3.4.6.1 Dimensional Requirements
Sleep and rest are an indispensable condition for the proper course of all life processes. While sleeping, the muscles and the skeletal system relax, as does the nervous system and many other processes relevant from the point of view of physiology. The proper conduct of all these processes is primarily dependent on comfortable, healthy sleep and therefore on individually suited, proper construction of the furniture intended for lying down, providing comfort of the bed in a place that gives a sense of security (Grbac 1985a, b, 1988; Grbac and Dalbelo-Basic 1996; Grbac et al. 2000).
As opposed to furniture intended for sitting, furniture for lying down is characterised by much simpler anthropometric determinants expressed by dimensional dependencies (Fig. 3.50). This takes into account mainly the height of the person and a specific movable space, and when designing, the spring elements must be chosen in such a way that muscle stresses are as small as possible (Kapica 1991, 1993a, b, c, d, 1988, Kapica et al. 1999).
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Fig. 3.45 Dimensions of a chair-table set including frontal hold reach on the worktop surface when the user is bent and space for the free movement of the lower limbs bent in the knee joint |
The bed is designed on the basis of the shape of a rectangular cushion of minimum dimensions:
• for one person: 1900 x 800 mm,
• for two persons: 1900 x 1200 mm, (PN-89/F-06027.04).
The minimum dimensions of length and width of the mattress should take into account the measures of people belonging to the population of the extreme upper position of statistical distribution of selected dimensions, resulting from anthropometric measurements (Smardzewski and Matwiej 2004). Furthermore, when determining the width, the place should be estimated which is taken up by a person lying down on the side with folded legs in a position when the thighs form an angle of 90° with the trunk (Kapica 1993a). Grbac (2006) suggests that the length of the bed was greater by at least 200 mm from the height of the user, while the width amounted to 1400 mm (for a single bed).
During sleep or rest, the Human body exerts pressure on the base and vice versa. Improper construction of mattress may cause: insomnia, sore parts of the body and/or the skeletal system, and even curvature of the spine. The largest loads occur
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Fig. 3.46 Ergonomic evaluation of a chair and oval table in the operational position S4 of a male of 50C (dimensions in mm) |
in the hip area and the shoulder joints, which require from the construction of the mattress the largest area in contact with the body and the appropriate degree of deformation. The forces caused by the weight of the body should be spread over on the largest possible surface (Karpinski and Deszczynski 1997).
Mattresses of a simple construction and inappropriate filling do not always adjust to every body shape, providing support only to its convex parts. This part of the furniture, on which one statistically spends 1/3 of the day, should be appropriately adjusted to the user’s body weight. While lying on one’s back, the body’s weight is transferred mostly to the area of the shoulder blades and buttocks, while the lumbar region which does not have support descends downwards (Fig. 3.51).
In Fig. 3.51, loads of the human’s body weight have been marked by vectors, as well as the forces of the elasticity of the mattress balancing them. As it can be seen, some vectors are not coaxial, which indicates the presence of shear forces and compressive and bending forces, affecting the human skeletal system, nervous system and muscular system.
A similar situation occurs in the position of the body on the side (Fig. 3.52). Above all, the shoulder joint and hip joint are burdened here. The lumbar area has no support, which results in its lowering under the influence of body weight.
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Table 3.15 Dimensional characteristics of the cross section of an armchair intended for use by healthy people and people with spinal disorders (Grandiean 1973)
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Resting on a mattress that is too soft, which under the influence of body weight adopts a basin form, may cause deformation of the spine caused by hours of unfavourable concentration of stresses, within its range, intensifying reflexive stress of the muscles surrounding it. In addition, the inadequate support of the cervical
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Table 3.16 Angular dependencies for armchairs and sofas at non-production activities (Slavikova 1988)
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Fig. 3.48 Errors in adjusting the human-seat system: a seat too high, b seat too low, c seat too deep (Nowak 1993) |
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Fig. 3.49 The impact of changes in the inclination and height of lifting the back when sitting on changes in pressure in the intervertebral disc (own development based on Nachemson 1976) |
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Fig. 3.50 The basic functional dimensions of beds and unfolding chairs: a single bed, b double bed (cm) |
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Fig. 3.51 The impact of the mattress while lying on the back |
spine is conducive to disorders of blood flow in the arteries, with all the pains, ailments, migraines and sleep disorders accompanying it.
All the previously presented effects of the sleeping space on a person in a lying down position are intensified by a very hard bed (Fig. 3.53).
Sleep on a hard base is very harmful, especially more uncomfortable in relation to sleeping on a mattress that is too soft. The influence of the base is more noticeable on the convex parts of the body, since they take on the entire load, while elastic forces of the base do not affect a sleeping person. A properly selected mattress, adjusted, variable elasticity of its surface ensures proper load distribution on the various parts of the body, reducing the harmful stresses arising around the spine and joints (Figs. 3.54 and 3.55).
The proper formation of the mattress enables to support the lumbar region correctly when lying on the back and on the side. This prevents this part of the body from “sliding down”, thanks to which the physiological curvatures of the spine are maintained (Karpinski and Deszczynski 1997).
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Fig. 3.52 The impact of a mattress that is too soft while lying on the side |
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Fig. 3.53 The impact of a mattress that is too hard while lying on the side |
Stresses occurring with lying down still for more than 2-3 h cause irreversible changes. Excessive point stresses caused by lying down on an inappropriate mattress can suppress and even stop the flow of blood in the skin above bone protuberances which leads to a lack of oxygen and nutrients supply of cells, and as a result, the death of the anoxemic cell. According to Krutul (2004b) in places where the bones push onto tissues harder, pressure increases and the lumen of blood vessels is reduced, which leads to the damage of skin tissue. The stresses of the external surface are 3-5 times smaller than internal forces that occur as a result. Therefore, the limit value of the pressure amounting to 32 mmHg (closing the lumen of capillaries) must be appropriately reduced and to the level from 6.4 to 10.6 mmHg. Each pressure greater than these values may result in closing the light of veins, then of arteries, which slows down the flow of blood or stops its circulation, causing local ischaemia and—in time—necrosis.
When designing furniture for lying down, a number of anthropometric and physiological rules should be taken into account, resulting from the conditions of their use.
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Fig. 3.56 Construction of an upholstered armchair: a furniture, b backrest, c seat |
In the case of sofas and armchairs, the seat and backrest must be designed
appropriately, which differ both in terms of functional and structural requirements (Fig. 3.56).
Both the seat and the backrest in furniture for sitting should have a specified and varying degree of softness adapted to the size of the load attributed to a given part. The load caused by the weight of the user is distributed unevenly on the backrest and the seat of the furniture piece and on particular surfaces of these parts (Sienicki 1954; Smardzewski 1993a, b, 2002c; Smardzewski and Grbac 1998).
3.4.6.2 Quality of Sleep
Sleep, according to the definition provided by PWN Encyclopaedia (2004), is the functional state of the nervous system, opposed to the state of being awake, which essence is the temporary loss of consciousness. Both in the human and higher animals, it occurs in a daily rhythm, alternating with being awake; it is regulated by the brain centres of sleep and inducing centres. Sleep is essential for the regeneration of the organism and the central nervous system.
Questions about the essence of sleep, which absorbs about 1/3 of human life (Pietruczuk et al. 2003), have appeared since the dawn of time: approximately 2400 years ago, Hippocrates advised to sleep at night and to be active during the day, and he claimed that sleep is caused by the inhibition of blood flow and its direction to the internal organs (Nowicki 2002). The scientific analysis of sleep began to gain interest at the beginning of the twentieth century, when Berger applied an encephalographic study, providing objective criteria of brain function when sleeping, and Legendre and Pieron put forth the hypothesis that sleep is the result of increased secretion of substances called hypnotoxin during the state of being awake (Nowicki 2002). In 1937, Harvey, Loomis and Hobart made divisions of the depth of sleep, and Aserinsky and Kleitman in (1953) discovered the diversity of sleep in two separate states: NREM sleep (non-rapid eye movement) and REM sleep (rapid eye movement) (Nowicki 2002).
Somnology—a new discipline of science concerning the issues of sleep, associated with all of its aspects, is increasingly important also in designing ergonomic furniture products intended for sleeping. It provides a wealth of information on both the essence of sleep, as well as the risks resulting from disturbances of its course.
Users sleeping or trying to fall sleep on improperly designed beds often complain about morning lack of sleep, backache, headache and migraines; they are irritated and feel fatigue. In extreme cases, this can lead to insomnia. Sleep begins with NREM which lasts on average 80-100 min, followed by REM which lasts approximately 15 min (this is the phase in which dreams appear and the muscles totally relax). In adults, this cycle repeats itself during sleep 4-5 times (Fig. 3.57). Along with the length of sleep, the deepest phase NREM decreases, and the duration of the REM phase increases, which near the end of sleep usually lasts approximately 40 min. A dream is remembered when one wakes in this phase. Depriving people who are sleeping of going into the REM phase can entail a particularly high risk to health (especially mental). After a few days of not sleeping well, one may have delusions or collapse into a state of psychosis. Such a risk may result directly from a poorly designed, uncomfortable construction of the bed. In addition, sleep disorders are classified as a state of stress, and this can disrupt mutual communication between two systems: the immune system and the nervous system (Pietruczuk et al. 2003), which in turn may have an impact on overall health, decrease of immunity, etc.
Studies conducted on representatives of the Polish population show that the length of sleep at night is approximately 7.5 h (Pracka et al. 2003). Sleep function disorders and diseases associated with insomnia entail serious economic
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Fig. 3.57 A graphic representation of the ideal course of an 8-h sleep of an adult (Nowicki 2002) |
consequences. This is confirmed by American studies, which demonstrate that in 1990, in the USA, the costs of hospitalisation associated with sleep disorders amounted to 16 milliard USD (Grabowski and Jakitowicz 2006). Women complain about insomnia much more often, and the quality of sleep deteriorates with age (Nowicki 2002). In the elderly, it is also found that the NREM phase shortens and REM phase of sleep gets longer (Pietruczuk et al. 2003). According to Polish data, as many as approx. 24 % of Poles suffer from insomnia (Kiejna et al. 2003).
Furniture designers should, therefore, take into account the above arguments and focus not only on the form, but also on functional and structural solutions (Smardzewski et al. 2008). The mattress, which constitutes an assemblage, subassemblage or finished upholstered product designed for sleeping and/or resting, consists of many layers, and these can be made of materials which differ not only in shape and dimensions, but particularly in the physico-mechanical properties. Therefore, specifying the technical characteristics of mattresses, the quality of sleep should be examined, and the parameters specified, which have an impact on it, as well as take into account the requirements concerning the geometry of the product and characteristics of the materials.
