There is a question I think about almost every day in my practice: what keeps you upright?
Not philosophically. Literally. Right now, as you read this, something is holding your skeleton against the constant pull of gravity. You are not collapsed on the floor. Something is doing work. The question is what, and whether it is doing that work well.
Feldenkrais had one of the clearest models I have ever encountered for understanding this. He divided muscular labor into two categories. On one side, the extensors. These are the muscles along the back of your body, broadly speaking. The erector spinae running along the spine. The gluteals. The hamstrings. The calves. Their primary job is to hold you up against gravity. On the other side, the flexors. The muscles along the front. Abdominals, hip flexors, biceps, the muscles that curl you forward. Their primary job is voluntary action. Reaching, gripping, pulling things toward you.
In this framework, your extensors handle support. Your flexors handle intention.
It is a beautiful division of labor. And for the most part, it is correct.
The Antigravity System
The term “antigravity muscles” sounds more dramatic than it needs to. All it means is this: certain muscles are specifically tasked with resisting the downward pull of gravity on your body. They keep your head from dropping forward, your spine from collapsing, your pelvis from tucking under, your knees from buckling.
These are predominantly extensors. The muscles that extend your joints, that open you up, that bring your body into its full upright length. They work constantly in standing and walking, often below conscious awareness. You do not decide to engage your erector spinae when you are standing in line at the grocery store. They just fire. Or they should.
Feldenkrais emphasized something important about these muscles. When they are working properly, they work quietly. You should not feel them. The effort of standing upright, in an organized body, is minimal. The skeleton does most of the structural work. The antigravity muscles make fine adjustments, small corrections, a constant quiet conversation with gravity.
This is not the same as “engaging your core” or “activating your glutes.” Those are voluntary commands. The antigravity system works at a level below conscious effort. It is reflexive, automatic, and deeply tied to your nervous system’s sense of where you are in space.
Feldenkrais understood this distinction with remarkable clarity. He knew that you cannot command these muscles into good function. You have to create the conditions for them to work. The nervous system has to learn, or re-learn, what upright actually feels like. Then the right muscles respond on their own.
Flexors Are for Doing
Here is the complement to the extensor story. If the extensors hold you up, the flexors are free to act. They reach for things. They pull food to your mouth. They grip tools. They generate the force for a punch or a pull-up.
In a well-organized body, flexors and extensors have clear roles. The extensors stabilize. The flexors mobilize. There is no confusion, no overlap, no competition.
Think of it like a building. The structural beams hold everything up. You do not ask the furniture to support the ceiling. The furniture is there for function, for use, for living in. The beams do the quiet invisible work.
When this division of labor is intact, movement is efficient. A person can walk, reach, lift, and bend with remarkably little effort because the right muscles are handling the right jobs. There is no wasted energy. No unnecessary tension.
I have seen this in certain clients. Usually people who grew up moving a lot, who did varied physical activity as kids, who never sat at a desk for eight hours a day. They walk in and you can see it. Their movements are clean, proportional, easy. They are not working hard to be upright. Their extensors are doing the job so their flexors can do everything else.
Where It Gets Complicated
Here is where Feldenkrais’s model, as brilliant as it is, runs into the reality of modern bodies.
The model assumes a body that has had the chance to develop this division of labor. A body that has spent enough time moving through varied positions, crawling, rolling, standing, walking on uneven ground, that the nervous system has a rich map of how extensors and flexors should collaborate.
Most of the bodies that come into my practice do not have that. Not because something is wrong with them. Because they live in the modern world.
When you sit in a chair for six, eight, ten hours a day, the extensors along your back do not just get weak. They get confused. They lose their role. The hip flexors shorten and tighten. The glutes go dormant. The thoracic spine rounds forward and the extensors in the mid-back get locked in a lengthened, inhibited position.
And here is the part that complicates Feldenkrais’s framework: the flexors start doing double duty. They are not just handling voluntary movement anymore. They are bracing. The hip flexors are gripping to provide stability that the glutes and deep hip extensors should be providing. The neck flexors are clamping down because the deep cervical extensors have checked out. The abdominals are holding tension not because they are “strong” but because the back extensors are not supporting the spine adequately.
The division of labor has broken down. The flexors have been drafted into a support role they were never designed for. And they are doing it badly. Tension, stiffness, pain.
When the Extensors Cannot Do Their Job
The pattern I see constantly in desk workers goes like this. Someone comes in with chronic low back pain and a posture that tells the whole story before they say a word. The pelvis is tucked under. The lumbar spine has almost no curve. The hip flexors are short and rigid. And the glutes, when I test them, are functionally absent. Not weak in the traditional sense. Absent. The nervous system has essentially stopped asking them to participate.
The extensors have been taken offline.
In their place, the hip flexors and low back muscles are working overtime in a pattern of co-contraction. Everything is gripping everything. The body has abandoned the extensor-flexor division of labor and replaced it with a strategy of generalized tension. It is the nervous system’s best available solution for staying upright given the tissue environment it is working with.
This is what I see over and over. It is not that people lack the muscles. The extensors are there. They just are not being recruited. The nervous system has lost access to them, or it has decided that the current arrangement, however inefficient, is the safest option.
Feldenkrais would say: teach the nervous system a better option. Create conditions for the extensors to reawaken. Through slow, gentle movement explorations, give the brain new information about what is possible.
And I agree with that. Up to a point.
The Limits of Awareness Alone
The challenge I keep encountering is this: when the tissue environment has changed significantly, when fascia has shortened and densified, when joints have lost range, when muscles have been inhibited for years or decades, the nervous system sometimes cannot find the better option on its own. Not because it is stupid. Because the hardware has changed.
Imagine you are trying to play a piano and three of the keys are stuck. You can explore all the musical possibilities you want. You can listen carefully. You can slow down and pay attention. But those three keys are not going to play until someone unsticks them.
That is what structural work does. It unsticks the keys. It restores options to the tissue so the nervous system has something new to work with.
In my practice, I often need to manually release the shortened hip flexors, address the fascial restrictions in the thoracic spine, and directly stimulate the glute tissue before the extensor system can come back online. The awareness piece, the Feldenkrais piece, is essential for integration. But the tissue has to be ready to receive the new information.
This is not a criticism of Feldenkrais’s model. It is an acknowledgment that the model was developed in an era when bodies had more baseline function to work with. The extensors in most of his students were probably functioning at a higher level than the extensors in most of my clients. Because his students were not sitting in cheap office chairs for a decade.
The Extensor System as a Whole
One thing Feldenkrais got deeply right was thinking about the extensors as a system, not as individual muscles. Your erector spinae, your glutes, your hamstrings, your calves. These are not separate players doing separate things. They are a connected chain that runs from your skull to your heels, all working together to keep you upright and moving forward through space.
In the Anatomy Trains model that informs my structural integration work, we call this the Superficial Back Line. It is a continuous fascial and muscular connection that runs from the brow ridge, over the skull, down the back of the neck, along the spine, through the sacrum, down the hamstrings, under the foot, and up to the toes.
When this line is working well, you feel tall. Not rigid. Tall. There is a sense of easy length through the back of the body. The head sits effortlessly on top of the spine. The pelvis is balanced. The legs push you forward without strain.
When it is not working well, you see the compensations everywhere. The forward head. The rounded upper back. The tucked pelvis. The knees that hyperextend because the hamstrings are doing the work the glutes should be doing.
Feldenkrais understood this systemically even if he did not use the fascial language we use now. He knew that the problem was never one muscle. The problem was always the organization of the whole system. And the solution was always reorganization. Teaching the entire extensor chain to participate, not just strengthening one muscle in isolation.
What This Means for You
If you are reading this and recognizing yourself in the description of collapsed extensors and overworking flexors, here is what I want you to understand.
First, this is incredibly common. You are not broken. You are adapted to an environment that your body was never designed for. Sitting in chairs all day is a very new phenomenon in human history, and your body is doing its best with a situation it never evolved to handle.
Second, the solution is not just “stand up straight.” That is a flexor command. When someone tells you to stand up straight and you pull your shoulders back and lift your chest, you are using voluntary flexor effort to mimic what the extensors should be doing automatically. It does not last. It should not last. It is the wrong muscles doing the wrong job.
The real solution involves two things that need to happen in sequence. The tissue environment has to change. Shortened hip flexors have to lengthen. Restricted fascia has to release. Dormant muscles have to be woken up at the tissue level. That is the structural intervention piece. I wrote about how strength relates to this in the next post in this series.
Then, and this is the Feldenkrais piece, the nervous system has to learn to use those newly available tissues. It has to discover that the extensors can work again. That upright does not require effort. That gravity can be a friend rather than an opponent.
Both pieces matter. Structure without awareness gives you a temporarily better body that does not know how to maintain itself. Awareness without structural change gives you a brilliant nervous system trying to work with hardware that cannot execute the plan.
The extensor-flexor model that Feldenkrais articulated is one of the most useful frameworks I know for understanding human posture and movement. It is elegant, it is grounded in real physiology, and it explains so much of what I see every day in practice.
It just needs updating for bodies that live in the twenty-first century.
In the previous post, I discussed how unnecessary muscular effort accumulates. The extensor-flexor confusion I have described here is one of the primary mechanisms through which that happens. When the division of labor breaks down, everything starts working too hard. And that excess effort becomes the new normal, invisible to the person living in it, until something hurts enough to make them pay attention.
The good news is that this is fixable. The extensor system can come back online. The division of labor can be restored. It takes work, both structural and educational, but it is one of the most satisfying transformations I get to witness. When someone’s extensors wake up and their flexors finally get to relax, you can see it in their face. Something lets go. They stand differently. They breathe differently. They did not try harder. They just stopped fighting gravity and let the right muscles do the job.
That is what Feldenkrais was pointing at. And it remains one of the most important ideas in movement education.