Ergonomics in sofa design

- Jun 15, 2018-

Ergonomics in sofa design is the use of sofas as an element of ergonomics. It focuses on the analysis of the interrelationship between the human-sofa-environment and the flexible use of ergonomics according to the requirements of users and the indoor environment. The theory, principles, data, and methods for determining the functions, scales, shapes, materials, and color combinations of sofas that meet human physiological and psychological needs.

"People-oriented" is the core of ergonomics in sofa design. "Comfort", "Functionality" and "Safety" are three basic principles of sofa design ergonomics. Among them, "comfort" is the primary principle of sofa design.

1 sofa design ergonomic content

1.1 Sofa Design and Human Behavior

The sofa is the main furniture for the modern living room home and office space reception area and meeting area. Because people and occasions using sofas are different, people are physically and mentally different from each other in terms of their design requirements for the function, scale, body mass, shape, and color of the sofa.

Take the home sofa as an example. A sofa with a soft seated backrest that supports the head and can be used under a variety of sitting postures. Suitable for middle-aged and young people who love leisure and freedom lifestyles, especially men. The elderly are physically weak and unresponsive. It is inconvenient to adjust the sitting position. Therefore, the couches used by the elderly should not be overly soft on their seats and backrests. The seats should not be tilted too much, the seat height should be slightly lower, and handrails with appropriate height should be set. For children who are naturally active, the sofa is a group of them. In large toys, children like to crawl and jump on the sofa. Therefore, when designing a sofa for a family with children, it is necessary to take into consideration safety, and to be flexible and easy to clean. In addition, if the sofa is designed specifically for children, more attention must be paid to “fun” and “entertainment”.

In the design of sofas, when designers analyze the needs of the indoor space and the humanistic environment, it is necessary to consider human behavior. This not only requires the designer to observe the user's behavior habits in detail, but also to further explore the causes of these behaviors from the surface behavior, and deepen the product's "comfort", "functionality" and "security."

1.2 sofa design and human sitting position

The ideal sofa should be comfortable and easy to sit up. When a person sits, the thigh is laid flat, with both feet on the ground, the body's center of gravity naturally leans slightly backwards, the spine assumes normal shape, body pressure is distributed reasonably, body muscles are relaxed, blood circulation is smooth, and posture is comfortable. (As shown in Figure 1: Spinal morphology under normal human sitting posture)

When the sofa is designed, the determination of functional dimensions such as the height of the seat, the depth of the seat, the inclination angle of the seat surface, the inclination angle of the backrest, the curvature of the seat surface and backrest, etc., need to fully consider the rationality and comfort of the human sitting posture. If not designed properly, it will not only affect the use of sofas, but will even affect people's health. Figures 2-5 illustrate four typical bad designs, respectively.

The sofa seat is deep and deep. When people use the backrest when sitting on the back of the seat, the heel does not support the ground, or even the entire foot is vacant. In this sitting state, the weight of the entire body is mainly concentrated on the buttocks and thighs, and the pressure on the inside of the knee socket and the lower leg interferes with the circulation of blood in the legs. In a short period of time, the numbness of the legs will occur, and the soreness will occur for a long time. In addition, if the angle of inclination of the seat surface of the sofa is too small and the seat cushion is rigid, at this time, human beings are only supported by two points of the back and buttocks under the sitting posture, and people will inadvertently slide down toward the front end of the seat surface. In order to prevent the decline, the entire body is statically applied, which accelerates human fatigue.

The back of the sofa is tilted too large. At this time, humans showed a downward trend and had a poor sense of seating. In order to stop the decline, humans need to increase the strength of the foot. At the same time, due to the lack of proper support for the head and neck, when people sit on the sofa, it is like keeping the momentum of the horse for a long time. This gesture gives People's fatigue can be imagined.

The sofa seat is over curved. This makes people feel less stable after sitting, and because the seat surface is not flat enough, it will cause abnormal compression of the lower part of the human buttocks and thighs and the seat surface, and it will give local people a sense of fatigue after a period of time.

The curve of the sofa curves too much inside. This is essentially the case where the angle between the sofa seat and the sofa back is too small. The unfavorable design of the figure makes the human sitting posture curled up. In this sitting posture, the human abdomen is squeezed, affecting the digestive system. At the same time, the shape of the human spine is severely deformed from a normal sigmoid shape to a concave shape, resulting in uneven distribution of human intervertebral disc pressure. In addition, the vertebrae are connected by their nearby muscles and tendons, and the positioning of the vertebrae is based on the force of the tendon. Once the spine deviates from the natural state, the tendon tissue that connects the vertebrae will be subjected to mutual pressure or pulling force, which will increase the muscle activity and lead to fatigue.

1.3 Sofa Design and Human Scale

The human scale is the most basic basis for determining the functional dimensions of furniture. The functional scale design of the sofa requires the use of existing human size data or human scale data obtained from human measurements of the user population as the design basis. In addition, since the sofa is a soft seat, the cushion effect causes its appearance size to be different from the size after the seat, so the principle and calculation formula for determining the functional size of the sofa is different from the normal seat.

1.3.1 Block height

The seat height is the distance from the highest point in front of the central axis of the seat to the ground. According to the low-body design, the lower limit of 5% is taken as the design basis to ensure that the heel of the shorter person can touch the ground.

Calculation formula: seat height = calf armpit height + foot height (heel height) + cushion sinking amount + appropriate margin

The high range of sofa seats is 420mm-500mm. The cushion with moderate hardness is generally 50-60mm, and the seat height of the sofa is approximately 420-430mm.

1.3.2 Seat depth Seat depth refers to the distance between the front edge of the seat surface and the vertical intersection line between the seat surface and the back surface. For sofas with backrests that directly support humans, the seat depth is designed for people of low stature, which means that the lower limit percentile value of 5% is used as the design benchmark to ensure that shorter people can effectively use the backrest.

Calculation formula: seat depth = seated thigh length - lumbar depression amount - knee gap + appropriate margin

The depth range of the sofa seat is 500mm-560mm. The seat depth of an ordinary sofa is generally 510mm-520mm. For a sofa that uses a lumbar pillow to support humans, the seat depth can be appropriately increased, typically 580mm-650mm.

In addition, the larger the backrest angle of the sofa, the greater the seat depth. For example, the seat of a highly casual sofa can be deepened so that the legs can be lifted and the person can assume a half-lying state. In addition, a separate foot pedal can be added to provide foot support. 1.3.3 seat width

Seat width refers to the front edge of the seat and the width of the seat surface inside the armrest. According to the design of the large size group, the 95% upper limit is taken as the design basis, so as to ensure that at least the vast majority of people have enough space to adjust their sitting posture.

Calculation formula: seat width (n seats) = shoulder width (*n) + activity allowance

The size calculated by the formula is the minimum reference size. According to the style of the sofa, the actual size can be increased. The seat width of the general unit sofa is generally 510mm-650mm; the seat width of the double seat sofa is generally 950mm-1150mm; the seat width of the three seat sofa is generally 1350mm-1650mm.

1.3.4 Backrest height

Backrest height refers to the vertical distance from the midpoint of the back surface to the intersection of the back surface and the seat surface. According to the design of the low-body group, the 5% lower limit value is taken as the design basis.

Depending on the style and function of the sofa, there are three types of sofa backs: low back, middle back, or high back. Among them, the low backrest provides the lumbar support, the height is not higher than the lower angle of the scapula of the sitting posture to ensure that the upper limb arm can be flexibly moved; the backrest provides the support of the waist and the back, and the height is not higher than the cervical spine point to ensure the head can be flexibly rotated ; High by providing the waist, back and head and neck three support.

Calculation formula: I. Low high = seated lower iliac angle - cushion sinking - appropriate margin

II. Middle height = seated cervical height - cushion sinking - appropriate margin

III. Height by Height = Height - Seat Depth - Appropriate Balance

The backrest height calculated by the formula is only an approximate reference value. According to the style of the sofa, this value can be adjusted appropriately, but it must be based on the premise of realizing the backrest's effective support function. The low height is generally 310mm-350mm, the medium height is generally 400mm-450mm, and the height of the high is generally 490mm-550mm. In addition, the heights of the center points of the three support areas of the back, waist, back, neck and head are approximately 150mm-180mm, 380mm-420mm and 450mm-480mm, respectively.

1.3.5 seat and backrest tilt angle

For a sofa, the seat surface tilt angle refers to the angle between the horizontal line and the horizontal line on the central axis of the seat surface. The backrest tilt refers to the point corresponding to the back end of the hip on the midline of the seat surface, and the tangent line of the convex arc surface of the waist, and the angle between the back surface and the horizontal line.

The tilt angle of the seat surface of a general sofa is about 5°, and the tilt angle of a seat surface of a high-rise leisure sofa can reach 10°-15°; the tilt angle of a backrest of a general sofa is about 105°-110°, and the backrest tilt angle of a high-backed sofa can be Achieves 125°-135°.

1.4 Sofa cushioning

The sofa is mainly composed of three parts: skeleton structure, inner filling structure and outsourcing structure. (As shown in Figures 7 and 8, the structure of the sofa) The cushioning of the sofa is determined by the joint action of the three structural parts. Take the sofa structure shown in Figure 6 as an example. Among them, the functional scale of the wooden frame, the slackness of the serpentine spring and the elastic band, the matching and modeling of the sponge, the flexibility of the outer wrapping material, and the tightness of the finished fabric, etc., all of the above factors determine the cushioning of the sofa.

Figure 8 sofa cushion structure composition

Subjective items for evaluating sofa cushions include touch, seating, and leaning. In the design practice, the amount of subsidence of the cushion can be determined through simple tests, and this data is used as the basic reference for the design of the cushion.

In terms of softness, the sofa should not be too soft, otherwise, humans have a sense of falling down and are not conducive to adjusting the sitting posture. Sofa seats, backrests, handrails and other support parts are different, there are differences in body surface feeling and pressure distribution, and thus the requirements for cushioning are also different. Generally, the backrest of the sofa should be softer than the seat cushion; the backrest head and neck support area is softer than the waist and back area; the contact area between the front side of the sofa seat and the knee socket and lower leg should be relatively soft.

2 Sofa Comfort Evaluation Method

Comfort is an abstract concept. We use the words “comfortable” and “uncomfortable” in everyday life to describe the comfort of a sofa. In the practice of sofa production, the technologist is also accustomed to relying on personal experience to describe the comfort factor of the sofa with terms such as "soft", "hard", and "seat can be felt." However, these abstract vocabularies do not really express our feelings clearly, and even cause differences in understanding due to individual differences.

In the scientific evaluation, we listed the comfort factors of the sofa, such as the hardness of the seat, the softness of the backrest, the resilience of the seat, the support of the backrest, and the feeling of sitting. The indicators (as shown in Table 1 : seven-grade evaluation table for hardness) quantitatively evaluate the subjective perception of each element, and finally statistically analyze the obtained data.

2.2 Objective Evaluation

2.2.1 Body pressure distribution evaluation method

When the human in the seat is in a stable posture, the pressure distribution of the human weight acting on the seat cushion and the backrest is called a body pressure distribution. Whether the distribution meets the requirements of human physiological comfort, especially the distribution of body pressure in the buttocks, is an important factor affecting the comfort of the seat. Therefore, we can evaluate the comfort of sofas by the distribution of body pressure.

A reasonable distribution of body pressure is not an even distribution. For the seat, the maximum pressure is located at the ischial node, and the pressure around it gradually expands and decreases until the back of the thigh and the back of the hip. For the backrest, its height is different, and the main stress location is also different. In general, the major pressure points located at the head, thoracic region, and lumbar region were the occipital bone, the scapula, and the 3rd and 4th lumbar vertebrae, respectively. On the backrest, the pressure on the main stress point is the greatest and decreases toward the surroundings.

In the body pressure test, we used the body pressure distribution instrument and the configured software to obtain the body pressure distribution map and related data on the sofa. In the end, the rationality of body pressure distribution was analyzed from two aspects of intuitive graphics and objective data, and used as a basis for evaluating the comfort of sofas.

2.2.2 Surface EMG Evaluation Method

Surface Electromyography (SEMG) technology is a physiological index measuring technology. It records and analyzes the bioelectric signals accompanying the central nervous system's innervation of muscle activity through surface electrodes, and thus analyzes neuromuscular function status and activity level. Make an evaluation. Surface electromyography has a strong objectivity, can overcome the problems in subjective psychological assessment, quantitative analysis of workload and muscle function status.

In the electromyography experiment, the device used was a surface electromyography. The leads of the surface electrodes are placed on the abdominal muscles of the monitored muscle group. The myoelectric signals are collected at a certain observation time, and the generated electromyogram (Figure 11) is converted into data by professional software and the scientific data is used. Methods The data were processed to analyze the muscle activity and fatigue of the tested muscles.

3. Summary

Ergonomics is one of the most important principles in sofa design. This article only briefly expounded on a few aspects of sofa design ergonomic content, but its content is far from the depth and breadth. For humans, sofa design ergonomics involves many aspects of human physiology and psychology; for sofa products, it involves shapes, structures, materials, functional dimensions, etc.; in addition, indoor functions, scales, and environmental factors People's use and psychological feelings also have requirements for sofa styles, fabrics, and dimensions.

We need to use scientific design theory, design methods and evaluation methods in the design of the sofa as a guide to design excellent sofa products to beautify our living and working environment, improve people’s behavior and improve people’s performance. The rest and work efficiency are to achieve the unity of man--sofa--indoor environment.