The Law of Flexion and Extension


The Law of Flexion and Extension

Jim Freda


The law of flexion and extension is a basic principle of human movement. This simple idea has powerful lessons to teach us about posture and the sitting disease epidemic.

Flexion is the position of the infant in the womb.  It is the fetal position. Flexion is the position we adopt when we are tired or fatigued. Flexion is submissive – bowing and the kowtow are excellent examples of flexion in the body.  Flexion is exhalation, release, and the collapse of the body in on itself. Flexion is defensive: it is the position the body adopts in moments of trauma. Flexion is reflexive. It is the default—the position the body assumes in the absence of other intention or effort. It is the position of sitting. Flexion is not bad. It is simply one of two basic directions of human movement. It must be balanced with extension.


Extension then is active power and exertion. It is the body reaching out, lengthening, pushing away from the earth. It is inhalation and expansion. Where flexion puts us in a good position for detailed work—bent over something, calm, and focused on fine motor control and close visual observation, extension in contrast emphasizes gross motor control—the big muscles—and give us large, expansive movements that carry us where we want to go. Extension is the simplest possible cure for sitting disease. However, it requires effort and intention. It does not happen automatically.


Drawing by fellow TM contributor, Chris Davis.


Extension must be trained while flexion is our default. The infant has the ability to curl up into fetal position, almost reflexively, but it must train the patterns of extension. An infant has no reflex for kicking back, but easily brings its knee up in defense (Michaud 2011). Similarly, the infant is not born with arches in its feet. It has the genetic inheritance of the bony landmarks and the muscular attachments necessary to form arches, but these have to be activated through the baby’s exertion against gravity. This happens in the first year of life on the ground as the child explores its body, its world, and its ability to move. By the time the child is able to stand and walk, those arches and the muscular structure that supports the arches, are firmly in place.

What we call the secondary curves of the spine, the lumbar and cervical curves, are also not present at birth. The bony landmarks and the genetic predisposition are there, as are the muscles designed to create that wonderfully responsive and balanced tensional structure of the spine. However this is potential until it is trained in. Like the arches of the feet this too only forms through effort against gravity. It has to be trained and built as the child grows in its first year of life.

While the infant naturally curls up and rests in the fetal position, extension, on the other hand, is the first thing we see the baby do in response to gravity; lifting the head upward, pushing the body away from the floor, building the curves of the spine and structure of the body through a process of fighting against gravity.


Floor Play and Extension

Floor play is an important concept. The floor in our culture has been covered with chairs and made undesirable and unwelcome. But it is on the floor that the child develops it’s wonderful posture and natural movement skills. Our structure has to be activated in opposition to gravity, by pushing against the earth, playing safely and freely on the floor. No other surface can provide this.  Throughout most of human civilization the floor has been preserved as a space that invited movement and welcomed the body. Persian rugs, Japanese tatami, Korean ondol are all examples of sophisticated technologies of floor surfacing that reflect a more enlightened understanding of the body.

Extension involves big and forceful movements that require the freedom of an open floor. These movements do not have to be athletic like those gymnasts practice. In fact it is best to think of them as the natural activities of daily life.

For the baby play on the floor is work; the infant has no choice—it’s working hard. It is exploring, testing, failing, trying persistently. It is is training itself to move, building the ability to reach out, to change its position, crawl, stand and walk.  The doorbell rings, it can’t lift it’s head, it has to train that. There is something on the table, it can’t reach it, it has to actually train that. The ground is the most natural and safest place for this to happen. Proximity to the ground even helps the child to develop its immune system.

We can easily adopt a position of flexion. We do it by default. However, extension requires constant effort to maintain. This effort is called life, and it is fun. Sitting too much violates this basic principle. Of course it also cannot ultimately deliver the comfort and ease it promised. Extension, we now understand, does not come to us without effort. Extension is what we need to correct the biomechanical imbalance created by sitting disease. Extension does not address the full scope of the effects of the sitting disease epidemic. It only provides a missing and vital starting point as well as a sense of the larger context for our understanding of movement. It is a starting point. The law of flexion and extension reminds us of a basic skill we have lost in our pursuit of modern life.

The study of the development of the motor skills of the infant is called developmental kinesiology.  It informs our understanding of the optimal biomechanical functioning of the body as well as how to address the biomechanical effects of sitting disease (Frank 2013). This is a wonderful insight because many of us need this training again in our lives as a kind of fundamental rehabilitation as we seek to recover from decades of sitting disease. We did it once. We can do it again.


In my next installment I will discuss the single most powerful activity you can undertake to reverse sitting disease. This is nothing more than standing while raising your hands overhead. I will explain the mechanical effects of chairs on our body in more detail as context and I will talk more specifically about the biomechanics of raising our hands and how this simple activity is a concentrated reversal of many of the effects of sitting too much. I will provide some cues to help you get the best possible stretch, but simply standing and raising your hands for a good and even overly long time works wonders. This activity is very useful and relieving. It does not take a great deal of space and is no substitute for nutritious movement on the open ground, but it is also only a beginning point.

The purpose of the No Chair Pledge is to advance understanding and activism around the sitting disease epidemic. By removing a chair from your space, whether it’s your home or your office, you can create the conditions for more free and nutritious movement. Please visit to learn more and sign our petition to end mandatory prolonged sitting for children ages 5-13.



Thomas Michaud. (2011). Human Locomotion. Newton Biomechanics, 87.

Frank, C., Kobesova, A., & Kolar, P. (2013). Dynamic neuromuscular stabilization & sports rehabilitation. International Journal of Sports Physical Therapy, 8(1), 62–73.






Share this post

Leave a Reply

Your email address will not be published. Required fields are marked *