Ergonomics has a great influence on the design of the office chair, and good ergonomic performance can make the user get a good sitting experience. Excellent ergonomic performance is closely related to the size of the office chair. This article mainly aims at the ergonomic requirements of the office chair. In this article,we puts forward relevant suggestions for the design of the main components of the office chair, and analyzes the different requirements of ergonomic dimensions of the office chair .

Key words: office chair, size, standard.

As an industrial product, office chair is closely related to human life, which provides effective support for people's work and satisfies the daily desk work. According to the data given by the TED research institute, we will spend 9.3 hours a day in a chair. Sitting for a long time brings about a decrease in work efficiency and a series of health risks. According to German statistics in 1996, 85% of patients with low back pain were caused by prolonged sitting. Therefore, the appropriate size of the office chair is a very important factor. In recent years, ergonomics has been paid more and more attention to the design of the office chair, and the main standards of office chairs at home and abroad have also made relevant requirements and specifications on the size.

1.  Analysis of key Dimensions in Office chairs.

Although the office chair has various forms, its structure is basically similar. in order to reflect the health elements of the office chair and provide a comfortable office environment for office staff, the office chair has the following key dimensions.

Chair height.

The height of the office chair has the greatest impact on the user, which will cause sitting posture distortion, waist fatigue or lower limb numbness caused by compression of the thigh arteries. Therefore, it is a trend to design the height of the office chair as an adjustable type.

Inclination  of the seat

The design of the inclination angle of the seat pan can adjust the body pressure distribution around the ischial tubercle. If the inclination angle is too large, which increases the force on the buttocks and brings a sense of pressure to the user, at the same time,if the inclination angle is too small, or there is a forward inclination angle, which may lead to sit-slip phenomenon.

Seat height.

Seat height mainly affects the range of movement of the human hip on the seat. According to ergonomic requirements, the width of the office seat should be as wide as possible, so as to adapt to more people, but the beauty and the inner width of the armrest installed on both sides should also be taken into account in the design.

Seat depth.

The depth of the seat pan also has a great influence on the user. In the design, the depth of the seat pan should be fully taken into account, which leads to the loss of the function of the backrest, the inability to support the back, the suspension of the back, the tension and soreness of the back muscles, and the shallowness of the front thighs. The weight of the human body will be more distributed to the calves, making people tired.



Armrest depth.

The purpose of designing the armrest of the seat is to effectively support the arms when they droop naturally, reduce the muscle tension of the upper limbs, improve the comfort of the user, and at the same time, it is convenient to support and stabilize the body when standing up or changing posture. The height of the armrest should be reasonably designed in the design, so as to achieve a reasonable support function.

Lumbar support height.

The function of the lumbar is mainly to support the lumbar, so that the lumbar muscles can be relieved, and at the same time, the lower point support and the upper two points support in the form of the back of the human body, so that the back of the human body can get a complete rest. According to human physiological data, the most suitable height of the lumbar is at the 4th to 5th lumbar vertebrae. The design of the lumbar curve should be in line with the physiological curve of the human spine and lumbar vertebrae, so as to ensure the comfort of sitting posture. The role of lumbar support can make the lumbar arc curve slightly kyphosis, if there is no lumbar support or unreasonable design, it will make the normal lumbar vertebrae show kyphosis shape or make lumbar vertebrae obviously kyphosis, resulting in lumbar vertebrae deformation. in order to avoid the abnormal state of kyphosis and excessive kyphosis, reasonable lumbar support should be used to reduce the pressure of intervertebral disc and reduce the load of muscle. Lumbar design has become a necessary part of office chair  design.

Shoulder support

Similar to the lumbar support, the main function of the shoulder support is to form two points of support on the upper back, and its approximate position is at the height between the 5th and 6th thoracic vertebrae. According to the analysis of natural sitting posture, when the human body is working in the sitting position for a long time, but the shoulder does not have support, it will make the human shoulder feel uncomfortable or sore. Therefore, the office chair must design the shoulder rest to decompose the shoulder pressure and relieve human fatigue.

Requirements for the size of office chairs in BIFMA X5.1/EN1335/ China QB/T2280

At present, the important office chair standards at home and abroad are: China's QB/T2280 (office chair), American market BIFMA X5.1, European market EN1335 (office chair).   From the current overseas market buyers, the Middle East and Africa region recognized China's office chair standard, the Australian market reference BIFMA5.1 standard, therefore, this article  also makes a comparative analysis of these three standards.
It is worth noting that BIFMA X5.1 has almost no requirements for size, except that it is mentioned in the latest version (version 2011) that the lumbar of office chairs can not be smaller than 200MM, but it is learned from export enterprises that American customers will provide standards on their own, or have requirements for several important aspects of office chair size.
China's office chair industry standard QB/T2280 (office chair) has its own regulations on size, which mainly include seat height, seat width, seat depth, backrest, gaslift distance, inner width of armrest, armrest  height dimensions and so on

European office chair standard EN1335 main part is the size requirements of the office chair, compared with other office chair standards, EN1335 size requirements are more specific, first of all, EN1335 according to the size requirements of the office chair will be divided into three grades, different levels of reference is different, EN1335 standard , divided into seat size requirements, backrest size requirements, armrest size requirements, underframe size requirements. Specifically, there are seat height, seat width, seat depth, seat plate depth, backrest inclination angle, backrest support point S height, backrest plate height, backrest width, backrest Radian radius, backrest inclination angle, armrest length and width, armrest width, armrest height, armrest front-end dimension from the front of the seat, inner width of the armrest, maximum compensation dimension of the base, stability dimension.

It is known from the above that the EN1335 standard takes into account the ergonomic requirements, especially the back 3-point support, backrest Radian requirements, seat back angle, seat inclination angle,armrest height, and seat higher requirements. In contrast, the American BIFMA 5.1standard makes almost no stipulation on the size of office chairs, but overemphasizes the life mechanics requirements of office chairs. From the perspective of comfort, this standard is defective. Ignoring the most essential requirements of the seat, although the domestic office chair standard QB/T2280 has certain size requirements, it is not detailed and complete, especially it can not well meet the ergonomic requirements. I hope it can be used in the next version.



Conclusion

The ideal office chair should take the natural physiological state of the human body as the ultimate goal. By using human physiological data and designing strictly according to the principles of ergonomics, the user can get full support and rest in the course of work. Make the human body feel comfortable, and finally reduce the fatigue of work and the pain caused by long-term sitting posture, which is an important aspect that should be paid attention to in the design of office chair. At the same time, it also embodies the humanized side of industrial design.