The optimal design of school desks depending on the height and weight of students

Fikret Veljovic, Avdo Voloder, Senad Burak, Benjamin Kulovac, Reuf Karabeg 1 Faculty of Mechanical Engineering, University of Sarajevo, Sarajevo, Bosnia and Herzegovina 2 Faculty of Medical Studies, University of Sarajevo, Sarajevo, Bosnia and Herzegovina 3 Private Clinic «Karabeg», Sarajevo, Bosnia and Herzegovina 4Faculty of Medical Studies, University of Sarajevo, Sarajevo, Bosnia and Herzegovina


Introduction
New approach to the design of the school bench should be based on rational utilization of the working surface as a function of pupils' spinal column loading. It should be taking care of ergonomic principles, optimal consumption of material, furniture price and pedagogical effects [1].
It was shown that most of the tables and chairs were not made in accordance with the ergonomic considerations. This results in a number of health problems, especially as Samuel, Joel, Freivalds found that primary school students spend over 30% of their time at school, mostly sitting at desks [2]. On the other hand, Rungtai Lin and Yen-Yu Kang concluded in their study that the usual school chairs and desks were not consistent with the fact that the classes were not homogeneous in terms of student body weight and that more and more students were overweight [3].
This paper covers an innovative study of the optimal positioning of desktops for students of different body masses using advanced numerical analysis and computer simulations of the CATIA software package (Dassault Systèmes, Vélizy-Villacoublay, France.

Aim
To present the optimal design of the student's school desk by ergonomic analysis using numerical optimizations and computer simulations and to determine solution for improving working conditions and prevention of musculoskeletal disorders.

Methods
This research presents an ergonomic analysis performed in the CATIA software package (Dassault Systèmes, Vélizy-Villacoublay, France). The analysis is performed on a total of 5 subjects of the same height (180 cm) and various weights (60, 70, 80, 90, 100 kg). The task is to determine at which weight and at what angle of the workbench with standard height will not exceed the permissible loads of the spine, specifically referring to the L4/L5 vertebrae whose stresses should not exceed 3400 N In the work, The RULA Assessment Tool was used to analyze and interpret proposed solution.

Results
The ideal desk design for students and the values of the optimum slope of its surface for safe student work that does not compromise the permissible maximum spine load of 3400N are calculated for different body mass values using the CATIA V5 -R18 software (Dassault Systèmes, Vélizy-Villacoublay, France).   Other results were analyzed and for all possible variants are presented in Table 1.
Solving equation (2) we get that m gr = 82.638 kg (3) meaning that the weight of the subjects for this case must be less than 82.638kg.
The limit mass for which the force FL=3400 N is obtained is similarly obtained for the curve relating to the slope of the desk from α = 20 0 :

Loads in the lumbar L4-L5 vertebra -FL (N)
Tilt angle of

Discussion
The work environment in which students spend most of their time in schools and colleges is most often characterized by work chairs and desks in classrooms [4], [5]. The desk at which students sit and work in the classroom is considered an important element in improving their concentration and overall learning process [6]. An ergonomic analysis of a desk with an optimal design of the position of its surface is therefore considered to be a very important aspect both from the point of view of raising the efficiency of the teaching process and from the point of view of the optimal environment for healthy student development. In doing so, the design of chairs and tables must comply with ergonometric principles as well as the biomechanical characteristics of the human body [7]. We found in our research that persons weighing up to 82.638 kg can safely work at a desk whose work surface is tilted 0 0 , without compromising the permissible spine load of 3400N. Also, research showed that persons weighing between 82.638 kg -89.800 kg can safely work at a desk whose worktop is inclined 20 0 without compromising the permissible spine load of 3400N. Finally, our study showed that persons weighing between 89.800 kg and 99.960 kg can safely work at a desk whose worktop is tilted 30 0 , without compromising the permissible spine load of 3400N. The disadvantage of student sitting caused by inadequate chair and bench design results very often in the overload of the student's spinal column, and consequently the resulting skeletal deviations that can result from it [8]. It has been found that more and more problems occur when office workers are sitting in a modern business environment [9]. Daneshmandi et al. state that office workers are in a sitting position for about two-thirds of their working hours, with neck, lower back and shoulder symptoms being the most prevalent problem among workers [10], [11], [12]. The recent study [13] showed the mismatch between university tables and chair dimensions and anthropometric characteristics of students in classrooms.

Conclusion
Based on the results of the research, appropriate measures can be recommended when planning the procurement of desks in schools and colleges, especially in the situation where student population is increasingly characterized by overweight, including those over a critical size of 82,638 kg.
The proposed ergonomic design of the desk will result in students being adequately positioned during their activities at school with the minimal risk of permanent deviations and other health problems. Considering the above analysis, it would be advisable to introduce the possibility of correcting student desks in classrooms and amphitheaters in accordance with their anthropometric values.