joint motion.
Sometimes this happens because we are cued over and over to do things that may actually not be the best option for our particular body and needs (self-regulation is key here!).
I will repeat once again that stability is context dependent and you are the “system” and the task you are doing can be as simple as walking down a set of stairs or as complex as standing on your head and moving your legs around. Both of these actions/movements require stability, but in very different forms due to our relationship to gravity and how our tissues must respond.
I don’t think of stability as something we have achieved after we are already in a “stable” position or yoga posture, but more as a natural process of moving into and out of movements, whether sitting down to use the toilet, reaching up to a shelf, twisting to look behind ourselves before changing a lane, or moving into and out of warrior 2. This requires a certain stiffness created by a change in the tension and compression of the tissues of our body.
The stiffness part of stability is about active muscles creating tendon force at a joint. The brain has to communicate to those muscles in order for them to activate and do their job of stabilizing a joint.
It’s both reflexive and voluntary muscle activation.
I’ve been talking a lot in class lately about the patellofemoral joint of the knee complex. Here’s an example of tendon force at the knee joint in both flexion and extension.
The knee requires “stiffness” for both flexion and extension, but the quadriceps muscles act differently upon the patellar tendon to make that flexion and extension happen.
When I ask you to squat into utkatasana, you rely on the quadriceps to use their inherent ability for elastic deformation under load while you flex/bend your knees and lower towards the ground. It’s an eccentric contraction where the quads are “tensioning” the patellar tendon by slowly lengthening – like slowly letting out a fishing line so that the patellar tendon stays tensioned enough to keep the knee stable while you bend, but gives you enough mobility to transfer loads between the trunk/pelvis/hip and the lower leg/ankle/foot. All hell would break loose if the quads just let go!
Now that the knee is bent/flexed in utkatasana there is elastic stored energy in the tissues of the quadriceps, which have resisted the gravitational pull to bring you to the earth, by deforming under load. Kind of like a rubber band being stretched.
You stand up, or extend your knee (often called “straightening” the knee), as a result of using this potential elastic stored energy in the tissues. The quads exert force on the patellar tendon a bit differently on the way up, creating a concentric or what we might call a “shortening” contraction.
Your knee joint must be stabilized in both the flexed and extended position and this has to do with the stiffness created by muscle force working on the patellar tendon and the ability of your motor system to read the proper cues given by the peripheral nervous system. It’s all about elements of tension and compression.
So, if I’m talking about spinal stability, why did I use the knee as the example here?
Well, the knee is a pretty straightforward joint complex. The spine, however, is comprised of 33 bones – 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 in the coccygeal region. That’s 5 regions working interdependently! Stable behavior is necessary for the spine to do its job of bearing loads, allowing for movement/mobility in many directions, protection of vital tissues, and steering clear of injury.
It’s going to take a bit more dissection of this topic to bring it all into view for the spine and stability (ok, I had to use the word dissection because I am a total anatomy nerd).
In regards to biomechanical stability of the spine, you may hear lots of opinions and verbal cueing about what stability is, what a spine “should” do, how you “should” move, and what is best. That information, however, is coming from a human being who has processed their experience and education in certain ways, although I truly believe that most of us teaching yoga asana really do have your best interest at heart.
Remember, the parable goes like this… six blind men feeling different parts of the elephant explained what they were touching in six completely different ways even though they were all describing the same elephant.
And this is one elephant we are speaking about.
Most yoga teachers I know are teaching classes of 10 – 40 students!
Even with a scientific understanding of spinal stability, the variation and disparity among our students is astounding, and not simply on the physical level. It’s astounding on the biopsychosocial level, which considers the complex interaction between the biological, psychological, and social factors of each of us. Context dependent, folks.
Earlier in this post I talked about all physical systems needing a plan for stability in order to carry out its functions in the best way possible. Ultimately, your body is designed with its own inherent plan for stability and I’ve talked a bit about that so far.
Remember that stability is based on potential energy and our ability to create the proper amount of “stiffness” in the tissues of a joint or joint region to adapt to the loads/forces being applied, which is, of course, context dependent on the system and the task.
It’s just that as we live in our bodies over time we develop interesting habits and strategies that sometimes re-route the original plan. We adapt (our tissues adapt) to the task, whether we are sitting 8 hours a day, wearing high heels, sitting on the furniture we have in our homes, exercising, etc. That’s why a certain yoga posture or a movement we make in our everyday life is not necessarily good or bad for us in terms of stability. It’s how we have adapted and will adapt to the loads placed on our joint system at any particular time and over our lifetime.
This is why movement variability is a really important human movement theme. Read my blog post – Variable Movement is Nutritious Movement, Yoga asana and the Cardinal Planes.
In the next few blog musings, I’ll try to break down and present what the spine as a whole is designed to do and how each of the separate regions are dependent on one another for “normal” human movement (and can I just say that the word “normal” is pretty hard to define because, yep, you got it, it’s all context dependent).
Here’s to more exploration!
