International Design Conference - Design 98

INTERNATIONAL DESIGN CONFERENCE - DESIGN ‘98

Dubrovnik, May 19 - 22, 1998

THE ERGONOMICS OF LYING AS A FUNCTION OF HEALTHY SLEEP

Ivica Grbac, Stjepan Tkalec, Silvana Prekrat

Keywords: lying ergonomics, furniture design, construction and materials, functionality of beds

Abstract: The major function of the lying furniture is to meet the needs of one's rest and sleep, and to fully regenerate the body energy by its correct positioning. This can be obtained only if the structural types of such furniture comply with the anthropometric measures of the user and other physical and mechanical properties in the interaction man - bed. The paper describes the results of research on bed ergonomics as the function of healthy sleep. Presented are the latest requirements considering the relation between the bed and the anthropometric and other requirements in use. The results show that the shape and dimensions of the lying furniture are determined by the structural shapes, materials and other factors such as the building standards for flats, furniture types, frame structure etc.

1. Introduction

The bed is a basic functional furniture item for lying which is used for rest, sleep etc. That part of the furniture is most frequently utilised, and can be found in bedrooms, hospitals, hotels and all the other places intended for rest and sleep.

There is a dual significance of the bed. The first aspect is anatomical and physiological, and presents a requirement that a bed supports the body in a correct position, which is also linked with a comfort. The other aspect is a neurological and psychological approach to the bed.

As the body takes a lying position, it becomes released of its own weight and the resting phase is more intensive. Each of the two aspects has sets requirements which can be met by the technical design of a product (bed), by its shape and dimensions. We can generally distinguish between three phases in the use of bed: a) rest (immovable), b) change of position, c) loading the lying surface.

The initial role of the lying furniture is to serve for men's rest and sleep and for a full regeneration of his strength. To meet these requirements the lying position must be supported in such way that the energy consumption is minimal Rumjancev, 1979. The fulfilment of these demands is possible only if this type of furniture has the appropriate dimensions in concordance with the anthropometric proportions. The bed must also fulfil other requirements in use such as the comfort of sleep, rest, physiological and hygienic demands etc.

To obtain the results in the analysis of beds functions it is necessary to transfer the medicinal requirements into a strict code of numerical values and measures Grbac, 1988. The basic requirements are divided into two groups:

a) anthropometric requirements

b) requirements in use (rest, sleep and other physiological and hygienic requirements).

2.Anthropometric requirements

The basis for anthropometric requirements is a specific constitution of the human body Grandjean 1975, Grbac 1988 and the measures of its parts. These parameters have been established by anthropometric investigations Prokopec 1972, not only through statistical analysis but also on the basis of minimal and maximal anthropometric measures of the human body. The relationship between the height and mass of the body is reflected on the increased bodily weight, principally at men. This factor influences the dimensioning of the objects of everyday use. The results of anthropometric investigations are the sublimation of the national results obtained during research in the institutes of a number of countries Grbac 1988, Prokopec 1972. Among other things these results prove that the imaginary "citizen of the world" would need next 100 years to grow for 10 cm. The trend seems to be even more pronounced in Croatia. In last 30 years the generations of recruited young Croats grew up for almost 5 cm, and the trend continues at approximate rate of 2 cm in a decade. If some changes do not happen at the turn of the millennium, the average 80-years old Croat shall be 178 cm high.

Various authors recommend different sizes of beds. The national standards also treat this issue in different ways. N. Wolanski (quoted by Grbac 1988) claims for example that the width of the bed must be such that it responds to the smallest length from the spine to the end of the stretched hand (single beds). S. Dworakowski and K. Žurawski (according to Grbac 1988) recommend that the measures are determined in a following way:

bed width = width in shoulders + 20 cm + 2 cm

bed length=human height + 15 cm

height of the bed surface = the length of the lower leg.

The experts from the VPKTIM Institute Grbac 1988 claim that the length and width of the bed are of a great importance for the normal exploitation in use. The norms for the furniture design developed in this institute determine that the maximal dimensions of the sleeping areas are 1860 x 900 mm. Grandjean (1978) recommends the minimal length of the bed of 1900 mm. The determination of the width of a particular bed must take into account the position which a man occupies when lying on side with his legs bent. For taller person it takes 700 mm; allowing additional 150 mm on each side it results in a total width of an area of body movements of 1000 mm. The results of research of other authors show that the width of a bed for adults ranges between 900 and 1000 mm; Grandjean (1978) recommends the width of 1000 mm, whereas most people find that comfortable beds have width of 1100 to 1200 mm.

It must be stated that the level of anthropometric research on Croatian population is very poor, and that even the measurements on recruits, although the most complete and most regular measurements so far, can only indicate the trends since they cover only half of the population. It is therefore obvious that an urging need exists for an investigation that should first record the present situation, and than continuously monitor the characteristics of the entire population. That could lead to the permanent insight into the growth characteristics, the development and physical properties of the people. Such data proved essential for the designers and technologists in the area of furniture design and development.

The shape and dimensions of the lying furniture is also influenced by other factors such as the norms for the residential building, type of furniture, technical design, material etc.

1.1. frontal reach

1.2depth of the body

1.3upper reach with both hands

1.4height of the body

1.5height of the eyes

1.6height of the shoulder

1.7height from the floor to the elbow

1.8height of the step

1.9height of the leaning arm

1.10bright of the shoulders

1.11width of the chest

Figure 1. Anthropometric dimensions of the human body which influence

the dimensions of the lying furniture

3. The research into the bed requirements as a function of healthy sleep

The climate which exists in bed is defined by the immediate surroundings of the sleeper and is mainly expressed in terms of the temperature and the humidity of the bed materials and the air in it. The best sleep with the necessary rest is obtained in a comfortable climatic conditions. The ideal temperature between the mattress and a blanket ranges between 31 and 35 °C. Higher or lower temperatures are much less acceptable Grbac 1988.

The optimal climate of the sleeping room is essential because the head of the sleeping person is always exposed to the climate of the surroundings. The temperature in the bedroom should ideally range between 14 and 18 °C which depends on the bed equipment and the sleeping clothing. People dream more intensively in cold than in dry and warm room. The circulation of the surrounding air has a positive effect on the sleeping person, but the bed should not be positioned beneath the window. The aeration of the room during sleep is of the utmost importance Atzinger and Werberstofer 1992, Becker 1983, Grbac 1988.

The relative humidity of the air in a bedroom should amount to 50 % in winter and 60 % in summer. The higher humidity causes the sleeper to feel damp and this negatively influences the sleep. The lower humidity values cause the nasal secretion, irritation of the throat and respiratory system and impaired function of lungs. That implies that the humidity which is lower than optimal also influences the sleeper and his sleep.

Most of our bedrooms do not have ideal humidity and normal evaporation (without heating) is usually insufficient to maintain the desired level of air humidity. The problem can be solved by the use of electric humidifiers, and this is always recommended for the bedroom.

The support to the body by the bed base must not disturb normal bodily functions, on contrary - it must ease them. The stiff muscles must relax, as well as cramps, ant the brain must be unthinking. The sleep regenerates the human organism. Therefore the bedroom must conform to modern positive standards.

Taking a right position during the day can disburden the spine. The man during the day loads and burdens the intervertebral discs, and the spine can be shortened for as much as 3 cm. That reduction in length is compensated during the sleep on an appropriate bed. Loss of bodily fluids also influences the volume of the discs, and the liquid must be regained during the night. Therefore it is crucial to realise that the bed which does not adjust to the natural shape of the spine is not appropriate and can not assure the rest and reinforcement of the body Grbac 1988.

The bed base must be adjustable in order that it enables the movements of the sleeper. It must be even, it must have an ability to vibrate and not to creep under the permanent load. Together with the mattress it must follow the movements of the body and protect it from deformations. Traditional spiral springs could not meet this requirement in the best possible way. The best solutions are offered by the modern elastic and flexible bed slat bases Grbac 1988, Grbac 1991.

The construction of the bed must be based on the fact that the person does not sleep still. Therefore the recommended size of the bed should be 100 x 200 cm. Smaller beds (90 x 190 cm) are accepted only for smaller flats and for the people shorter than 175 cm, because it is clear that a person bent like a worm can not sleep well and healthy.

Bearing all that in mind the question is being raised about the optimal size of the bed. The experience tells that the good and comfortable bed is longer than its user for at least 20 cm. According to that modern beds are manufactured in a range of lengths between 200 and 240 cm.

The position of the body is changed 20 to 60 times during the nights sleep Grbac 1988, hence the bed width plays an important role for a good sleep. Too narrow beds reduce the freedom of movements. The recommended width of a bed for a person is 95 cm on single beds and 90 cm on double beds.

The height of a bed should not be neglected. It determines whether the getting up, lying down or making a bed is comfortable and relaxed. Therefore the recommended height of the upper rim of a bed is 45 5o 55 cm. The optimal height of a bed for its making and arrangement (e.g. in hospitals) is 65 to 80 cm. It is also important that the bed frame is noiseless in use Grbac 1988.

The mattress must have an elastic and quality core which supports the back and ensures the natural position of the body during sleep. It must be so designed that its pliability is greater in the places of the greater load i.e. on the positions of the shoulders and hips, and that its deformations are smaller in the places when the feet or the arms are laid. This pliability, however, must not be excessive and exerted by even small movements. After disburdening the mattress must bounce back into the initial position. The most important property is the uniform distribution of the load of the body over the entire mattress regardless of its softness or hardness. The mattress which is adjusted to the body must be made in one piece with various levels of hardness which are determined according to the habits, age and usual positions of the sleeper. The mattress must also be soft so that the load is evenly distributed depending on the weight of the person. If not so, the circulatory system can be affected, although the movements of the body and the motorics of the sleep egalize uneven compression. Sleeping on a too hard mattress causes the body to move too often and the consequence is a nervous and light sleep Burfeind 1993, Grbac 1988.

The mattress and its base must have a good thermal insulation, and hollow and transpirant mattresses are advantageous. It is important that the bed transmits the adsorbed moisture, i.e. that the vapour that has been taken up from the sleeper's body during the night is being released during the day. That is also one of the conditions for the establishment of the healthy sleeping climate. In order to ensure a healthy sleep it is also important to equip the bed with an adequate pillow and blanket.

The best lying comfort is achieved when the human body is evenly supported on the bed and the physiological shape of the spine is natural.

German physician Wilhelm Ackermann established during his five years research the results which are important for the proper position of the spine in a lying position Becker 1983, Grbac 1988,. A segment of the results is shown on figure 2 a - f.

Figure 2a shows the position of a spine on a too soft bed. The heaviest part of the body sinks into the bed, and the pelvis lays too deep and causes the spinal column to be unnaturally bent, which causes the back pain.

Figure 2b shows the side position of a body on the same type of bed. It is obvious that the body weight is unevenly distributed, and the too low position of a pelvis deforms the spinal column and causes pains and problems of circulation.

Figure 2c presents too hard bed which causes tension because the back can not be supported and impressed. This causes the nervous pains and the feeling of shattering.

Figure 2d presents the side position on the same hard bed. It can be seen that the hip and the shoulder are not impressed enough. This changes the form of a back, and therefore the position of a spinal column. Shoulders and arms become numb, the occiput aches and an rotated pelvis causes further damage.

So-called "Ackermann bed" is presented on figures 2 e - f. Figure 2e presents a lying position on the back. The shape of the back is uniformly supported, so that the spinal column and the pelvis are in anatomically correct position.

Figure 2f shows a side lying on an "Ackermann" bed. The shoulder and pelvis are so impressed that the pelvis and a spine rest in their natural form.

Intensive research is performed today to obtain a bed which can adjust to the human body in a best possible way. We have at disposal pocket-spring beds, lattoflex beds, latex mattresses, water beds, air beds, double-spring beds etc. Many papers describe the effect of the particular type of bed on the human rest and the results of research in this field Atzinger 1992, Biener and Jäggi 1982, Grbac 1988, Grbac 1991.

Figure 2: a - d: Position of a spine on various types of beds

e - f: "Ackermann" bed

5. Conclusion

New understandings about the quality of sleep make new demands upon the bed designers and manufacturers. The market of beds and mattresses cover a large number of producers and each of them has his own strategy in the promotion of his products. The production of beds in near future shall be based on the individual selection of the bode for particular person and his characteristics. The results of research show that the be must be designed so that it fulfils the following:

- it must have a good aesthetic value

- its dimensions must be appropriate to the user

- the body during sleep must always be in anatomically correct position; the bed must support the body in natural position and assure the correct shape of the spine and other parts of the body

- the lying surface must be evenly compressed over the entire area of the body contact

- supply of fresh air must be undisturbed

- it must conduct the heat and moisture

- it must be adjustable for users of various weights, especially for those with spinal problems

- it must fulfil all the hygienic and health requirements.

New methods for determination of the optimal bed design and new findings about the interaction of the bed properties and healthy sleep are described in this paper and they indicate the need of the interdisciplinary approach to the investigation of the complex problems of bed and sleep.

The results of research indicate that the shape and size of the bed can be influenced by the structure of the construction, type of material used and other factors such as the new standards for residential building, type of furniture, technical design etc.