Relationship between seat surface shape and pressure distribution in school seat models

Abstract

BACKGROUND:

Students remain in a sedentary position inside classrooms for 60% to 80% of their school day. As such, research has associated students’ prolonged seated posture on school furniture with their discomfort and musculoskeletal pain. The correlation between the shape of the seat surface and the zones of the body making contact with these surfaces constitutes an essential factor in determining comfort-enhancing seat design.

OBJECTIVE:

This study’s primary aim lies in contrasting the standard, current school chair against two seat prototypes, both designed and built from digital models, comparing seat-user pressure distribution and contact area during students’ performance of different school tasks.

METHODS:

Participants (n = 13), ages 7 to 19, performed school tasks sitting on three different chairs’ seat surfaces during the test: the current seat used in schools and two seat prototypes, each designed based on body anatomy. The seats were evaluated through a force-sensing array pressure-mapping system.

RESULTS:

The measurements from designed seat prototypes provide a better distribution of pressures and greater contact area with the students’ anatomical areas (buttocks and thighs) than the seat currently used in the specified schools. The improvement in pressure values and contact area as seen in the second designed seat prototype is due to its inclination angle and contact with students’ sacral zone.

CONCLUSIONS:

This research work found that a seat’s shape based on human anatomical features (buttocks and thighs), compared to a completely flat seat, creates a higher reduction of body pressures and an increase in the body contact area, with the intent to increase comfort and reduce musculoskeletal pain.

Keywords

School furniture pressure measurement seat surface shape design comfort

1 Introduction

Multiple factors can affect students’ health and performance in a school environment, such as daily schedules and specific school activities, exposing students to physical load risk-factors associated with their adoption of postures for prolonged periods [1]. Schoolwork can require students to sit for multiple hours daily [2, 3]; more specifically, they often write for up to six hours per day plus log two hours of computer use, not only at school but also at home and in other environments [4]; on average, they remain sitting seven to eight hours daily [5], remaining seated in static and stooped postures for extended periods [3]. Inside classrooms, students remain in a sedentary position for 60% to 80% of the school day [6], corresponding to 25 to 30 hours a week as cited in the country of Colombia [7]. Fallon and Jameson found that the postures students maintain while sitting on school furniture for prolonged periods are associated with discomfort and musculoskeletal pain [8] detected in their neck, right shoulder, and back [9], increasing both tension in the posterior ligaments of the spine and pressure on the intervertebral discs [10]. These postures also reduce the nutrition of the discs and restrict blood circulation [11].

In addition to impacting pedagogical dynamics during school classes, furniture design proves a key factor affecting student posture and, therefore, student productivity and performance [8, 11]. In this sense, the chair should provide stable support for the back, buttocks, and thighs, with characteristics of the backrest and seat serving as determining factors in students’ adoption of body postures and noticeably affecting comfort. Reed [10] has identified three chair design parameters associated with comfort: 1) the dimensional adjustment with respect to population anthropometry; 2) the support provided by the shape of the seat and backrest related to the anatomy of the sitter; and 3) the factors of physical contact, relating the surface building-material properties (hardness, temperature, texture) to the pressure resulting from bodyweight distribution.

Regarding the third parameter, Hostens et al. [12] noted that prolonged pressure peaks resulting from body compression against the chair comprise the main cause of discomfort and back pain. These compressive loads occlude the skin capillaries, thus preventing adequate blood flow and ultimately producing pain or discomposure [13], the two indicators of discomfort. One of the most important and useful ways to improve comfort lies in increasing the contact area between the body and the surface of the chair. In the specific case of the school chair seat, the area of the ischial tuberosities should bear maximum pressure, with the buttocks and periphery muscles gradually reducing this load [14].

In the styling or creating of contact surfaces, chair designers should take several key characteristics into account: geometry, functional angles, shape, and foam formulation [15], as well as the building material (its hardness, resilience, and cushion capacity) and its contour (i.e., correlation between students’ anatomy and the seat and backrest shapes). Springle and Schuch demonstrated that a cushion with contours adapted to the individual results in a better distribution of pressures than does a completely flat cushion [12].

Concerned with product durability, the Colombian technical standards for school chairs require building materials for contact surfaces to consist of plywood or polymers [16]. Because this standard prevents changes in the building material, the available means to improve pressure distribution lies in the anatomy-based design of the chair shape. This design is expected to accommodate compression in specific areas: the lumbar area, which comes in contact with the backrest; and the sacral-coccygeal, ischial, and thigh areas, which come in contact with the chair seat. In this respect, the aforementioned regulations establish a few specific requirements, namely, “the surfaces of the seat and backrest of the chair must be anatomical and uniform in appearance and protect the user from slipping” [16].

This paper is part of the research entitled, “Defining formal parameters of school-chair seat and backrest through the measurement and evaluation of pressure points,” conducted by the research groups Diseño, Ergonomía e Innovación (Design, Ergonomics and Innovation) and Centro de Estudios de Ergonomía (Ergonomics Studies Center), at Pontificia Universidad Javeriana (Bogotá - Colombia).

Researchers designed and built seat prototypes from rigid materials, based on the school-age population’s body curvature and anthropometry. Thus, the aim of this paper focuses on contrasting the designed seat prototypes with the currently used chair in terms of seat-user pressure distribution and contact area during the performance of different school tasks.

2 Methodology

2.1 Participants

The study’s participants included 13 students (four boys and nine girls) between 7 and 19 years of age; at the time of the study, each was attending one of three educational institutions in the city of Bogotá. Researchers selected the participants from different school grades and considered the students’ different physical statures according to the compatible height range for each chair size (Table 1), in compliance with the Colombian technical standards [16].

Table 1
School chair sizes according to the Colombian technical standard. Adapted from ICONTEC [16]

Chair size Age (years) School grade Compatible height range (cm) Seat Surface height (cm)

1 3–5 0 110–137 30

2 6–9 1–4 138–151 36

3 10–13 5–7 152–165 40

4 14–19 8–11 166 or more 44

Chair size	Age (years)	School grade	Compatible height range (cm)	Seat Surface height (cm)
1	3–5	0	110–137	30
2	6–9	1–4	138–151	36
3	10–13	5–7	152–165	40
4	14–19	8–11	166 or more	44

The Research and Ethics Committee of the Architecture and Design Faculty of Pontificia Universidad Javeriana approved the research protocol. Researchers also obtained authorization from the institutions and either the students or their parents (for under-age students).

2.2 Instruments

2.2.1 Chair types

Because of the anthropometric variability resulting from the participants’ age ranges, three chair sizes specified by Colombian technical standards [16] were employed for the trial research (Table 1).

The trials were conducted on two sets of chair seats in sizes 2, 3, and 4:

The type of chair currently used in the Colombian educational institutions, which is equipped with a flat-surfaced wooden seat (Current Seat or CS) and complies with the Colombian technical standards (Fig. 1); the study used this type of chair in all three sizes.

Curved-surface seat prototypes (DP1 and DP2) designed as part of the present research and constructed out of MDF (medium-density fiberboard), which exhibits a similar hardness to that of the seats students currently use. Note: DP1 was designed in all three seat sizes while DP2 was designed only in seat size 4.

Fig. 1

Left: chairs currently used in classrooms; right: their corresponding flat–surface seat (CS).

2.2.2 Pressure measuring equipment

Researchers performed measurements using an FSA (force-sensing array) pressure- mapping system, designed by Vista Medical (reference NEX-SBS), which uses a 5-Hz-measuring-frequency sensing mat equipped with a 530 mm×530 mm network of 256 piezoelectric sensors. The two mats are covered with a polyurethane-coated nylon fabric, and each includes an attached ribbon cable sleeve and a 34-pin plug [17]. The interface module of the system has two 34-pin receptacles (A and B) to connect the mats’ cables [17]. Data capture was performed using a laptop equipped with an FSA 4.1 software package. One of the mats was placed only on the seat surface, not on the backrest, to measure pressure distributions under the students’ buttocks and legs.

2.3 Procedure

2.3.1 Digital seat model design

To design the seat based on the body anatomy, the researchers employed a ManneQuin Pro (Nexgen ergonomics) software package to build human digital models. For this purpose, the extreme values of the compatible height ranges of school chairs in sizes 2, 3, and 4 were used, following the dimensions specified by the corresponding technical standard [16]. Visualized in skeleton mode, the models were configured to a seated posture, with a 15-degree backrest inclination angle. Making use of an AutoCAD V.2014 (Autodesk) software package, the ischial tuberosity (point A in Fig. 2) was drawn on the sagittal plane. This approach allowed superimposing two digital models per chair size, corresponding to the lower and upper extremes of the height range in question. To that end, the SRP (seat reference point) was used to match the two models, thereby allowing the drawing of the curves intended for the seat surface to maximize the students’ physical contact area and the anatomical correspondence. In this manner, the external shape of the seat was adapted to chairs in sizes 2, 3, and 4, according to the buttock and sacral zone anatomy, to identify the ischial tuberosities that coincide with the deepest concavity of the seat.

Fig. 2

Seat configuration with respect to the projections of the ischial tuberosities (A).

Based on this information, digital seat models were prepared for the three chair sizes in question, making use of an AutoCAD V.2014 (Autodesk) software package; as discussed, DP1 was created in all three seat sizes while DP2 was created in seat size 4.

2.3.2 Test-prototype building

Based on the digital seat models, sections of medium-density fiberboard (MDF) were cut using a laser machine and then glued together to build the seat prototypes according to the dimensions specified in chair sizes 2, 3, and 4. These interchangeable components were assembled on a test chair, allowing control of the seat surface height from 30 cm to 44 cm with respect to the floor, according to the dimensions specified by the technical standard. In order to produce appropriate contact with the participants’ back and to make adjustments in accordance with their different buttock-popliteal lengths and lumbar heights, the test chair also permitted the movement of the backrest both vertically and horizontally (Fig. 3).

Fig. 3

Left: test seat used for pressure measurement; right: designed seat prototypes (DP1 and DP2).

2.3.3 Measuring process

Researchers conducted the first pressure measurement with eight students (P1-P8), using sizes 2 and 3 of both the CS and the first test prototype (DP1) and placing the pressure-measuring mat on the surface of the tested seats. First, researchers weighed each participant to calibrate the equipment accordingly; before this step, each participant was asked to remove any objects from his or her pockets and not to wear a belt. Once they were seated, the students’ sitting posture was also checked to ensure the foot palms were supported on the floor and the thighs were in the horizontal position. Although the body weight in the seated position is distributed between the seat and the backrest, the same backrest prototype (taken as a control variable) was used for all tests as the seat itself supports most of the weight [18]. Furthermore, Floyd and Ward concluded in their research that children during the performance of different tasks spent up to 80% of their time in sitting positions without ever leaning on the chair backrest [3]. Notably, the researchers always adjusted the height of this backrest prototype to ensure it made contact with the participants’ lumbar lordosis. Table 2 shows the distribution of the participants across chair sizes according to their body height and weight.

Table 2
Distribution of the sample of participants according to body weight and chair size (n = 13)

Chair size Participants (n) Age (years) Height range (cm) Mean (cm) Weight range (kg) Mean (kg)

2 3 7–9 128–134 130.5 26–34 30.7

3 5 10–13 141–149 145 39–54 45.8

4 5 17–19 166–182 170.2 42–72 56.8

Chair size	Participants (n)	Age (years)	Height range (cm)	Mean (cm)	Weight range (kg)	Mean (kg)
2	3	7–9	128–134	130.5	26–34	30.7
3	5	10–13	141–149	145	39–54	45.8
4	5	17–19	166–182	170.2	42–72	56.8

Pressure measurement on the CS spanned five minutes per participant, during which the participant received an explanation displayed on a blackboard. After a 10-minute rest, the mat was transferred to a DP1 seat of the same size as the previously used CS so researchers could repeat the measuring process (Fig. 4).

Fig. 4

Pressure measurement on the seat prototype.

Based on results obtained for both the CS and the DP1, the researchers modified the prototype shape to reduce the pressure peaks; they did so by increasing the radii of the curves that corresponded to the sacral and ischial zones. Based on these results, they designed a new prototype (DP2), which corresponded only to chair size 4. Then, all three chair seats—DP1, DP2, and the CS—were utilized in chair size 4 with five university students (P9–P13) to repeat the previous procedure, ensuring their anthropometric measures fitted chair size 4. In the test, the participants carried out two tasks: they paid attention to a person in front of the board for five minutes as they would a teacher; and they wrote on a paper on the table surface for 10 minutes. A pause of 10 minutes occurred between the two tasks performed in each type of seat.

The pressure data recorded during the tests were imported into MS Excel macro for their analysis. Because the assessment equipment operated at a frequency of 5 Hz, each sensor on the mat obtained an average of 1,500 readings during the five minutes of the attention task and 3,000 readings during the 10-minute writing task.

2.4 Statistical analysis

The statistical analysis was performed using IBM SPSS Statistics 24. The averages of the total pressure as well as of the effective contact area on the seat surface were calculated for each participant as recorded during the “attention” task measured on seat sizes 2 and 3, and the “attention” and “writing” tasks performed on seat size 4; the software recorded approximately 1,500 readings per participant for each task, corresponding to a readout every 0.2 seconds.

The possible atypical values of each measurement were eliminated through a box diagram, according to the interquartile range of the data recorded for each participant. Likewise, the 0 (zero) mmHg pressure values were not considered for the statistical analysis because they imply no contact between the body and the seat surface.

To determine significant differences between the values recorded for the CS and the prototypes designed for this research (DP1 and DP2), the researchers tested data normality through the Kolmogorov-Smirnov test, followed by a Wilcoxon matched-pairs test (P < 0.05), depending on the data at hand.

3 Results

The results of the Kolmogorov-Smirnov test show that the findings of pressure and contact area do not behave normally (P < 0.001). When applying the Wilcoxon matched-pairs test (P < 0.05), researchers found no significant differences between the pressure means of the CS seat with respect to the DP1 (T = 20), but differences in the means of the contact areas (T = 7) did exist, after comparing the data of the 13 participants.

Table 3 shows the mean pressure values obtained with CS and the DP1 (sizes 2 and 3) during the attention task as well as the percentage difference between the two seats. In the case of participant 1, for example, the mean pressure data for DP1 are 22% lower than those of the CS. Researchers calculated this percentage based on the arithmetic mean of the pressure values during the full trial. Table 4 shows the contact area (cm²) data for DP1 and the CS, together with their percentage differences. In the case of participant 1, findings indicate that with the DP1 seat, the contact area increased by 22% compared to the CS seat.

Table 3
Mean and standard deviation pressure values (mmHg) from CS and DP1 seats of sizes 2 and 3

CS surface DP1 seat surface Percentage difference (DP1–CS)

Total pressure Pressure on the ischial zone Total pressure Pressure on the ischial zone Total pressure Pressure on the ischial zone

Participant Sex Chair size Mean SD Mean SD Mean SD Mean SD

P1 F 2 43.08 3.84 73.88 10.44 33.69 3.61 52.59 7.45 –22% –29%

P2 F 2 37.00 3.93 66.37 9.76 39.84 4.60 55.44 11.26 8% –16%

P3 F 2 38.58 2.97 84.00 4.85 29.46 2.32 51.03 4.79 –24% –39%

P4 M 3 42.65 1.33 85.63 2.03 39.77 1.79 72.93 4.98 –7% –15%

P5 F 3 42.63 1.58 83.28 3.47 49.80 2.02 79.61 4.38 17% –4%

P6 F 3 47.50 2.71 64.72 5.03 45.06 2.80 69.34 5.17 –5% 7%

P7 F 3 59.24 2.86 81.82 5.01 49.24 2.68 78.91 3.45 –17% –4%

P8 M 3 30.34 1.80 76.86 3.75 27.03 3.81 42.31 6.83 –11% –45%

				CS surface	DP1 seat surface	Percentage difference (DP1–CS)
P1	F	2	43.08	3.84	73.88	10.44	33.69	3.61	52.59	7.45	–22%	–29%
P2	F	2	37.00	3.93	66.37	9.76	39.84	4.60	55.44	11.26	8%	–16%
P3	F	2	38.58	2.97	84.00	4.85	29.46	2.32	51.03	4.79	–24%	–39%
P4	M	3	42.65	1.33	85.63	2.03	39.77	1.79	72.93	4.98	–7%	–15%
P5	F	3	42.63	1.58	83.28	3.47	49.80	2.02	79.61	4.38	17%	–4%
P6	F	3	47.50	2.71	64.72	5.03	45.06	2.80	69.34	5.17	–5%	7%
P7	F	3	59.24	2.86	81.82	5.01	49.24	2.68	78.91	3.45	–17%	–4%
P8	M	3	30.34	1.80	76.86	3.75	27.03	3.81	42.31	6.83	–11%	–45%

Values showing pressure reduction are shaded in gray.

Table 4

Mean and standard deviation contact-area values (cm²) from CS and DP1 seats sizes 2 and 3

Participant	Sex	Chair size	CS surface				DP1 seat surface				Percentage difference (DP1–CS)
			Total area		Ischial contact area		Total area		Ischial contact area		Total area	Ischial contact area
			Mean	SD	Mean	SD	Mean	SD	Mean	SD
P1	F	2	717.25	26.68	247.86	0.00	872.59	58.52	275.52	5.72	22%	11%
P2	F	2	821.77	71.76	210.55	7.39	937.19	40.66	282.22	3.86	14%	34%
P3	F	2	947.81	59.61	229.79	9.93	898.05	53.70	348.46	5.09	–5%	52%
P4	M	3	746.36	10.28	262.44	0.00	959.57	15.72	282.40	5.39	29%	8%
P5	F	3	746.70	16.16	218.70	0.00	1089.80	14.93	400.95	0.00	46%	83%
P6	F	3	954.35	28.13	349.92	0.00	1087.26	22.75	291.60	0.00	14%	–17%
P7	F	3	915.58	5.70	240.57	0.00	1063.49	32.33	364.50	0.00	16%	52%
P8	M	3	1031.22	25.96	192.17	3.55	1011.23	59.80	457.33	3.29	–2%	138%

Values showing increases in contact area are shaded in gray.

Measurements from DP1 in sizes 2 and 3 (Table 3) showed that the mean pressure values on the ischial zone fell within the discomfort area for seven out of eight participants (i.e., they were above the threshold value of 46.5 mmHg) [13, 19], accompanied by elevated pressure values. This led to the researchers of this study performing a series of additional tests with a female student, for whom a size 3 DP1 seat surface was inclined at different angles (0°, 3°, 6°, and 8°) to reduce the pressure in the zone in question and distribute it toward the thighs. Thus, a 6° inclination angle allowed for the reduction of pressure on the ischial zone by 10.2%, when compared to that observed at 0°, which correlates with findings by Rincón et al. [20]. Similarly, the backend portion shape of the DP1 was modified to increase the contact area in the sacral-coccygeal zone, thus reducing pressure on the ischial zone. Hence, seat DP2 incorporated a 6° forward inclination angle and a curvature that increased the sacral-coccygeal contact area. Tables 5 and 6 compare the mean pressure and contact area, respectively, for CS, DP1, and DP2.

Table 5

Mean and standard deviation pressure values (mmHg) from CS, DP1 and DP2 seat size 4

Participant	Sex	CS surface				Seat surface DP1				Seat surface DP2
		Attention		Writing		Attention		Writing		Attention		Writing
		Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD
P9	F	45.12	4.59	48.64	2.29	24.61	1.48	23.17	0.79	27.61	4.18	26.90	2.68
P10	F	36.48	3.21	43.33	2.31	25.69	1.27	28.48	1.55	19.83	0.87	23.78	1.19
P11	F	26.73	2.81	35.58	2.10	42.30	2.22	38.36	1.47	31.44	1.85	37.75	1.22
P12	M	59.95	7.80	73.04	1.66	48.99	4.32	51.10	2.21	34.27	0.97	40.04	2.51
P13	M	37.73	3.61	40.03	1.15	33.20	1.32	33.16	1.04	25.42	1.44	25.82	1.22

Table 6

Mean and standard deviation contact-area values (cm²) from CS, DP1 and DP2 seat size 4

Participant	Sex	CS surface				Seat surface DP1				Seat surface DP2
		Attention		Writing		Attention		Writing		Attention		Writing
		Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD	Mean	SD
P9	F	1048.96	56.51	980.78	61.31	961.07	15.93	951.98	19.45	908.86	118.01	826.79	18.00
P10	F	857.72	27.36	877.07	17.18	970.87	12.20	991.74	13.60	916.03	15.19	918.40	12.89
P11	F	999.13	22.37	1026.79	13.48	1060.90	33.65	1080.65	11.66	856.41	26.84	904.76	28.33
P12	M	873.98	80.29	1004.66	6.70	1148.36	19.12	1022.86	25.55	1269.16	26.20	1132.70	28.56
P13	M	911.85	53.90	908.90	23.06	1095.66	14.67	1085.95	12.57	1125.64	34.92	1091.47	14.78

Table 7 shows the percentage differences between the pressure values and between the contact areas of seat size 4, on the seat surfaces of CS, DP1, and DP2. In fact, the results of testing with DP1 and DP2 show they produce lower pressure and provide higher contact area values. For example, in the case of participant P10, the CS mean pressure during the attention task was reduced by 30% with DP1 and by 46% with DP2. Conversely, the contact area for the same participant increased by 13% during the attention and writing tasks when using DP1 compared to the CS. In turn, the contact area of DP2 decreased by 6% and 7% during the attention and writing tasks, respectively, compared to that of DP1. However, one should take a key finding into account: when applying the Wilcoxon matched-pairs test, no significant differences between the pressure means were found.

Table 7

Pressure and contact-area percentage differences between size-4 CS, DP1 and DP2 seats

Participant	Pressure (mmHg)						Contact area (cm²)
	Percentage difference (DP1-CS)		Percentage difference (DP2-CS)		Percentage difference (DP2-DP1)		Percentage difference (DP1-CS)		Percentage difference (DP2-CS)		Percentage difference (DP2-DP1)
	Attention	Writing	Attention	Writing	Attention	Writing	Attention	Writing	Attention	Writing	Attention	Writing
P9	–45%	–52%	–39%	–45%	12%	16%	–8%	–3%	–13%	–16%	–5%	–13%
P10	–30%	–34%	–46%	–45%	–23%	–17%	13%	13%	7%	5%	–6%	–7%
P11	58%	8%	18%	6%	–26%	–2%	6%	5%	–14%	–12%	–19%	–16%
P12	–18%	–30%	–43%	–45%	–30%	–22%	31%	2%	45%	13%	11%	11%
P13	–12%	–17%	–33%	–36%	–23%	–22%	20%	19%	23%	20%	3%	1%

Values showing improvements in contact area or pressure are shaded in gray.

According to the pressure maps (Figs. 5 and 6), findings of the CS testing indicate higher measures of this parameter on ischial bones as well as little contact on thighs, while findings associated with DP1 and DP2 favor more homogeneous distributions. The higher pressure on the thighs, as observed with DP2, responds to both the 6° inclination angle employed with this prototype and to the seat-surface contact with the sacral zone.

Fig. 5

Pressure maps corresponding to participant 10 (female).

Fig. 6

Pressure maps corresponding to participant 13 (male).

4 Discussion

Proper pressure distribution on the seat and backrest of the chair occurs when the body parts in question (back, buttocks, and thighs) make broader contact with their supporting surfaces [12, 21]. This situation results in lower force application per area unit because the student’s body weight does not tend to distribute uniformly over the entire contact surface of the seat [22, 23]. Therefore, a better-fitting seat surface shape that more effectively aligns with the body zone with which it makes contact provides for better pressure distribution and, consequently, holds potential to give the student more comfortable learning experience.

Although the literature does not define a specific value for the maximum acceptable pressure in capillaries, some studies propose 32 mmHg as a general limit, which corresponds to heart capillary pressure. Above this value, capillaries can be obstructed, which can lead to oxygen deprivation in tissues [24]. Hostens et al. [12] recommend pressures below 20–30 mmHg to prevent capillary occlusion in tissues under pressure. For the particular case of children, the present research has not found any defined capillary maximum allowable pressure values. A study conducted by Vaisbuch, Meyer, and Weiss [25] with 30 children, 15 healthy and 15 with a congenital disability affecting the spine, considered a 32 to 40 mmHg safe range, deeming higher pressures risky.

Table 3 presents the mean pressure values in seat sizes 2 and 3 (for participants P1 to P8) during students’ attention tasks. The mean values in the flat-shaped surface (CS) exceeded the safe range in five participants. In contrast, the DP1 seat surface was shown to reduce the mean pressure values in six participants but exceeded the limit of 40 mmHg in three. The highest mean pressure was 49.8 mmHg. For this reason, the DP1 prototype was modified to increase the contact area with the body, and thus, the DP2 prototype was developed.

Figure 7 presents mean pressure values per participant in the three seats under study. The flat-shaped surface (CS) produced mean pressures that exceeded 40 mmHg in three participants. In turn, DP2 determined pressures that remained within the literature-recommended range (20 to 40 mmHg) during both attention and writing tasks, even in the case of participant 12, who had the highest body mass (72 kg). This same participant reached the highest mean pressure in the CS, thus exceeding the comfort limit (40 mmHg).

Fig. 7

Mean pressure across different seat types and recorded tasks for participants 9–13.

Regarding the contrast between the studied tasks, most mean pressure values were found higher for students engaged in writing than when paying attention to an instructor. This result is due to the students’ differing postures when performing different tasks: leaning back against the chair’s backrest when listening to the instructor but leaning forward when pursuing writing tasks, thus applying pressure only to the chair seat. Research by Pope, Goh, and Magnusson [26] adds clarity to the biomechanics of different types of seated posture by defined or classifying it into three types: a) anterior posture (inclined forward), a position assumed when working at a desk, with the center of mass located forward of the ischial tuberosities; b) medium posture (facing forward), with the center of mass directly above the ischial tuberosities being unstable as they act as a pivot; and c) posterior posture (leaning back), wherein the center of mass is behind the ischial tuberosities. Likewise, Quintana et al. [6] stated that, in the anterior posture, the ischial tuberosities and the posterior face of the thighs provide support; whereas in the medium posture, the weight of the trunk is concentrated on the ischial tuberosities; and in the posterior posture, the tuberosities and the posterior faces of the sacrum and the coccyx offer support. As a result, both pressure distribution and contact area with the seat surface change with posture as the center of mass and the body’s support zones vary depending on whether the person is writing (anterior posture) or paying attention (medium and posterior postures). Notably, the contact area is inversely proportional to discomfort. Hence, the larger the contact area between the chair surfaces and the body, the lower the discomfort [12]. Fig. 8 shows that in the case of participants P12 and P13 (male), the contact area of DP1 was noticeably increased in the design of DP2 to that observed in the CS.

Fig. 8

Effective contact area according to the seat type and the task for participants 9–13.

In contrast, the other participants represented in the figure—P9, P10, and P11 (females)—exhibited similar values for this parameter in all three seats (P10). Furthermore, some of these participants exhibited contact area decreases in DP1 and DP2 with respect to the CS (P9 and P11). Such behavior may be due to sex-related anatomical differences in the characteristics of buttocks and thighs. For example, the distribution of fat is different between men and women; compared to men, women have more subcutaneous fat, which is typically distributed over the buttocks, thighs, and behind the upper arms, giving them a more rounded form [27].

While this study’s results can serve to inform future seat design for classroom settings, one should also note its limitations. For example, the postures that the participants adopted during the tests were determined by the attention and writing tasks; however, neither clothing nor arm position and placement were controlled. These factors may prove important considerations as pressure values may change in specific scenarios, such as if the upper extremities are crossed or supported on the table. Another limitation surrounded the ability to obtain pressure contact measurements for a wide range of students; as this study was performed inside schools during class hours and thus required the consent of directors, teachers, and parents, only 13 participants were confirmed for testing. One can see a related drawback of the study associated with the sex-related anatomical differences in the characteristics of buttocks and thighs since data from participants (four boys and nine girls) do not provide enough information to impact findings. The limited participant pool hinders the procurement of conclusive results regarding the changes in pressures and contact areas with the three evaluated seat surfaces. A larger pool of participants could add increased validity to findings and better serve to inform future research in furniture ergonomics.

To validate findings of seat surface DP2, the study’s next stage may include the controlling of posture and clothing in a larger sample of participants, thus facilitating the evaluation of the relationship between the seat surface design and the anatomical characteristics of the contact areas of the two sexes. Likewise, to improve future pressure distribution results in combination with the seat surface shape of DP2, researchers of this study recommend using softer building materials in the seat’s design.

5 Conclusions

The compatibility between the shape of the school seat surface and the zones of the body that make contact with the seat (mainly the students’ buttocks and thighs) has emerged as one of the most important variables that determine a student’s level of sitting comfort. The use of rigid materials, such as plywood or polymers, is common in the design of some types of furniture (i.e., school furniture), mainly for reasons of durability. This research work found a reduction of the surface pressures and an increase in contact area when using a seat prototype whose shape is based on human anatomical features (buttocks and thighs) as compared to a completely flat seat. This finding is independent of the material hardness.

The pressure distribution on the seat is dependent on the postural conditions during the school activities in class and its relationship with anthropometric and behavioral characteristics of the students, as well as the pedagogical dynamics. In the design and evaluation process of seat prototypes, researchers must take these variations into account to find the best morphological correspondence between the user and the seat.

6 Last considerations

Based on the findings of this research, the authors of this paper directly participated in the upgrading of the Colombian technical standard [16], suggesting the addition of a new requirement that mandates “the shape of the seat surface should produce contact with the sacral zone, buttocks, and thighs.” This provides an important starting point to improve the existing standards for school seat design in the country of Colombia, providing more comfort-enhancing furniture features that can benefit the learning environment.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

The researchers express their gratitude to the people and entities that made this study possible: Pontificia Universidad Javeriana, Banco Santander, students, teachers and directors of Calasanz Femenino School and Jaime Garzón School (Bogotá), and the industrial design students Laura Durán and Laura González at Pontificia Universidad Javeriana.

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