The perfect posture
When we sit in a chair, the greatest burden is on our hips. We sit in chairs every day, but in the posture we use while sitting, there are points that are problematic for our bodies. This may overstate the case just a bit, but when we sit in a chair, it is really only the legs that have it easy. The same upper-body weight load removed from our legs while sitting now has to be borne by our hips. To help support that load, our backs also take on some of the burden. Furthermore, by sitting in a chair, the natural standing-position S-curve gives way, and we end up with an arch form. This breakdown of the natural back S-curve is the reason you feel your hips, back, and neck get tired when sitting in a chair.
When sitting, it is difficult for us to maintain the best posture for a long period. We are generally inclined to cover our habits, by tilting certain parts of our body forward, backward or left and right. Most of the time, you can protect yourself by putting your back, shoulder and head in line with your hips. Your legs can then support your hips to maintain the appropriate balance.
Pressure distribution
If we sit without firmly placing our hips against the chair back, the body is supported only by the buttocks, part of the thighs, and the upper back, which means the pressure is excessively concentrated in a small area.
When sitting in a proper posture, the lumbar region and pelvis are supported and the back approaches a natural position so that pressure is suitably distributed in the vicinity of the important chair-back point, and the back is supported at an appropriate height. Looking at the body pressure distribution on the chair, we find that pressure is properly concentrated in the region of the ischial tuberosity, a fleshy area considered an insensitive part of the body and a point that stabilizes the posture. In addition, the fact that the pressure is gradually declining in this vicinity shows that the form of the chair conforms to the body.
Okamura's advanced ankle tilt reclining and its focus on ergonomics

Ankle tilt reclining is a mechanism by which the chair seat sinks backward in sync with the reclining of the chair back. When reclining, the gradual opening of the ankle, knee, and hip joints without moving the ankle itself serves to create a relaxed state and promote better blood circulation. In addition, the fact that the chair seat does not push the body upward makes this a body-friendly reclining mechanism which reduces the likelihood of leg swelling and numbness. Since the chair moves in unison with the body, even if reclining is repeatedly done, your concentration will not be broken by feelings of discomfort, and you will not feel the need to make unnecessary movements to change your seating position.

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