Does a prototype ‘Experimental’ chair facilitate more postural changes in computing adolescents compared to a normal school chair?

Abstract

BACKGROUND: Prolonged sitting, such as when computing, has been linked to adolescent spinal pain. A chair should fit the user’s body dimensions and aid the user in frequently changing sitting posture, rather than support one ‘ideal’ posture. Such a chair is not currently available to high school learners.

OBJECTIVES: To develop a novel experimental school chair. To determine whether the experimental chair encourages regular small range movement in all directions whilst students work at computers in class. To compare the number of postural changes between the normal static school chair and the experimental chair, whilst students do computing work.

METHODS: A development and validation study was conducted. Twelve high school students were randomly selected from a conveniently selected school. Fifteen minutes of 3D posture measurements were collected in both the prototype and school computer chair. The analysis focused on the frequency of postural movement.

RESULTS: Data of eleven learners were analysed. There were statistically significant increases in the number of postural changes on the ‘Experimental’ chair compared to the school chair, for both pelvic side-flexion and pelvic rotation (ρ= 0.01).

CONCLUSION: The experimental chair provided more postural movements when compared to the school computer chair. Future studies should use this experimental chair to determine the effect of increased postural movement, of especially the pelvis, on the prevalence and intensity of musculoskeletal symptoms of high school learners in the Cape Metropole area, Western Cape, South Africa.

Keywords

High school sitting biomechanics sitting posture movement experimental chair

1 Introduction

Adolescents with back pain have been shown in longitudinal studies to become young adults with back pain [1 –3]. Given the huge cost of back pain to society in terms of disability, lost workplace productivity and healthcare [4 –6], it is important to recognise and intervene in causes of back pain in adolescents to reduce the high prevalence of adult pain in future generations. High school students sit for long periods in class in relatively static postures, at both writing and computer desks [7, 8]. Lengthy periods of static sitting have been hypothesised as a reason for poor adolescent spinal postures, which may contribute to the reported high prevalence of adolescent spinal pain [2 , 8]. High school furniture (chairs and desks] should therefore facilitate good sitting postures and minimise back pain. Well-designed school furniture should match the changing anthropometric profile of adolescents, by taking account of the significant growth spurt changes, which occur over their high school years [9 –12].

The best design for school chairs is controversial as there is no universal definition of ‘good’ adolescent sitting posture [13 –15]. There are three main qualitative descriptions of ‘good’ sitting posture:

adopting a flat lower thoracic and lumbar posture [16], particularly with a chair backrest to support a flat lumbar posture [17]

adopting a thoracic kyphosis and lumbar lordosis [18 –20]

adopting a lumbar lordosis, without a thoracic kyphosis [21, 22]

To add to the complexity of how to maintain ‘good sitting posture’, it is uncertain whether any ‘good’ posture can be maintained for prolonged sitting periods. There is evidence to suggest that a specific siting posture can only be maintained for approximately five seconds [23, 24], which is well short of the times students are required to sit in lessons. Sitting in the same position for any length of time is believed to place excessive physiological load on the spine, and this could lead to micro-damage in spinal structures, and consequent pain [25 –28]. Regular changes in sitting postures, underpinned by kinetic and kinematic variability of movement, could thus be protective of pain. While this concept has received considerable attention in sports research [29, 30], it is less discussed in the field of ergonomics [13 –15]. If adolescents’ postures could be intuitively and dynamically changed while sitting in class, to alleviate prolonged load on spinal structures, it is reasonable to postulate that a chair that facilitates regular movement while sitting may enhance ‘good posture’ [13 –15].

The current literature is inconclusive regarding the effectiveness of dynamic sitting posture on maintaining ‘good posture’, or preventing back pain. A number of researchers [25–27 , 31–33] support the notion that an increase in the number of postural movements could positively affect learners who sit for prolonged periods of time. A systematic review by O’Sullivan et al. [34] indicated a lack of evidence to support postural dynamism as a stand-alone approach in the management of lumbar spinal pain, as the nature of spinal pain is multi-dimensional. However, in an isolated study of 105 subjects (aged 8–12 years), dynamic sitting was shown to decrease spinal pain in scholars [35]. Increased postural movement whilst sitting is associated with less intervertebral disc compression and reduced loss of disc height due to radial bulging in the lumbar region [36]. It is possible that a similar response may occur in the cervical and thoracic spine with increased postural dynamism. This underlying mechanism may partially explain why dynamic sitting is associated with reduced spinal pain.

There is considerable international research that suggests that the current design of school furniture is not optimal for adolescent anthropometry or spinal posture [9 , 37–42]. School furniture is often static (allowing no individual dynamic movement), and it is one size only, producing a mismatch to many students’ anthropometric profiles (i.e. one size does not fit all) [9 , 37–42]. To our knowledge, there is little research into adolescent posture and high school chairs, particularly in terms of how best to support the developing spine. While little is known about the effect of traditional (non-adjustable) school chairs on body mechanics, we identified one study [43], which investigated the effect of a height adjustable saddle-type school desk and chair on sitting and standing postures, muscle tension and pain levels. These researchers reported that the adjustable school desks and chairs promoted better sitting and standing postures, increased muscle strength, alleviated pain and appeared to be associated with better overall academic marks.

The costs of purchasing and maintaining school furniture are high in any country, and these costs are particularly burdensome in developing countries such as South Africa where the overall education budget reflects 6% of the national budget in 2013 [44]. Our previous research into furniture used in computer laboratories in state high schools in the Western Cape, South Africa, found that the usual school chair is plastic, rigid and non-adjustable, built for adults, stackable, and resembles garden furniture (Fig. 1) [45].

This type of chair is clearly not supportive of growing adolescent spines and potentially has contributed over time to the high prevalence of South African adult spinal pain [46, 47]. Given the need to boost productivity and reduce disability in South African communities, focus needs to be put on high school furniture to promote healthy adolescent spines. This paper reports on two studies: the development, and subsequent validation, of a novel experimental school chair, which encourages regular small range movement in all directions whilst students work at computers in class.

2 Methods

2.1 Study 1. Chair development: The Experimental Chair

2.1.1 Aim

To develop a novel low cost experimental dynamic chair, made of recycled material, for use in computer classrooms.

2.1.2 Evidence-base

The dimensions of the prototype ‘Experimental’ chair were determined from the findings of an anthropometric study conducted in 2011 on approximately 700 adolescents in the Cape Metropole area, South Africa [45]. The following body dimensions were measured as part of the anthropometric study [45]: Popliteal height (To compare with chair seat height), Buttock-to-popliteal length (To compare with seat depth), Hip width (To compare with seat width).

2.1.3 Design

Based on the notion that sitting posture should allow for dynamic sitting behaviours that encouraged many postural changes (rather than the assumption of one consistent ‘ideal’ posture) [48 –52], the prototype chair had a dynamic seat. The seat mechanism used a four- spring-based system with 0.87 daN/mm tension four-based spring system to create the dynamic nature of the chair. This dynamic seat mechanism made movement in any direction possible. We hypothesised that if the seat could tilt in all directions, it would potentially encourage the learner to adjust his/her sitting posture with little (or even unconscious) effort. The multi-directional tilt degree was based on Mandal’s (1981) recommendation that a chair seat should have a slope of between five and ten degrees [31]. Based on chair design guidelines [28] and findings that students do not consistently use the backrest of a chair even if one is present [53 –55], the prototype ‘Experimental’ chair had neither armrests nor backrest.

2.1.4 Manufacture

The prototype experimental chair was designed using recycled materials from other chairs (such as the five-point base, casters and seat) (Fig. 2). This approach was taken to minimise prototype manufacture costs, and to scope whether subsequent inexpensive local manufacturing was possible, should the experimental chair concept prove to be effective.

2.2 Study 2: Validation of the experimental chair

2.2.1 Aim

To determine whether the experimental chair encouraged more frequent spinal postural changes (encouraged ‘good’ sitting posture) whilst students sat at computers in class, compared with the current standard school chair.

2.2.2 Ethics

The Health Research Ethics Committee at Stellenbosch University and the Western Cape Department of Education provided ethical approval for the study to be performed at the selected schools. Written informed consent was obtained prior to testing from parents or guardians, and written informed assent was obtained from students.

2.2.3 Study design

Validation study using within-child comparisons of spinal movement on the experimental chair (adjusted to student preference) and standard school chair.

Sample recruitment: All interested male and female high school learners aged 13–18 years, attending one conveniently-selected school in the Western Cape, South Africa, were invited to take part in this study. Learners were excluded if they had any diagnosed movement disorders. The list of eligible consenting learners was collated, and this was categorised by gender and age (12 categories). Names were randomly sorted within these categories, and the first eligible student in each category was recruited to participate in the study (sample size = 12). All invited students participated.

2.2.4 Study setting

The movement laboratory of FNB Movement Analysis Laboratory (Cape Town, South Africa) at Stellenbosch University.

2.3 School chair and workstation set-up

2.3.1 Sitting posture measurement procedure

A table and computer display unit, similar to those used in school computer laboratories, was used to standardise the tasks undertaken by subjects during data capture. Neither the table nor the display unit was adjustable and was thus at the same height for all subjects. This replicated the real-life environment found in Western Cape school computer laboratories. Each learner was given the same task (typing and mouse activity tasks) whilst posture was measured on the control, and then experimental chairs [56]. Foot position during sitting influences the knee angle, which, in turn, influences the hamstring length, which impacts on the pelvic position [50]. Therefore, for all testing, the learners were instructed to keep both feet in the same position on the floor at all times whilst typing. This ensured that the movement detected at the pelvis was a primary movement due to actual pelvic movement, and was not secondary to movement of the learner’s feet. Data capture on each chair took place over 15 minutes, split into three five-minute sessions. The five-minute sessions were consecutively captured without a break, on each chair. This procedure was followed due to the immense size of files, which resulted from 15 minutes of continuous data capture.

After the first 15 minutes of posture capture on the usual plastic school chair was completed, the learner was requested to relax for 15 minutes, by walking around leisurely, having something to eat and drink, but without removing the reflective markers. The period of ‘relaxation’ was incorporated into the testing protocol to prevent the effect of fatigue from influencing the posture outcome of the second testing condition. After the resting period, the learner was asked to sit on the prototype ‘Experimental’ chair. The principal researcher adjusted the chair height according to the popliteal height of each learner [57]. The same testing procedure was then followed for the prototype ‘Experimental’ chair. For testing purposes, the back and armrest of the control chair were removed to allow pelvic marker visibility. Table 1 provides an overview of the characteristics of the experimental and control chairs used in this study.

Figure 3 illustrates the usual chair and Experimental chair and workstation set-up in the laboratory.

2.4 Data-capturing procedures

For posture assessment, 3D posture measurements were taken of each subject on each chair using the VICON Motion Analysis (Ltd) (Oxford, UK) system. The VICON has demonstrated high accuracy and reliability [58] and has less than a 1.5-degree error [59]. For this study, the eight-camera T-20 VICON MX system running NEXUS 1.7 software was used and trials were captured at 30 Hz.

2.4.1 Anthropometric measurements and marker placement

The Conventional Gait Model was used, as this provided the angle output sought for the analysis undertaken in this research [60, 61]. The Conventional Gait Model had not yet been validated for sitting. However, due to its established validity for standing, and the likely transferability of the model to sitting, the same model was used.

A trained Motion Analysis Laboratory technician was responsible for the marker placement and for the essential VICON-specific anthropometric measurements. The technician was specifically trained in placing VICON markers by using a standardized VICON marker placement protocol (Table 2). The markers, which were not removed between chair conditions, could not have introduced marker-placement errors between testing conditions. Standard infrared reflective markers (of 21-mm diameter) were placed on standard anatomical landmarks (Table 2).

The output angles for all joints are calculated from the YXZ cardan angles derived by comparing the relative orientations of the two segments. A trained laboratory technician performed the data processing by using Nexus Version 1.1.7 software. Gaps in the captured motion data were, by preference, filled by means of the Pattern fill option in the VICON, which was patterned to a marker on the same rigid body segment.

2.4.2 Choice of angles for analysis

The 3D sitting posture analysis focused on the frequency of movement, which occurred whilst sitting on each chair, and not on sitting posture per se. Analysis focused on the pelvic, thoracic and head angles, as the angles are classified as absolute angles, and the angle calculation was therefore orientated according to the laboratory coordinate system. These absolute angles are defined as follows: head angle is the angle between the head and the laboratory coordinate system, the thoracic angle is the angle between the thorax and the laboratory coordinate system and the pelvic angle is defined as the angle between the pelvis and the laboratory coordinate system.

2.4.3 Statistical procedures and outcome measures

A number of statistical procedures were undertaken.

Sample demographics: Basic demographic information of the pilot sample was reported descriptively per subject.

Basic descriptive analysis of sitting posture behavior: The median and inter-quartile range (IQR) for the neck, thorax and pelvic angles were reported as measures of central tendency and variability for the total capture duration as reported by Van Niekerket al. [62]. The difference between the smallest (min.) median value and the largest (max.) median value was calculated for each subject. The median could therefore be found anywhere within the range presented. Figure 4 illustrates how this statistical procedure was accomplished. (Also refer to Tables 4–6 in the Results section).

Descriptive analysis of the number of postural changes: A postural change was defined as the difference in degrees between one turning point (change in movement direction) and a successive turning point in the data of a given angle. As each angle is a continuous function, the derivative of the data was used to calculate the turning points concerned. The absolute difference between successive turning points was calculated per subject. The histograms for the absolute differences were grouped into categories between zero and 180 degrees in increments of one degree. The categories were then clustered into categories of two to five degrees, between five and ten degrees, and more than ten degrees of movement. Histograms of the categories were created for descriptive comparison [62]. These movement categories are referred to as movement bins. All ‘movements’, smaller than two degrees, were disregarded as system error[63].

Comparative statistics: The ratio between the prototype ‘Experimental’ chair postural change and the school chair postural change bin categories was then determined. If the prototype ‘Experimental’ chair bin was larger than the school chair bin, then the count for the prototype ‘Experimental’ chair bin would increment by 1 and vice versa. The following formula was used to calculate the ratio:

Ratio = (Prototype ‘Experimental’ chair count / School chair count) –1

The ratio illustrates in which chair more movement takes place for specific sized postural changes, as categorised in the bins. The interpretation of the ratio is that a positive value indicates more movement in the prototype ‘Experimental’ chair and a negative value indicates more movement in the school chair. A positive ratio indicates that a greater number of postural changes occurred in the prototype ‘Experimental’ chair compared to school chair and a negative ratio indicates that the prototype ‘Experimental’ chair had a smaller number of postural changes occurred compared to school chair.

Statistical differences between the number of postural changes occurring on the experimental and control chairs were assessed using the Wilcoxon Matched Paired test.

3 Results

3.1 Sample demographics

All twelve invited subjects participated in the study. However the data of only eleven subjects could be analysed. The data from the 12th learner (S08, 14 year old male) was discarded, as poor visibility of some markers meant that the VICON Nexus software could not calculate the kinematics. For ease of identifying the learners according to their subject IDs, the numbering of the IDs was not changed, and this subject’s ID was simply omitted from reporting. The sample thus consisted of five boys and six girls. Table 3 describes each learner.

3.2 Sitting posture behaviour

The per-minute minimum and maximum median for the full testing duration for the pelvic, thoracic and head angle in all three movement planes on each test chair is reported in Tables 4 to 6. A negative value indicates a posterior pelvic tilt or head/thoracic extension, and a positive value indicates the opposite direction.

3.3 Number of postural changes

Figure 5 provides an example of differences in the three-dimensional pelvic angle for one subject. In order to be more concise, we focused on reporting the ratio of the number of movements. A positive ratio indicates that a greater number of postural changes occurred in the prototype ‘Experimental’ chair compared to school chair and a negative ratio indicates that the prototype ‘Experimental’ chair had a smaller number of postural changes occurred compared to school chair.

Table 7 describes the ratio of the number of postural changes (total of all categories of data combined) of all three angles, for all subjects, between the test chairs. Pelvic rotation illustrated a substantially higher ratio (∞) of postural changes, as well as a higher thoracic rotation ratio (2.0) in the ‘Experimental’ chair, compared to the control chair. Head rotation (–0.6) in the control chair indicated an increase in the number of head rotation movements compared to the ‘Experimental’ chair.

There were statistically significant increases in the number of postural changes on the ‘Experimental’ chair compared to the school chair, for both pelvic side-flexion and pelvic rotation (ρ= 0.01) (See Table 8).

Count ratio of the number of postural changes and IQRs for the entire sample: The count ratio comparison for the IQRs and the histogram data for the entire sample, for the pelvic, thoracic and head angles are presented in Table 9. In general, the IQR was greater for all three angles in all three of the movement planes in the ‘Experimental’ chair. The pelvis showed a higher number of postural changes in all three movement planes, with most postural changes occurring in rotation.

4 Discussion

To our knowledge this the first published study, that tested the frequency of 3D postural change in adolescents whilst sitting to undertake standard computing tasks over 15 minutes on a dynamic, experimental chair compared with a plastic control chair, similar to the one used in computing laboratories in Western Cape high schools. This study found that the ‘Experimental’ chair produced significantly more postural adjustments in all three of the movement planes for the pelvis, as well as the transverse plane of the thorax, and markedly more rotational movement, than the control chair. This paper contributes new information for the sparse research evidence-base on school chair design. Further research is now needed to test whether the increased frequency of spinal movement is associated with fewer reports of spinal pain.

The pragmatic approach taken to analyse and compare the pelvic, thoracic and head angles between chair conditions, was in response to the large volume of data, and to best present meaningful summaries of the differences between the two chair conditions. We have reported this approach previously [62]. As was anticipated because of the large variability in anthropometric measurements between learners, a large difference was detected in the minimum and maximum median values per subject, with (for one subject (S02)), the difference being up to 93.2 degrees in head flexion/extension whilst sitting in the ‘Experimental’ chair. Since neither chair had a backrest during testing, the reduced movement observed in the school chair could not have been due to a backrest. As chair backrests are used in the school computer laboratories, the amount of movement observed in the school chair may actually be an overestimate of the actual movement occurring whilst sitting in the school environment. Future studies should consider using wireless sensors, which would address the current limiting nature of marker visibility. Wireless sensors would also be useful when capturing data in students’ usual ‘real life’ school environment.

The differences between chairs was most obvious in pelvic side-flexion and rotation, which showed the greatest increase in frequency of postural changes on the ‘Experimental’ chair.

Research into the effect of dynamic sitting on the spine has proposed that pelvic rotations have a positive impact on the spine by varying the load of the spinal structures, which, in turn, serves to decrease the number of compressive forces on the disc [64 –66]. This is believed to aid disc height recovery and increased physical stature [64 –66]. The increase in stature is seen as a positive effect of pelvic rotation in sitting, as it reflects an influx of fluids and, consequently, of nutrients into the avascular disc [67]. Although the findings from our ‘Experimental’ chair are promising, in that larger pelvic movements occurred compared to the standard control chair, more descriptive and prospective studies are needed to ascertain whether the experimental chair effects produce similar positive influences on the underlying spinal structures.

Moreover, the greater range (IQR) of movement, which occurred in the ‘Experimental’ chair than the control chair, may have occurred because the dynamic nature of the experimental chair provided subjects with the opportunity of moving through a larger range of movement. This might encourage students to move into end-range positions of the neck, thorax and pelvis, which might strain spinal structures and, consequently, lead to pain [46]. It is therefore important to determine in future prospective studies, what the effect of this increased range of movement, as well as the increase in the number of movements, may have on the prevalence of pain in learners.

Increased pelvic movement also appeared to facilitate movement of the thoracic spine, as we found more thoracic postural changes in the ‘Experimental’ chair compared to the school chair. Prolonged computer usage is typically associated with pain in the thoracic region [68]. The enhanced frequency in postural change may also be advantageous, as it could help to prevent stiffness of the spinal structures, which could also lead to pain [69].

The head was the only segment that showed an increase in the number of postural changes in the control chair compared to the ‘Experimental’ chair. Although spinal movement is advocated, it is not advisable to limit movement to only one segment (like the head), as this can lead to localised strain of the neuromusculoskeletal structures, and, consequently, to pain [70]. The reduced frequency of head movement in the ‘Experimental’ chair could have been due to the unstable nature of the chair, which in turn would require subjects to seek stability in one body segment. This might have been achieved by maintaining the head in more or less the same position.

The fact that a statistically significant increase in frequency was only observed for pelvic rotation places limitations in terms of our expectations of what a dynamic chair can offer. Simply giving someone a dynamic chair does not mean that they will change their posture more frequently than in a chair without a dynamic seat. One implication is that behaviour needs to change to encourage movement. A comprehensive approach that includes education, exercise and feedback for instance is therefore still important [71]. The effect of providing learners with a dynamic chair and education on postural movement needs to be assessed in future studies.

5 Limitations

This validation study used a small sample of learners, which limits generalizability. However, given the measurement protocols, which were developed, and the indications of differences, which might be found between the test chairs, larger studies with adequately calculated sample sizes can now be undertaken. We have concerns about a possible overflow of fatigue from the first testing condition (control chair) to the second condition (experimental chair), despite our attempts to reduce fatigue by allowing a 15 minute rest period between testing. In future studies, randomisation of the chairs is advised. Due to the lack of adjustability of the school chair, the height of the chair could not be set at an optimum seat height for each individual learner. We did however adjust the seat height of the “Experimental” chair to fit the anthropometric profile of each learner and thus have him or her sit in a more optimal position. A future study question would be to ascertain if the differences between chairs were influenced by preferred chair height.

Whilst the chair allowed movements in all directions, it is of note in this small sample that a statistically significant increase in frequency was observed only for pelvic rotation. This may not be found in subsequent larger samples. However this finding could flag limitations in terms of our expectations of what a dynamic chair can offer. Simply giving someone a dynamic chair does not mean that they will change their entire posture more frequently, than in a chair without a dynamic seat. Maybe the implication is that behaviour needs to change to encourage movement. A comprehensive approach that includes education is therefore still important.

6 Conclusion

In conclusion, we report encouraging findings from our small sample validation study of an experimental chair based on adolescent anthropometrics. This information is a small; first and necessary step in testing the effect of increased postural movement on the prevalence and intensity of musculoskeletal symptoms of high school learners in the Cape Metropole area, Western Cape, South Africa.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

The authors would like to thank the Medical Research Council (MRC) for providing funding for this project.

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