Study on new three-hole ribs bedframe to evaluate sleeping position comfort

Kai-tuo ZHANG, Qing TAO, Jin-sheng KANG, Zhao-bo LI, Shou-dong WANG

(1Department of Mechanical Engineering, Xinjiang University, Urumqi 830047, China)(2Department of Engineering Design and Physical Sciences, Brunel University, London UB83PH, UK)

Abstract: To study the sleeping comfort bed, a new three-hole ribs bedframe and the stress and strain model was proposed. The combination of subjective and objective experiments is used to evaluate sleeping position comfort. The result shows all of the mechanical properties satisfy the technical requirement. The pressure distribution measurement system of human body was used to make a scientific comparative analysis of the three-hole ribs bed and the ordinary plate bed. The data in two sleeping positions were deeply analyzed by MATLAB. Meanwhile，a subjective comfort questionnaire was conducted to statistically analyze the subjective feeling of volunteers. It was found that the maximum pressure of the three-hole ribs bed reduced by 19.6%, the average pressure decreased by 2.4%, the contact area enlarged by 5.3%, and the total pressure increased by 3.3%. The distribution curve of longitudinal pressure and contact area of human body was relatively reasonable. It was shown that the contact area of bone protrusions such as the back, hips and feet increase obviously, and the local pressure decreased greatly. The waist and legs were provided with sufficient support. At the same time, the subjective comfort evaluation level was improved by two levels. The results showed the three-hole ribs bed greatly reduces the local pressure and increases the contact area of the whole body. By dispersing the local pressure, it improved the comfort of the local muscles and corrects the spinal alignment to decrease the compressive sense of the bone protrusion. The results of subjective and objective evaluation experiment were consistent, which verify the sleep comfort of the three-hole ribs bedframe, and provided a new method for the comfort analysis of bed design in the future.

Key words: Three-hole ribs bedframe, Pressure distribution, Subjective evaluation, Sleeping comfort

1 Introduction

Sufficient sleep is essential for physical health, optimal physical and cognitive function of human body. On the contrary, lack of sleep not only hinders people’s daily life and overall well-being, but also endangers personal and public health[1]. Sleep quality has an important impact on people’s social interactions, mood, quality of life and work efficiency. Sleeping comfort is an important aspect of sleep quality[2]. Nowadays, the consumers become more health-conscious, the competitiveness of manufacturing depends on humanized products. Ergonomic factors are playing an increasingly important role in the sleeping position comfort[3]. Moreover, it is gradually found that poor design can lead to backache or general sleep disorders, which will lead to abnormal deformation of muscle structures or even blood circulation obstruction and excessive compression of nerve tissue[4]. It seriously affects the sleep quality and physical health of sleepers. Adhering to the people-oriented and full considering the human-bed support surface comfort have become an important content in bed design[5]. Therefore, it is crucial to study the sleeping comfort bed.

It would produce a comfortable or uncomfortable physiological response when the human body was stimulated, which is difficult to test objectively. Therefore, researchers had conducted some great studies on the determination method of comfort or discomfort. Fan-zhe LOW contrasted the human body contact pressure curve of latex and polyurethane mattresses, and found that latex mattresses could reduce the peak body pressure and evenly distribute the pressure. It could give users better sleep comfort[6]. Alessandro Naddeo studied the effect of spine posture on the perception of postural comfort or discomfort and proposed a method to balance the influence of spinal discomfort on overall posture comfort[7]. Qing TAO used vibration simulation and digital human body modeling to establish a sitting comfort evaluation method with various anthropometric parameters[8]. Yu-xia CHEN analyzed the factors affecting mattress comfort and sleep quality from the perspective of human-bed interface pressure distribution[9]. Zhao-bo LI established a human upper limb exercise comfort index evaluation model through in-depth analysis of kinematic data, and verified that the backward movement of the book in the sitting position is a comfortable category[10]. Guang-bin WANG studied the influence of various parts of the car seat on the pressure distribution of the human body through the simulation of sitting pressure distribution, and proposed the category of seat comfort[11].

Sleeping comfort bed is an important factor that affects the quality of human sleep. Common board beds and row skeleton beds in China are characterized by excessive or insufficient support. It is easy to cause local excessive pressure of human body and insufficient support of body spine, which will lead to impaired blood circulation or serious abnormal deformation. People have tried to study the comfort of various mattresses, but it is difficult to adapt to more people[12]. Accordingly, how to design a more comfort bed is urgently needed to solve. This paper designs a three-hole ribs bedframe based on ergonomic, which use its unique support structure to achieve accurate response and the most effective support for the human body. No matter what the sleeping posture is, the body is always in a balanced state, so as to maintain the most comfortable sleeping state.

2 Three-hole ribs bedframe and its supporting structure

The preliminary experimental study found that the ordinary board bed mainly supports the human body in the point field of the bone protrusions. Such as back, buttocks, feet, which not only causes the excessive partial pressure, but also causes the waist, legs, ankles in the suspended state, and the spine is unnaturally deformed. It is easy to cause backache and abnormal deformation of muscles, which can cause great harm to human body.

To improve the existing ordinary board bed shortcomings of excessive or insufficient support by studying the sleeping comfort bed, a unique three-hole ribs bedframe support structure is designed.(see in Fig.1)[13-14], which is divided into three parts: a three-hole limit sleeve, a rotating shaft and an arched base. The three-hole limit sleeve and rotating shaft are made of PA (nylon) material with strong absorption capacity for impact and stress vibration. The arch-shaped base is formed by high-strength rubber polyurethane heating and melting mold. The limit sleeve is designed into a separate three-hole structure, and filled with polystyrene foam to provide a buffer effect for separate deformation of the ribs. The size of single rib is 92 cm long, 3 cm wide and 0.7 cm high, and its material is made of beech wood with strong elastic deformation and bending forming ability. The base adopts an arched support structure, which can offset most of the force transferred downward by the rotating shaft and reduce the radial deformation of the connecting shaft between the base and the bedframe. The middle rotating shaft is hinged to ensure that the ribs can rotate at a small angle following the limit sleeve. The support structure of the three-hole ribs is fixedly connected with the bedframe by the snap type, which is convenient for disassembly and stable support.

Fig.1 Three-hole ribs bedframe support structure

The three-hole ribs bedframe system like bow. It consists of three ribs connecting the two ends of the three-hole ribs bed support structure (see in Fig. 2). Its main innovations are that each rib can rotate individually at a small angle and deform slightly. Also, the bow ribs bedframe system can rotate freely in two directions around the rotation axis following the limit sleeve. The arched base gives the arched ribs bedframe system a strong elastic deformation capabilities and support functions. Based on this, the bow ribs bedframe system is given the ability of rotating at a small angle and elastic deformation along with the skeletal curve when it is subjected to pressure from the human limbs or bones.

Fig.2 Bow ribs bedframe system function chart

The three-hole ribs bedframe is composed of 14 bow ribs bedframe systems. It is endowed with ultra-high flexibility based on the strong elastic deformation and support ability of the bow ribs bedframe system. Depending on the precise rotation and deformation of each rib, its support surface can perfectly fit the human skeleton curve and provide the most effective support for the human body (see in Fig.3). These can avoid excessive compression of back, hips and feet, and abnormal deformation of waist and legs.

Fig.3 Support effect diagram of three-hole ribs bed

In order to verify the rationality of the design of the three-hole ribs bedframe, this paper used scientific subjective and objective experiments to compare and analyze the lying comfort of the three-hole ribs bed and the ordinary plate bed. About 15 volunteers take part in the two kinds of sleeping position by the human body pressure distribution test. Its pressure distribution index was calculated, longitudinal pressure and contact area distribution curve of human body was drawn by MATLAB, which was used to analyze the pressure distribution of the human-bedframe contact surface. The subjective experiment referred to the statistics of the feeling of volunteers lying on two kinds of bedframes through the design of subjective comfort questionnaire. The data was used to analyze the local and overall comfort of the two kinds of bedframe surfaces by SPSS.

3 Human body pressure distribution experiment

In the process of human sleep, the common bed can easily break the balance relationship of human sleep. It will cause seriously abnormal deformation of spines and excessive compression of local muscles. In particular, long-term local pressure concentration will not only hinder normal blood circulation and lead to local ischemia, but also stimulate the nerves to force the body to turn over for many times. Lying comfort of the human body is seriously affect to reduce the occurrence of local pain. Moreover, if the sleepers are people with motor behavior disorders, decubitus will be caused by prolonged ischemia due to inability to turn over. Therefore, the defect of the bedframe not only easily causes the uncomfortable prone position, but also seriously affects the health of the human body.

A new three-hole ribs bedframe designed. The aim is to improve the discomfort of the ordinary board bed. It can reduce excessive rolls due to severe deformation of spines and muscles due to insufficient support. In order to verify the comfort of the bed, the objectively analyzed through the comparative analysis of the human pressure distribution data.

3.1 Experimental object

The human body pressure distribution test was completed at the Industrial Design Laboratory of the School of Mechanical Engineering of Xinjiang University. Fifteen students were randomly selected as volunteers, including ten males and five females. Their average age was 22 years old. The average height of men was 174cm and the average weight was 73kg. The average height of women was 162cm and the average weight was 52kg. All volunteers were in good health without any history of musculoskeletal disorders.

The common board bed (size 195cm × 90cm) was selected as the test object to be compared with the three-hole ribs bed. It was No. 1, and the three-hole ribs bed was No. 2.

3.2 Experimental methods

The human body pressure distribution measurement system, which was produced by Tkscan Company of the United States. It was used to test the human lying position pressure distribution on the three-hole ribs bed and the ordinary board, the experimental scene was shown in Fig.4. The experiment selected the supine and side sleeping postures. Its sampling time was 3 minutes, and the sampling frequency was 10 f·s-1. In order to intuitively compare the pressure distribution of the two kinds of bedsteads, the sensor sensitivity was adjusted to the appropriate value for both bedsteads.

Fig.4 Experimental scene of human pressure distribution test

During the experimental data collection, the pressure distribution of the three-hole ribs bed and the ordinary plate bed was separately collected after it was relatively stable. Saved the human pressure distribution map of the whole animation and the two-dimensional diagram. The whole-body pressure was exported in the form of excel. And the basic pressure indexes such as maximum pressure, average pressure, total pressure, and total contact area were calculated[15]. The longitudinal pressure and contact area distribution curves of the human body were derived by BPMS Research 7.60.

4 Experimental results and analysis

4.1 Human lying posture pressure distribution index

Study on the support performance and elastic deformation ability. The human body pressure distribution index changes with the difference of subjects and sleeping positions. In this paper, the maximum pressure, average pressure, total contact area and total pressure are selected to analyze the support performance and elastic deformation ability of both bedframe systems. The maximum pressure shows the stiffness, that is, the ability of the support surface to resist elastic deformation, which reflects its support performance. The average pressure not only shows the rigidity of the human-bedframe system, but also reflects the deformation ability of the system. The total contact area represents the support and deformation capabilities of the system. The total pressure is determined by the contact area and the average pressure, which also characterizes the support and deformation abilities.

Table 1 shows that the maximum pressure value and average pressure value of the three-hole ribs bed are smaller than the ordinary plate bed in the two lying position, and the total contact surface area and total pressure are larger than the ordinary plate bed, which indicates that the three-hole ribs bedframe not only has sufficient support ability, but also increases the contact area, disperses local pressure peaks as well as reduces the possibility of lying discomfort.

Table 1 Index of body pressure distribution

4.2 Human longitudinal pressure and contact area distribution curve

The longitudinal pressure distribution curve of the human body refers to the distribution of pressure in the longitudinal direction of the human body. The longitudinal contact area distribution curve of the human body is the distribution of the contact area between the human body and the bedframe in the longitudinal direction. The combination of them can locally analyze the support effect of bedframes on the human body. This paper mainly focused on the distribution of pressure and contact area of the human head, back, waist, buttocks, legs and feet.

The comfort of the head is mainly affected by the pillow comfort, and the same pillow is used in the experiment, there is no significant difference in the pressure. Fig.5 gives a comparison curve of longitudinal pressure and contact area distribution of the human body supine lying on two kinds of bedframes. It shows that the ordinary board bed mainly supports at the bone process points such as the back, hips, calves and feet, which shows the characteristics of partial pressure concentration and smaller support area, while the waist and thigh are suspended. The buttocks are the most insensitive part of the human body, so it is reasonable to withstand a large amount of pressure. The greater pressure on the back and calves may cause strong contraction of the muscles and discomfort. On the contrary, the three-hole ribs bed not only increases the support area of the back, buttocks, calves and feet through the super-high flexibility of the self-supporting structure, but also disperses the pressure of the bone process parts to all sides, and reduces the possibility of local ischemia or bedsore due to stress concentration. Meanwhile, it increases adequate support at the waist and thigh to avoid abnormal deformation of the human spine and musculoskeletal due to insufficient support. Fig.6 shows the comparison curve of longitudinal pressure and contact area distribution of the human body side lying on two kinds of bedframes. It can be seen that the bony point in the leg of the human body changes from the calf to the thigh when side lying on two kinds of bedframes, while others are consistent with lying position.

Fig.5 Contrast curve of longitudinal pressure and contact area distribution of human supine Longitudinal pressure distribution curve is above; longitudinal contact area distribution curve is below

Fig.6 Contrast curve of longitudinal pressure and contact area distribution of human side lying Longitudinal pressure distribution curve is above; longitudinal contact area distribution curve is below

Fig.7 is a two-dimensional diagram of the pressure distribution of the human body lying on the two bedframes in the supine and lateral positions. It can be seen that the ordinary board bed only supports the back, hips, legs and feet, which is characterized by the red point of peak pressure. In contrast, the three-hole ribs bed uses its own super high flexibility to convert the point support into its surrounding surface support and turn the red point into a light green area. It increases the contact area of the bone protrusions and reduces the local compression, and greatly improves the lateral comfort of the bone protrusions. In the supine and side position, it can give enough support for the waist and legs to avoid the abnormal deformation of the spine caused by insufficient support of the ordinary board.

Fig.7 Human body pressure distribution map of two bedframes in the supine and side lying posture

Compared with the ordinary board, the three-hole ribs bed relies on its unique support and elastic deformation ability of the support structure. It greatly increases the support area of the human bone protrusion part and reduce the local support discomfort. It provides adequate support in the waist and leg to avoid problems such as back pain and musculoskeletal deformation, which has improved the disadvantages of excessive support of the bony prominence and insufficient support of the waist as well as legs. It is verified that the three-hole ribs bed can provide the most effective support for human body in different lying positions.

5 Subjective comfort experiment

When people sleep, the uncomfortable contact part will stimulate the sensitive nerve end. So, people must constantly adjust the sleep posture to reduce the pain, which means that the human body can subjectively feel the comfort of contact surface. Annet used statistical methods to prove that the subjective comfort test had predictive validity and was an indispensable part of the evaluation of lying comfort[16]. Subjective comfort evaluation generally used reasonable evaluation contents and methods to design questionnaire, and taked the subjective feelings of volunteers as the evaluation basis.

Based on Corlett and Bishop’s discomfort assessment technology (BPD)[17], this paper designed a subjective comfort questionnaire by using the seven-level quantifier assignment method (see in Table 2) with the highest reliability, the questionnaire was seen in Fig.8. The subjective comfort of the two bedframes was tested by recording the subjective state or feel of the volunteers lying on the two bedframes.

Table 2 Seven-level scoring standard for subjective feelings

Fig.8 Bedframe subjective comfort questionnaire

About 15 volunteers were asked to maintain a regular life routine one week in advance. And sleep normally on two bedframes in a uniform time (Beijing time 23 o’clock to the next day 9 o’clock) within two days. They were forbidden to eat beverages or foods that affect sleep before going to sleep. During the experiment, the temperature of room was 23～26 ℃, and the relative humidity was 50%～60%. The next day, after waking up for a period of time, 15 volunteers were asked to fill in the questionnaire about the comfort level of both bedframes according to their own subjective experience. The scores of 15 volunteers subjective comfort were shown in Table 3. Statistical analysis of the mean value of 15 volunteers’ subjective comfort scores for both bedframes (see in Fig.9)

Fig.9 Average line chart of subjective comfort for 15 volunteers

Table 3 Summary table of subjective comfort of 15 volunteers

Continued Table

From the comparison of the average values of the subjective comfort scores, it can be clearly seen that the comfort of the three-hole ribs bed in the 11 parts and the whole of the human body is in the comfort category. The three-hole ribs bed is two levels higher than that of the ordinary slab in the shoulder, upper arm, middle back, waist, thigh, feet and overall. Especially the shoulder and the feet of ordinary board are typical local point supports, which are characterized by small support area and partial pressure concentration. On the contrary, the three-hole ribs bed not only increases the support area of the shoulder and the feet to reduce the pressure peak and improves the comfort of the single point support, but also provides suitable support for the waist to avoid severe abnormal deformation of the spine.

The average comfort value of the 15 volunteers only reflects the overall concentration trend of the comfort. It can not represent the evaluation results of different volunteers on the comfort of the same bedframe. The standard deviation of comfort values refers to the differences between the average comfort and comfort evaluation of each volunteer on the same bedframe. It reflects the differences of 15 volunteers’ judgment on the comfort of the same bedframe, which represents the reliability of bedframe comfort (see in Fig.10).

Fig.10 Standard deviation line chart of subjective comfort for 15 volunteers

It can be clearly seen from Fig.10 that the standard deviation of comfort evaluation of two bedframes is only large difference between the waist and the calf. However, the standard deviation of other human body parts is very small. It is indicated that the 15 volunteers have less difference in the comfort evaluation of both bedframes, that is, the two bedframe comfort trends presented by the comfort average can reflect the subjective comfort of 15 volunteers. In summary, the three-hole ribs bedframe has been greatly improved in sleep comfort compared with the ordinary slab bed, and the support for each part and the whole body of the human body belongs to the comfort category.

6 Conclusion

This paper introduced new bedframe function and support comfort in detail. The experimental results confirm that a three-hole ribs bedframe improve the discomfort of the existing common board. Use a combination of subjective and objective experiments to verify that the three-hole ribs bed had good sleep comfort. The human body pressure distribution measurement system is used to analyze the contact pressure and contact area distribution, it verified that the three-hole ribs bedframe could increase the contact area of the bone protrusion, disperse the local pressure, and provide reasonable support for the spine, so the surface of the bedframe was perfectly matched to the curve of the human skeleton to achieve the most effective support for the human body. By subjectively feeling the sleep comfort of both bedframes, the volunteers obviously preferred the three-hole ribs bed, especially in shoulders, waist, legs, and feet. The current study shows that this combination of subjective and objective experiments will provide a reliable method for the comfort evaluation of bed design.