A sitting position is defined in space by
1- the width, height, depth and inclination of the seat
2- the height and inclination of the backrest
In this article, we are going to focus on the angles of the backrest and the seat from an anatomic and ergonomic point of view.
What is the purpose of the seat tilt?
When we sit on a horizontal surface and the backrest is vertical, the weight of the upper body is mainly applied to the ischiatic area. The ischia are two bony protrusions of the pelvis that help us to maintain our balance. If the weight supported by the ischiums is too great, blood circulation is no longer properly performed and leads to a decrease in the supply of oxygen, which is responsible for tissue degradation. To avoid asphyxiation a pain signal invites our body to lift itself up so that circulation is restored. This pain signal is in fact what we usually call discomfort.
In the image below, borrowed from the company Mescan, the red zones indicate areas at risk because of high pressure closing capillaries . The two ischiatic zones are clearly visible.
If I recline my backrest without tilting the seat, some of the upper body weight is supported by the backrest. This reduces the load on the pins and delays the onset of the pain signal.
But in doing so I create a horizontal force in the seat that pushes me forward. If my seat is smooth I slide and fall off my chair. But my seat is not smooth and holds me back. This causes a shearing phenomenon that slows down the blood circulation and brings us back to an area of discomfort.
It is therefore essential to avoid sliding forwards when reclining the seat at the same time as you recline the backrest.
If our mass were evenly distributed from head to toe the best position to reduce pressure would be the reclining position we usually use to sleep.
But this lying position is not practical for reading, eating, watching TV or driving. So we have to find compromises depending on the activities we do in the chair.
Six angles for an armchair
The study of the proportions of the human body is an ancient subject, the most famous illustration of which is the Vitruvian Man made around 1490.
At the end of the 19th century surgeon Fowler studied sitting and semi-seated positions in bed and their effects on his patients.
In the 20th century designers sought to model the body in a seated position. Based on the work of Ohara in 1969 the Dutch designer Andries van Onck proposes in a course given in 2009 in Italy six types of sitting position
In the first type the seat is almost horizontal (3°). This position is practical for eating, working at a desk or for getting up easily.
At the other end of the spectrum is the sixth position with a very inclined seat (22°). The pressure is well distributed between seat and backrest and blood circulation is as free as possible. This feeling is confirmed by clinical studies using pressure sensors on so-called pressure sore prevention mattresses. This position allows recovery by allowing the blood to circulate freely without putting too much strain on the heart. The second position is suitable for car seats. It allows the driver to keep his or her eyes on the road while offering an acceptable level of comfort.
Positions 3, 4 and 5 are used for resting chairs depending on the activity.
Custom made armchair and recliner
Mon Siège à Moi makes custom-made armchairs from your dimensions. The seat inclinations are pre-defined according to the type of chair. But it is also possible to specify different inclinations depending on the use of the chair.
In this way you can optimize the comfort and ergonomics of your chair or armchair.