In the previous pages, we built the foundations of the human body (feet and legs). The next level is the ones of the hips, which support the spine. This page hence also discusses the "shape of the back".
I chose to consider two approaches, leading to the same result. This page discusses how the back should be held in sitting or standing while:
- Carrying out a "dangerous" activity (driving, playing music, working on the computer, etc.: Anything that you do with the hands in front of the body),
- Keeping the posture for a prolonged period without doing anything (e.g., while waiting for the train).
It is a "safe posture", which limits the global risk when other task-related risk factors are present.
However, wait until you have read the next pages to carry a load. Some more precautions need to be taken in this particular case.
Standing posture
Consider what happens if your thoracic vertebrae (represented by a point of the spine located between your shoulder blades) are behind the hips: Your back is hollow and the hips are blocked in rotation, i.e. are unable to compensate. Your posture is bound to be unhealthy.
Therefore, your thoracic vertebrae have to be kept "in front" of the hips: Your ability to control the shape of the back is entirely dependent on the relative position of these two reference points.
Hence, let first your hips move backwards while bringing the thoracic vertebrae (and the sternum) forward. Be careful not to create tension in the chest: Think in terms of movement and directions!
This creates a hollow low back, which you can flatten it by rotating the hips inward.
By bringing the thoracic vertebrae forward and then rotating the hips inward, we have protected the whole back (incl. the neck) and ensured that the posture is safe. Note how a hand placed at belt level, fingers spread apart, can be used to assess the shape of the low back.
Sitting posture
In sitting, the reasoning above applies as well. However, another approach, leading to the same result, helps getting a better understanding of the relationship between the hips, the spine and the back muscles.
The upper body is seen as a statue (the spine) on its base (the hips). Any architect knows that no building can be built on unstable foundations. Therefore, the hips needs to be under control at all times in order to avoid unhealthy spinal postures.
Let us carry a small experiment:
- Sit on your hands, palms up, so as to feel your "sitting bones" (actually the lower tip of the pelvis),
- Roll on your hips, bringing your thoracic cage to the front, creating a hollow low back: The sitting bones move backwards,
- Come back, still rolling on your hips, to an "upright" position until you feel maximum pressure from the sitting bones on the hands,
- Go even further backwards into a slouched posture (round back): The sitting bones move to the front.
This experiment, illustrated below, shows how rotating the hips influences the shape of the back. On the middle picture, the inclination of the back is exagerated: It should actually be only a few degrees forward.
Consider now the pictures below, showing the respective hips positions in the 3 previous pictures. Obviously, the middle position one is the most stable one, in the sense that the pelvis cannot collapse and "slide away" (blue arrows) when subjected to a vertical force (the weight of the upper body).
Sitting litteraly on the hips therefore minimizes the tension in the back muscles, needed to compensate for a lack of stability of the hips. Further, it avoids hollow and round backs.
You will see that this posture is the same as the one resulting from the previous paragraph.
Although details are given in further sections, note already that when sitting right (i.e. on your sitting bones and against your lumbar support), your shoulder blades are naturally away from the backrest of your chair. The same applies to driving a car, although the shape of the car seat may cause the backrest to come close to your shoulder blades. In any case, they should never be pressing against it.
Summary
In standing, assume the monkey posture by:
- Keeping your knees flexible and your weight distributed under the foot sole (not on the heels, but towards the heels);
- Bringing the thoracic vertebrae forward while pulling the hips backwards;
- Rotating the hips so as to flatten the low back.
In sitting, the same is achieved only by rolling on your hips until your weight rests litteraly on your sitting bones. Note that
- "vertical" sitting bones (i.e. with maximal stability and ability to resist vertical pressure),
- a flat back, hence avoiding a round back overloading the intervertebral discs as well as a hollow back overloading the facet joints, and
- being inclined slightly forward, hence minimizing the tension needed to reach tools placed in front (e.g., keyboard, mouse, steering wheel)
are 3 different aspects of the same monkey posture. These complementary advantages explain why it is so safe and therefore why it is the one to be adopted when minimizing the postural risk factor is vital, i.e. when carrying out a dangerous activity (e.g., computing, driving, playing music) or when remaining in the same posture for a prolonged period of time (e.g., while watching TV).
A lumbar support is an essential accessory in sitting. Its use is described in the section on computer work.
In other words, one should eliminate the task risk factor when increasing the postural risk, and one is allowed to take a controlled task risk only in a safe posture. This rule applies in sitting as well as in standing.
Counter-examples
On the picture below, the pelvis is not rotated backwards, so the low back is hollow.
Below, the pelvis is rotated backwards too much, so that the low back is round (reversed curvature). This typically occurs when sitting unsupportted for too long.
