In modern times, most office workers work in the office sitting for long hours.

With the development of science and technology and the continuous progress of society, more and more workers work in a sitting position, which will become the most important working posture for human beings. 

Due to the unreasonable design of the chair, if the structure of the chair does not conform to the principle of ergonomics, which will bring harm to people who deal with sitting posture for a long time, such as work fatigue and low work efficiency. a variety of cervical vertebrae, lumbar vertebrae, vertebrae and other diseases, thus affecting the health of the staff. Therefore, the ergonomic design of the office seating is particularly important. As a systematic analysis project, the close cooperation between the ergonomic effect and the staff participating in the development and design of the office chair has become a necessary process. At present, it is a common concern of the designers and related staff.

 The purpose of office chair design.

The design of the office chair should be based on the actual use value of the chair. the focus of the design is the rationality of the structure, focusing on the perfection and optimization of the function, and the appearance is also attached to the realization of the functional features. do not excessively pursue the sense of form, showing more rational and structural features, so that the chair can really adapt to people's physiological characteristics. The design should be based on ergonomic principles and anthropometric measurements, focusing on humanization, not causing physiological harm to the human body in the process of work, so that people can engage in thef work more conveniently and quickly.

Ergonomic  analysis in office chair

1 Seat height

Should be 380-480mm

Seat height is the vertical distance from the front of the seat to the ground. Seat height is one of the main factors that affect the comfort of sitting posture. Unreasonable seat height will lead to incorrect sitting posture which is easy to cause fatigue for the lumbar of the human body. If the seat is too high, the legs can not touch the ground in the air, and part of the body pressure is scattered in the thigh, which oppresses the blood vessels of the thigh, hinders the blood circulation, if the seat is too low, the knee is arched, and the body pressure is too concentrated on the ischium. Pain will occur over a long period of time. Ergonomic studies show that reasonable seat height should be equal to calf plus foot height plus 25-30mm heel thickness minus 10-20mm room for movement, that is, chair height = calf plus foot height.



Among them, the calf plus foot height should generally be suitable for all people above the 5th percentile, as far as the office seat is concerned, its seat height should be slightly higher than the rest seat, and it should be designed to be adjustable to adapt to the use of most people, generally 400-440mm, if it is adjustable, 380-480mm.

2.Seat cushion.

There are two ischial tubercles under the pelvis of the human body. in the sitting position, when the seat is approximately horizontal, the femur on the outside of the ischial tubercle can be in a normal position without excessive compression, and the human body will feel comfortable. When the sitting surface is in the shape of a bucket, it will make the femur rotate upward, which not only makes the femur in the oppressed position and bears the load, but also causes abnormal compression on the hip muscles and forces on the elbows and shoulders, thus leading to discomfort. Therefore, the design of the seat surface should be similar to the level and avoid the bucket design.

3 Width of seat.

The seat width is determined by the human hip size and the appropriate range of movement, and the seat width of the office seat should be as wide as possible, because it must be suitable for tall people, and its corresponding anthropometric dimension is hip width, therefore, the design of seat width is usually based on the 95th percentile of the female hip width. The seat width is generally not less than 380mm, and the international GB/T3326 stipulates that the front width of the armchair seat is more than 380mm.

4.Seat surface.


The seat depth refers to the distance from the front of the seat to the back edge of the chair. In the design process of the office seat, the seat surface should not be too deep, otherwise the back support point is suspended and the back has no effect, at the same time, the knee socket will be oppressed, making the calf feel numb; the seat surface should not be too shallow, otherwise the front thigh is suspended, the weight will all be pressed on the calf, and fatigue will occur quickly. Ergonomic studies show that the seat depth is slightly less than the horizontal length of the friend's legs, that is:

Seat depth = sitting depth-60mm (gap).

According to the national standard GB/T3326, the depth of backrest seat is: backrest seat depth = 340-420mm.

5.Seat tilting

Usually the chair seat tilts slightly backward, first of all, it prevents the buttocks from gradually sliding out of the seat surface, resulting in poor posture stability; secondly, the back can be supported to reduce the pressure at the ischial node, so that the whole upper body weight is borne by the lower limbs to improve the situation, reduce fatigue. The ergonomic study shows that the inclination angle of the working seat is 0 °- 5 °, and the inclination angle of the recommended working chair is 3 °

6.Armrest.

The function of the armrest is to reduce the burden on the arms, help rest the muscles of the upper limbs, increase comfort, support the body and help stabilize the body when standing up or changing posture. The height of the armrest should be designed reasonably, too high or too low can easily cause fatigue of the upper arm. In the design, the height of the chair armrest is determined according to the elbow height of the 50th percentile sitting posture. Generally, the distance between the armrest and the seat plane is 200-250mm. At the same time, the front end of the armrest is slightly higher and tilts with the inclination of the sitting surface and the backrest slope.

7 Lumbar support.

lumbar is at the height between the 4th and 5th lumbar vertebrae, which is composed of the physiological curve of the human spine, that is, lumbar curvature, and is also directly related to the comfort of sitting posture. The normal lumbar arc curve is slightly kyphosis, if the office chair  does not have lumbar support or lumbar support is too weak, it will make the normal lumbar vertebrae show kyphosis shape, lead to lumbar vertebra deformation, and lumbar vertebrae protruding excessively will also make lumbar vertebrae obviously kyphosis, and lumbar vertebra deformation will occur over a long time. Therefore, lumbar kyphosis and excessive kyphosis are abnormal, and reasonable lumbar support should be the physiological curve that makes lumbar arc curve in. This can reduce the pressure of intervertebral disc and reduce the load of thigh meat. Therefore, the design of lumbar support has become a necessary part of office chair design.

8. Shoulder support.

Shoulder resting is on the height between the 5th and 6th thoracic vertebrae, because the height of the 5th to 6th thoracic vertebrae is equivalent to the height of the scapula, the area of the scapula is large, and it can withstand greater pressure. When the human body is working in the sitting position for a long time, the shoulder part feels uncomfortable or sore. Therefore, the office chair must be designed to rely on the shoulder to decompose the shoulder pressure and relieve human fatigue.

9.Backrest tilting.

Backrest tilting refers to the angle between backrest and seat. Backrest tilting can affect intervertebral disc pressure and back muscles, and the increase of backrest titling can enhance human comfort, because when the body is leaning back, the load of the body moves to the lower half of the back and thighs. Ergonomic studies show that when the angle between the seat surface and the backrest is more than 110 °, the inclined backrest supports the weight of the upper part of the body, thus reducing the pressure in the intervertebral disc, so it is better for the upper body to lean backward 110 °- 120 °.

10 Seat cushion.

Cushions are also an important part of office chairs. According to ergonomics, the human ischium is stout and can withstand more pressure than the surrounding muscles, while the thigh seat has a large number of blood vessels and nervous system. Excessive pressure will affect blood circulation and nerve conduction and feel uncomfortable. Therefore, the pressure on the cushion should be designed according to the principle that different parts of the hip bear different pressure, that is, the pressure is the largest at the ischium, gradually decreases around, and decreases to the lowest at the thigh. In addition, the cushion material should be breathable and non-skidding to increase the comfort of the seat surface.

Conclusion.

In a word, the ideal office chair should be based on the measurement data of anthropometric measurements and be designed strictly according to the principle of ergonomics, so that people can feel comfortable in the process of work. reduce work fatigue and diseases caused by long-term sitting posture, which is also the important embodiment of humanization mentioned in the design.