(1) Ergonomics’ analysis of comfort and health in chair design
Starting from the rational analysis of the morphological changes of the lumbar spine and the stress on the lumbar spine in sitting posture, it points out the impact of different sitting postures on human health and comfort, clarifies some details of the humanized design of the seat back, and emphasizes the correct sitting posture and its "habit formation". The positive meaning of nature.
Reasons to pay attention to sitting posture research:
Sitting is the most common and important form of work and life for modern people. In developed countries, sitting is the norm at work, with a ratio of more than 2/3. As ordinary people, we spend about 1/3 of our time lying on the bed and 1/3 sitting in a chair. ,on the sofa. In view of this normal situation, it can be said that 1/3 of our lives are "survived" on seats.
Sitting has its negative effects. Sitting posture is extremely damaging to the human spine, especially the lumbar spine! Undoubtedly the most tiring posture, prolonged sitting brings many health risks to the lumbar spine and lower back muscles. In the past, in people's cognitive field, lumbar spondylosis, lumbar muscle strain, and low back pain were only common diseases of the elderly. Now, they have become common diseases of many young people. Most of the people who suffer from low back pain are sitting workers. They sit in seats all day long and maintain the same posture for a long time. For those who sit for a long time with bad posture, it is even worse.
Based on the above understanding, sitting posture is not trivial and should be paid enough attention and studied. There is much potential to improve sitting posture and pay attention to "sitting comfortably".
Sitting position and seat:
Sitting posture refers to the posture formed by people sitting on "seats" such as chairs and sofas. Seats are the main "hardware" of sitting postures. The study of seats is fundamentally a study of people’s sitting postures. Therefore, it is necessary to combine the research on “seats” with “people’s soft landing on seats” (sitting posture).
Discuss the comfort of seats and sitting postures in the name of health:
The design of the seats is to meet the needs of different levels. Below, we only discuss our personal feelings from one point, that is, from the perspective of comfort and health.
Understanding of seat comfort and health (comfort):
The comfort of a seat refers to the intuitive feeling and experience of the human body's sitting posture, which mainly reflects sensory perception.
There are difficulties in judging and expressing seat comfort. First, the judgment of comfort is highly subjective and varies from person to person. Even if factors such as height and body shape are excluded, it is difficult to be completely consistent. It can be seen that it is difficult to find a recognized and unified standard for comfort objectively. standard.
Second, they are often not very sensitive to the comfort of seats. They are also daily necessities. People judge the comfort of shoes and clothes relatively quickly and clearly. If you put on a pair of shoes that do not fit your feet, your feet will react immediately. Therefore, there is a phenomenon of "wearing small shoes" that reflects certain aspects of interpersonal relationships. A word for "body discomfort"; for example, if the size of clothes is too small, the body will immediately feel it. On the contrary, the "chair feel experience" lags behind. It is difficult to have a "real feeling" after sitting for ten minutes. Even if the chair is not ideally designed, it is difficult to judge whether it is comfortable or not. It takes a long time to sit for a long time before you can feel the real feeling. Taste it slowly.
Third, the judgment of chair comfort often lacks quantitative data indicators, and it is difficult to express it using quantitative methods. Therefore, descriptive definitions are often used and conceptual descriptions are relatively vague.
Therefore, the judgment of comfort often adopts a comparative method to determine the advantages and disadvantages of comfort and discomfort. This is like saying that there are two birds on the tree, a sparrow and a crow. The one next to the sparrow is just a crow, and conversely, the one next to the crow is just a sparrow. To put it bluntly, it's a bit like a circular argument in logic, causing and causing each other. Of course, you can use logical elimination to identify all uncomfortable feelings and eliminate them one by one. The result will be comfort. However, this is more troublesome. It can be seen that whether the seat is comfortable or not, there is indeed some "unspeakable secret" in terms of judgment and expression.
Healthiness:
The health of the seat refers to the health of the human body's sitting posture, which is concentrated in the judgment and analysis of the force on the human spine. Different from comfort, health is a deep and essential understanding. It has quantitative data indicators for judgment, and it is one of the main indicators for the humanized design of seats. The understanding of seat health can be studied from the perspectives of ergonomics, physiology, and human mechanics.
The relationship between seat comfort and health:
In most cases, the judgments on the comfort and healthiness of the seats are consistent, and most comfortable things are good for health; however, in a few cases, the judgments on the comfort and healthiness of the seats are not completely consistent. Comfortable and pleasant, but not necessarily healthy.
For example, a sofa that is too soft is very comfortable to sit on, but when you sit down, the body's center of gravity is in an unstable state and the whole body cannot exert strength. At this time, the body will unconsciously adjust its center of gravity and frequently change positions, and the muscles are always in tension. This condition will cause diseases of the lumbar spine system, which is detrimental to health.
For another example, a large sofa is luxurious and comfortable, but the seat surface is too deep and the person's lower back cannot be supported by the backrest of the sofa, leaving the waist hanging in the air. The usual remedy is to add cushions to a wide and deep sofa, but relying solely on cushions for lumbar support (lumbar support) may not be the best solution. Many cushions give people an unstable feeling, and the waist will feel sore after sitting for a long time.
To make a similar comparison, if you sleep on a bed that is too soft, your body will sink into the mattress and the pressure will be dispersed. It may seem comfortable, but it is extremely inconvenient to turn over and change positions. The support force of the spine is unevenly distributed, which especially affects the physiological curvature of the lumbar spine. , causing muscle tension and fatigue, making you feel uncomfortable after going to bed. Not only does it not relieve fatigue, it will cause backache and back pain. It can be seen that a bed that is too soft may look comfortable but is not good for your health.
Therefore, the comfort and health of seats are not coordinated concepts. They are both related and different. Comfort may not necessarily be healthy, but health will definitely breed comfort and pleasure. The study of seat comfort should start from studying the health of seats and sitting postures, that is, taking the deformation and stress of the human spine as the starting point. Only by studying comfort can we combine psychological feelings with objective essence, and be able to Expressing more clearly and accurately means that the problem can be explained clearly.
Analysis of the stress on the lumbar spine when the human body is in different postures (the stress on the lumbar spine in the three postures of sitting, lying, and standing):
The human spine is a skeletal structure that bears the weight of the upper body of the human body and maintains its stability. On this basis, the lumbar vertebrae and cervical vertebrae are body organs that carry a larger range of motion. The four most basic movement postures of the human body refer to sitting, lying, standing, and walking. In the three postures of sitting, lying, and standing, we use the indicator of the pressure in the third lumbar intervertebral disc to reflect the stress on the lumbar spine:
Among the most basic human postures, the sitting posture is the posture with the greatest pressure on the lumbar intervertebral disc, which is also the posture that puts the most effort on the lumbar spine.
Bad posture will multiply the stress on the lumbar spine:
As mentioned before, poor sitting posture will multiply the force on the lumbar spine. Where does the force come from? For this reason, it is necessary not only to see the conclusion, but also to analyze it. Some mechanical principles are involved here, namely the role of torque. When the body is hunched over and sitting forward, the person's center of gravity stretches forward, and the center of gravity deviates from the fulcrum, forming a moment. At the same time, it also increases the moment of the lumbar back muscles that competes with the gravity moment, thus increasing the tension of the lumbar spine. burden. Therefore, bad posture is harmful to health because it violates the principles of mechanics.
Once you understand the mechanical concept of torque, you can pay attention to avoid bad postures in daily life and work to reduce physiological load. For example, the weight of a human arm is about 4% of the body weight. For a person weighing 70kg, one arm weighs about 3kg. When a person stands, his upper limbs are suspended on his shoulders, and one shoulder bears the 3kg gravity of the arm. If you change the posture and extend your right arm flat to the outside, in a cantilever beam posture, most people will have difficulty holding this posture for more than ten minutes. Although the arm still weighs 3kg, the torque becomes larger, so the force on the shoulder is Magnified exponentially, far more than 3kg. Based on this principle, when carrying heavy objects, the heavy objects should be kept close to the trunk to reduce the gravity moment acting on the trunk and the torque of the low back muscles that compete with it.
Study on sitting posture - analysis of lumbar spine morphological changes and lumbar intervertebral disc force changes in commonly used sitting postures (human body shape changes from standing to sitting):
It is planned to analyze the outside and inside of the human body respectively. From the outside of the human body, when standing, the human body uses its feet to support the body's weight. When the standing posture is changed to a sitting posture, the stress-bearing parts of the whole body also change, and the buttocks are the main supporting parts to support the weight of the upper body. About 75% of the upper body weight is concentrated on the left and right ischial tuberosities of the buttocks. At this time, the feet change from supporting the whole body weight to mainly supporting the weight of the calves. Therefore, the sitting position is relatively comfortable, which is only for the feet, and the burden is greatly reduced. But for the lumbar spine, the waist has always been in an unreasonable state, and the burden has become heavier.
From the inside of the human body, when the body is standing upright, the human spine assumes a normal S-shaped physiological curvature. At this time, the cervical and lumbar segments protrude forward, and the thoracic and sacral segments protrude backward. When a person's posture changes from a standing posture to a sitting posture, the bones will change accordingly, and the pelvis will rotate backward, causing the sacral vertebrae at the lower end of the spine to also rotate. At this time, the human spine will change from the normal S-shaped physiological curvature in the standing posture to the normal S-shaped curve. The arch (C shape) changes, especially the physiological curvature of the lumbar spine, which will produce different deformations in different sitting postures. The lumbar spine tends to straighten from forward convexity, or even slightly backward. Changes in the morphology of the lumbar spine cause the lumbar intervertebral discs to be pushed and rubbed, resulting in uneven pressure distribution. The stress on the lumbar intervertebral discs cannot maintain normal and even distribution, and the internal pressure increases, resulting in discomfort such as waist soreness and fatigue. This is caused by changes in human body posture. Reasons why the lumbar spine and lower back muscles are negatively affected.
Straight back chair and straight waist sitting posture:
People's sitting postures can be classified based on the overall trend. From the perspective of the changes in the angle of the upper body in the front-to-back direction, the straight-waist sitting posture is the middle node, which distinguishes the forward leaning and backward sitting postures of the upper body. These three sitting postures (leaning forward, sitting straight, and leaning back) are the most commonly used sitting postures.
When a person sits on a straight-backed chair (the backrest inclination is a 90° right angle. The backrest inclination refers to the angle between the backrest and the seat surface) and adopts a straight-back sitting posture (upright sitting posture), the chair backrest actually has no impact on the person's back. No matter how strong the support is, the weight of a person's upper body is entirely borne by the spine. Sitting with the trunk upright will greatly deform the normal S-shaped physiological curvature of the spine, greatly increasing the pressure in the lumbar intervertebral discs (data in the data); not only that, sitting upright requires continuous muscle tension, so this The posture is difficult to sustain, even for a quarter of an hour. Therefore, sitting with a straight waist is not comfortable at all and is an inappropriate sitting posture. A 90° straight back chair is a bad design.
Sitting forward:
Sitting with the upper body leaning forward means sitting with the back flexed and hunched over. Sitting forward makes the person's back completely separate from the back of the seat, and all the weight of the upper body is concentrated on the lumbar spine, which is the only point of support. The more severe the forward lean, the further the center of gravity of the body's upper body will extend forward, beyond the balance point. The resulting additional torque must be borne entirely by the lumbar spine and lower back muscles. Sitting forward will change the normal S-shaped physiological curvature of the spine. Especially when leaning forward too much, the lumbar spine will deform from the normal forward protrusion to the backward protrusion. The pressure in the lumbar intervertebral disc will increase extremely, which will lead to the degeneration and herniation of the lumbar intervertebral disc. ; Sitting forward does not require any force on the abdominal muscles, but the lower back muscles must continue to tense and contract. After a long period of time, the lower back muscles will be overwhelmed, causing pain and spasm. Therefore, sitting forward (hunchback) is an unhealthy sitting posture and should be avoided as much as possible.
Sitting back:
When the seat backrest inclination angle is greater than 90° and the human body adopts a reclining posture with the back close to the seat backrest, the functional characteristics of the backrest inclination angle are not only to share and support part of the upper body weight, but also to reduce the ischial tuberosity of the buttocks facing the seat. The supporting force, and more importantly, is to maintain the spine's extended posture. Compared with the straight waist sitting posture and the forward leaning posture, the backward sitting posture can better avoid the kyphosis deformation of the lumbar spine, make the spine closer to the normal S-shaped physiological curvature, and relatively reduce the pressure in the lumbar intervertebral disc. Therefore, when the backrest inclination angle is greater than 90 °, the greater the backrest inclination angle, the more comfortable the human body's sitting posture will be.
Backrest inclination greater than 90° also has an impact on reducing muscle load. When there is no chair back to lean on, the upright human upper body has no lateral support, so it can only rely on the strength of the back muscles to position the spine to maintain the balance and stability of the upper body, causing muscle tension and fatigue. Take dozing off in a chair as an example. If you do not lean on the back of the chair at this time, the person's upper body will tilt forward and backward and lose balance. This is the result of the involuntary relaxation of the back muscles. On the contrary, if the upper body leans back and the lower back and buttocks are naturally pressed against the back of the chair at an angle greater than 90°, the tension of the back muscles can be relaxed and the load on the back muscles can be reduced. It can be seen that sitting backward is beneficial to health and comfort.
Through the above health analysis of the three most commonly used sitting postures: forward leaning, straight waist sitting, and backward sitting, on the one hand, for rational considerations, forward leaning and straight waist sitting postures should be avoided, and healthy and comfortable leaning back postures should be preferred. Sitting posture; in addition, from a practical point of view, in the desktop working environment, people often have to adopt forward leaning and straight waist sitting postures. Of course, consciously and proactively adopt a reclined sitting posture to improve the "living state" of the lumbar spine and lower back muscles, which is of great significance to improving the health and comfort of sitting postures.
