MODELLING OF ERGONOMIC CONTRAUTIONS OF MATTRESSES IN UPHOLSTERY FURNITURE

Łukasz Matwiej¹, Medard Senski^2
1 Department of Furniture Design, The August Cieszkowski Agricultural University of Poznan, Poland
² Natural Chemical Products Ltd., Bydgoszcz, Poland

ABSTRACT

Healthy and comfortable sleep is an essential factor helping protect health and maintain high human physical efficiency. That is why mattress designers and manufactures should reach out for tools based on the latest technologies allowing appropriate design of furniture constructions and ensure their best ergonomics. The system ‘Force Sensitive Applications’ presented in this article makes it possible to design more human-oriented mattresses characterized by better properties than those designed using traditional tools without possibilities of a dynamic and easily accessible verification of the correctness of the employed materials in upholstered systems.

Key words: ergonomics, furniture mattresses, sensory mat, upholstered systems.

INTRODUCTION

Since times immemorial, together with successive generations and changing techniques, in an effort to improve the comfort of our sleep, never-ending attempts have been observed to improve the construction and the appearance of beds. The greatest progress in the design of the part which exerts a direct influence on the lying person – the mattress – took place during the Middle Ages [1]. First mattresses took the form of large pillows which were filled with grass, horsehair, down or feathers. The type of the filling depended frequently on the geographic location, availability of materials and – first and foremost – the need for comfort. Despite the fact that with the passage of time the original fillings were replaced by more elastic materials and the traditional materials constituted only minor additives, the majority of mattresses continue to be designed on the basis of intuitive selection of both materials (various kinds of springs, foams and mats) as well as shapes of the bed. This leads to many disagreements and inconsistencies with regulations and expectations of various users. The main cause of this lack of knowledge can be attributed to differences in expert opinions in the field of orthopaedics associated with the sources of ailments caused by sleep as well as contradictory data about mechanical properties of the human body and about acceptable pressures that can be exerted on this body [4, 6]. Both designers and manufacturers of mattresses should always remember that healthy and comfortable sleep is an indispensable requirement protecting out health and maintaining out best physical condition. However, a complete and maximal relief and relaxation of muscles occurs only when optimal adjustment of individual upholstery layers to the shape of the user’s figure is possible and the natural position of the line of the spinal column can be assured (Fig.1). The reclining position ensures the lowest energy consumption and, therefore, a complete regeneration of both physical and psychological human resources [2].

Recent scientific and technical advances open up totally new possibilities and have already introduced new tools based on digital technology which allow the proper design of furniture constructions combined with the appropriate attention to their ergonomics. A good example of this type of tools which allow measuring various pressures in the furniture intended for sleep is a sensor mat marketed by the ‘Force Sensitive Applications Company’. The system utilizing the piezo-resistance technology is designed to perform detailed analyses of the interactions between the human body and a given surface of support. Hence, it provides information about the impact exerted on the human body by the type of a given surface and the position that the body adopts while resting on it. For over 10 years now, the above-mentioned new tool developed by the Force Sensitive Applications Company has been supplying rehabilitants, motor-car engineers and designers, riding amateurs etc. with priceless information contributing, among others, to:

• the improvement of the health conditions of users,

• introduction onto the market of a new generation of car seats,

• improvement of achievements of both riders and their horses.

The above system opens up entirely new possibilities for mattress designers allowing them to develop new, improved, highly ergonomic (hence healthier) products which will better meet human needs and expectations.

Description of the system’s operation

The principle of the operation consists in the reading of resistance changes depending on the pressure exerted on the examined surface. The Force Sensitive Applications System employs a patented, piezo–resistant semiconductor polymer placed between two layers of tear-resistant nylon fabric of high conductivity. The set is placed in a special protective, polyurethane cover which, in turn, is closed in a flexible covering made of Lycra. In order to guarantee its ideal fitting to the frequently curvilinear surfaces of furniture, the layers can move freely which, when combined with the flexible covering, prevents the mat from dangling. Changes in resistance resulting from the differences in pressures exerted on the semiconductors are processed by the interface module and fed into the computer (Fig. 2). The effect of pressure measurements is displayed on the monitor screen in the form of a multi-coloured map portraying the distribution of pressure values and/or in the form of a 3D diagram (Fig. 3). Investigations carried out with the assistance of this system consists in positioning (either in a lying or sitting position) the given person on the sensor mat placed on the examined foundation and switching on the appropriate software which will record values and distribution of pressures developing between the body and the foundation (mattress). The obtained information is recorded continuously and dynamically which allows to observe various interactions taking place between the body and the mattress on which it rests, e.g. during changes of the lying position.

Comparative Investigations of Mattresses with the Assıstance of the Force Sensitive Applications System

Within the framework of cooperation with the Natural Chemical Products Company headquartered in Bydgoszcz which first purchased the Force Sensitive Applications system, and the Chair of Furniture Design at the Wood Technology Faculty of the Poznań Agricultural University carried out a series of comparative experiments of mattresses with the sleeping function. The following two mattress models were examined:

• One-sided spring-foam mattress 11 cm thick (bonell spring unit with a framing, padding fabric 500 g/m², polyurethane foam of high density and hardness 2 cm thick, quilted covering made of cotton mixture),

• One-sided foam mattress 15 cm thick (top layer – 7 cm thick: Viscoelastic foam, bottom layer – 8 cm thick: polyurethane foam, covering made of cotton mixture).

During experiments, the examined mattresses were loaded with a female body of 70 kg weight (mean weight of females in Poland – 66 kg). The expediency of the choice of the woman for the performed investigations was confirmed by the fact that differences in the construction of the body of men and women (dimorphic differences) estimated at 8% allow (in the case of experiments with mattresses) to achieve a more complex state of loads. The main differences in the construction of the female body important from the point of view of ergonomics include:

• height by about 8 cm shorter than in men (in Poland – even up to 17 cm),

• narrower shoulders,

• broader hips,

• centre of gravity of the body placed slightly lower.

The experimental mattresses were examined on a special station in three stages, namely, position ‘on the back’ (Fig. 4), ‘on the side’ and ‘on the abdomen’. Each time, the measurement of pressures lasted about 10 min.

RESULTS

In the case of the spring-foam mattress, the following maximum values of pressures were recorded in the position ‘on the back’:

• In the neighbourhood of hips – 91 mmHg,

• On the right shoulder blade bone – 84 mmHg,

• On the left heel – 78 mmHg.

The picture of pressure values is not distributed uniformly on the entire body and this exerts an indirect influence on the value of the recorded pressures (Fig. 5).

The investigations of the mattress with the body position ‘on the side’ provided some alarming information; the maximum pressure in the vicinity of the hip bone reached the limiting value of 100 mmHg and in the neighbourhood of the elbow joint – 91 mmHg (Fig. 6).

Similar values were recorded during the experiments carried out in the body position ‘on the abdomen’. However, in this case, the highest pressures occurred in the neighbourhood of knee joints (100 mmHg) and chest (61 mmHg) which, in the case of the female body structure, poses a serious health risk (Fig. 7).

It was expected that the experiments on the foam mattress with the top layer made of the Viscoelastic foam (thermo-elastic) would yield more favourable results in comparison with the spring-foam mattress and these expectations were confirmed. In the position ‘on the back’, the following maximum pressures were recorded:

• In the neighbourhood of hips – 58 mmHg,

• On the right shoulder blade bone – 45 mmHg

It is evident from Figure 8 that the pressure distribution is considerably more favourable – the majority of the body contact with the mattress surface is covered with contour lines with pressures ranging from 20 – 30 mmHg.

Also in the case of the body position ‘on the side’, a considerable improvement in the registered pressure values was registered with the maximum pressure recorded in the neighbourhood of the hip joint: 51 mmHg (Fig. 9).

The most significant differences in relation to the spring-foam mattress were observed during experiments with the foam mattress in the position ‘on the abdomen’. During the 10 min measurements, the Viscoelastic foam allowed the body to ‘sink into’ the mattress and, therefore, a greater body surface remained in contact with the foundation and the unit pressures were smaller. This is excellently illustrated in Figure 10.

It is easily noticed that the maximum recorded pressure recorded in the area of the chest was 35 mmHg. It is also worth noting that the software always registers the number of active sensors; in the case of the foam mattress in the position ‘on the abdomen’ 389 of the total number of 1024 sensors were active, while in the case of the spring-foam mattress, this number was 370. The greater number of active sensors indicates a greater surface of contact between the body and the ‘support’, hence a better, from the point of view of ergonomics, distribution of the force.

It is also worth comparing the two examined mattresses with regard to the occurring pressures (in the position ‘on the abdomen’) as shown by the 3D diagrams (Fig. 11).

It is evident from Figure 11 that the spring-foam mattress is not the best ergonomic choice. It poses a number of risks resulting from the fact that the acceptable pressures on the human body are exceeded by several time. The incorrect construction of the bed as well as the concentration of the human mass forces at the protruding parts of the figure may lead to sleeplessness resulting from the lack of comfort and appearance of various complaints. Pressures lasting for more than 2-3 h may lead to irreversible changes; increasing local pressures may block free blood passage in veins and vessels preventing the supply of oxygen and nutrients to cells and causing their hypoxia and death. The value of this constriction is the resultant of the body weight and the surface of its contact with the bed. Analyses revealed that the limiting value of the constriction which can cause a complete closing of the passage of capillaries (tiny blood vessels forming a network between veins and aortas) is 32 mmHg. Because the pressure of the external surface is 3 to 5 times smaller than the internal forces which it effects, therefore the value of the pressure must be reduced appropriately and should range from 6.4 to 10.6 mmHg [4].

CONCLUSIONS

It is evident from the performed investigations that the above, reduced value (6.4 to 10.6 mmHg) is very difficult to obtain even using the latest upholstery materials. However, ergonomic principles and constraints make it necessary for the mattress construction to provide the largest possible area of contact with the body as well as the appropriate degree of deformation. Forces produced by the body weight should be spread over the largest possible area [3]. Summing up, it is necessary to emphasize the comparative nature of the performed investigations. The obtained results are not unequivocal as they were influenced by too many factors, e.g. time of experiments, temperature of environment. Nevertheless, the experiments proved that the Force Sensitive Applications system is a good tool allowing to design such upholstery systems which can – provided the appropriate materials are used – eliminate long and static pressures caused by unfavourable distribution of loads of individual regions of the body in the lying position. Because of the utilization of the examined system, designers and manufacturers will be able to seek optimal solutions for the produced mattresses, both on the level of their construction as well as modification of mattresses themselves (e.g. chemical composition of base foams). Therefore, it can be said that the Force Sensitive Applications system allows designing mattresses with much better properties than those designed in a traditional way without possibilities of dynamic, continuous and clear verification of the appropriateness of materials used in upholstery systems.

REFERENCES

1. Bacia K., Witkowski B., 1971. Technologia Tapicerstwo [Technology of upholstery]. WSiP, Warsaw [in Polish].

2. Grandjean E., 1978. Ergonomia mieszkania [Flat ergonomie]. Arkady, Warsaw [in Polish].

3. Karpiński J., Deszczyński J., 1997. Aspekty biomechaniczne konstrukcji nowoczesnego materaca ortopedycznego [Biomechanical aspects of construction of new orthopedic mattresses]. Mag. Med., 7, 50-52 [in Polish].

4. Krutul R., 2004. Odleżyny. Pielęgnacja w warunkach szpitalnych [Bedsores. Maintenance in hospital environment]. Ogólnopol. Przegl. Med., 4, 38-42 [in Polish].

5. Smardzewski J., Matwiej Ł., 2005. Antropo-technical models in testing of upholstered furniture. Int. Conf. Calunnicke Dni, Zvolen, 18-26.

6. Sredina R., 1980. Dobra postelja za pravi odmor [Good bed makes perfect relax]. Ind. Oblik. Market, Belgrad, 55 [in Croatians].

Accepted for print: 05.07.2006

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.