Children spend a considerable part of their daily life in school. Children spend about 80% of their school time in the classroom performing various activities like reading, writing, drawing and other related activities, which require them to sit continuously for long hours [1]. School children are at special risk of suffering backache due to the prolonged periods spent seated during school and the formation of poor postural habits [2]. Therefore, they feel discomfort/pain in different parts of the body. Due to various discomfort, the students become inattentive, restless and fickle minded during their performances in the classroom. The origins of work-related musculoskeletal disorders are commonly associated with non-optimal postures resulting in postural stress, fatigue, and discomfort [3]. Comfort is a pleasant state of physiological, psychological and physical harmony between a human and its environment [4]. Therefore, a human compatible working environment should be given because it enhances the attentiveness and satisfaction. Sitting is the main posture of the students while attending the class. Greater contact with the support surface plays an important role in maintaining a comfortable stance which may be applicable to sitting [5]. Sitting in the same posture for a long time puts an extremely undesirable physiological strain on the muscles, ligaments and on the discs [6,7]. The muscle strain can be evaluated by analyzing electromyography signals. There were significant increases in the level of activation (integrated EMG) of the postural muscles when ischiofemoral contact was reduced [8]. The peak movement velocity of EMG voltage was faster when subjects were standing normally than when they were seated [9]. The base of support in sitting is substantially larger than in the standing position. Thus the task of maintaining the center of mass projection within boundaries of the base of support is less challenging [2].

The students of the primary school use bench and desk in the classroom in general. However, in a large number of schools, especially in rural areas, the students used to sit in the floor in the classroom due to infrastructural limitations as well as to maintain the traditional custom. Sitting on the floor with folded knees is an Indian traditional sitting posture. The students adopt this posture in the classroom. In addition to that the students also used to sit on the floor with stretched legs during attending the class. In the present study efforts have been made to evaluate the posture related problems of the students in different postures in the classroom as well as to find a suitable posture for the primary school children.

Selection of sites and subjects

The rural areas of West Bengal state (India) were the target areas of the present investigation. For conducting the present study, 20 rural secondary schools were selected at random from the randomly selected 9 districts out of 19 districts of West Bengal State (India).

The study was carried out in randomly selected ten rural primary schools, which were located at remote villages in West and East Midnapore districts, West Bengal (India). For conducting the present study, 10 rural primary schools were selected randomly. A total of 106 primary school boys having age range of 6 to 10 years were selected at random from the chosen schools for this study. Among the 106 children, 46 children were selected who used to sit on the bench and other 60 children were selected who used to sit on the floor in the classroom. The students, who were used to sit on the floor, generally adopted two sitting postures i.e., sitting on the floor with folded knees and sitting on the floor with stretched legs. Some of the variables, viz., body joint angle, center of gravity, electromyogram and base contact area were studied on 15 children who are randomly selected, on the basis of alphabetical order of their names, from the already selected subjects. The numbers of subjects selected for study of different parameters are mentioned in respective tables. The study was approved by the institutional human ethical committee. Consents for participating in the study were taken from the parents of the selected students.

The postures selected for this study were sitting on the bench, sitting on the floor with folded knees (Indian traditional posture), and sitting on the floor with stretched legs and standing erect as reference posture of the selected children. The bench, which was used by the students in the rural schools, was taken for the study. The bench had no back rest or footrest and the height of the bench higher than it was required for most of the students.

Evaluation of Musculoskeletal disorder

The musculoskeletal symptoms appeared during adopting different sitting postures were evaluated by the modified Nordic questionnaire technique [10]. During this study, the already selected children (n=60) who were adopting sitting postures on the floor were asked to maintain two sitting postures that were sitting on the floor with folded knees (38 children participated) and sitting on the floor with stretched legs (22 children participated) for duration of a single period (about 40 minutes). The same method was used for the 46 students who were performing their class work by sitting on the bench. The questionnaire was modified in simpler form in local language.

Discomfort rating of the students

The assessment of discomfort was administered with body mapping method where the perceived rate of discomfort was referred to a part of the body. In this study, the discomfort was assessed between the two sitting postures, i.e., sitting on the bench (n=46) and sitting on the floor (n=60). A 10-point scale was used for this purpose, which was graded from no discomfort at all to maximum discomfort [11]. According to the degree of severity, the scores of the 10-point scale were divided into three subgroups, i.e., mild (1–4), moderate (>4–7) and severe (>7).

Joint angle study in different sitting postures

Different joint angles of primary school children were measured by a goniometer in different postures during attending the class. The center of the goniometer was adjusted over the estimated center of the joint and the reference arms were aligned with the long axes of the adjoining body segments during measurement of different joint angles of the body.

The following body joint angles were measured: Neck flexion angle, Shoulder joint angle, Elbow joint angle, Hip joint angle, and Knee joint angle.

Recording of EMG

Electromyography signal shows its amplitude which is proportionate to the force exerted by the underlying muscle. RMS or Root Mean Square is a technique for rectifying the raw signal and converting it to an amplitude envelope, to make it easier to view. It represents the mean power of the signal. Study of the myoelectric activity of back and shoulder muscle revealed the stress on the muscles during adopting different sitting posture.

EMG voltages of back muscles (Latissimus dorsi, and External oblique) were recorded from the children with the help of BIOPAC system. At first electrodes were placed over the back muscle at latissimus dorsi and external oblique following the application of BIOPAC electrode gel. The electrodes were also placed on the shoulder muscle (trapezius muscle) in the same way. Then EMG and RMS values were recorded in the computer system by using BSL Pro 3.7 software of BIOPAC system. The EMG was recorded in four postures mentioned earlier.

Measurement of Base contact area

The base contact area of the body of the students with bench or floor was determined in four different postures, viz., standing, sitting on bench, sitting on the floor with folded knees, and sitting on the floor with stretched legs. White art papers were placed on the sitting / standing surface on which the subjects were asked to sit or stand. The outer contour of the contact area was drawn on the white art paper. The drawn boundary areas were measured with a planimeter by dividing the whole contact area into several subdivisions.

Measurement of centre of gravity

The whole body centers of gravity (CG) of the students were determined by segmental method [12] in normal erect posture, which was taken as reference posture, and in different sitting postures. The location of CG was compared between normal erect and other sitting postures. The location of whole body CG was determined by the following equation:

Percentage of location of CG = (Length of the CG from the ground / Total length of the body) x 100

Statistical analysis of data

Data were summarized into mean and standard deviation values using Microsoft Excel. The level of significance of difference between group means was determined performing t-tests [13].The ANOVA study and post-hoc analysis were made by the use of ORIGIN 6.1 software. Correlation coefficient between two variables was computed by the same software. To find the significance of difference in frequencies between two groups χ² test was employed.

The results of different parameters were analyzed for different sitting postures, viz., sitting on the bench, sitting on the floor with folded knees, and sitting on the floor with stretched legs (Figure 1) on a comparative basis. Sometime standing posture was also compared with sitting postures.

Figure 1: Different postures adopted by primary school children.

The occurrence of discomfort / pain in different segments of the body was found to vary in different postures among the primary school children (table 1). It was noted that the problems in the upper and lower back were the most prevalent and the occurrence of pain/ discomfort was significantly higher (p<0.001) among the children while sitting on the bench. The percentage of occurrence of neck problem was significantly lower (p<0.001) when the children sat on the floor than that of sitting on the bench. A lesser number of students reported thigh and knee problems during sitting on the floor with stretched leg than that of sitting on the bench.

Table 1: Prevalence of discomfort / pain reported by the primary school children in different sitting postures (the values in parenthesis indicate the percentage of discomfort) (n=106).

table 2 represents the perceived rates of discomfort of primary school children in two categories of sitting postures, viz., sitting on the bench and sitting on the floor. It was revealed that during sitting on the bench, the perceived rate of discomfort was comparatively higher than that of sitting on the floor. The comparison between two sitting postures showed that the children had significantly higher degree of pain / discomfort in neck (p<0.001), shoulder (p<0.001), upper arm (p<0.01), lower back (p<0.001) and buttock (p<0.001) during sitting on the bench than that of the children who used to sit on the floor. The mean value of scores (perceived rating of discomfort) of all segments was taken as the overall discomfort rating of the children. The overall rating of discomfort also indicated that the students adopting both category of sitting posture had moderate degree of discomfort and it was significantly higher (p<0.01) in the children who sat on the bench than that of the children who used to sit on the floor by folded knees.

Table 2: The perceived rate of discomfort (mean ± SD) in different segments of the body (in a 10- point scale) (n=106).

From table 3, it was observed that the position of CG (49.85%) was higher during sitting on the bench in comparison to other sitting postures, i.e., sitting on the floor with folded knees (41.24%) and sitting on the floor with stretched legs (45.16%). It was significantly lower (p<0.001) during sitting on the floor either with folded knees or with stretched legs than that of sitting on the bench.

Table 3: Location (Mean ± SD) of whole body CG in standing and different sitting postures adopted by the children (n=15).

Different body joint angles of 15 randomly selected children were measure for different sitting posture and standing posture have been presented in table 4. The deviation of each joint angle in different sitting postures from that of the reference posture, (standing erect) was calculated. The deviation of neck angle from the reference posture was greater during sitting on the bench (-34°) than that of sitting postures either with folded knees (-14.9°) for with stretched legs (-8.07°). Form the results (table 4) it was also observed that the deviation of the shoulder joint angle (Right side -57° , Left side -51.9°) and elbow joint angle (Right side 53.5°, Left side 54.5° ) were greater during sitting on the bench than that of sitting on floor with folded knees and sitting on floor with stretched legs. The subjects had larger hip joint angles during sitting on the floor than that of sitting on the bench. It was noted that the hip joint angles formed during sitting on the bench was lesser than 90˚, which indicated forward bending during attending the class. On the other hand the hip angle during sitting on the floor was slightly greater than 90°.

Table 4: Mean ± SD of joint angles (in degrees) of primary school children during adopting different postures (deviation means change from the standing posture) (n=15).

Deviation of knee joint angle is much lesser in case of sitting on the bench than in case of sitting on the floor with folded knees. However, in case of sitting on the floor with stretched leg there was least deviation of knee angle

The recorded EMG voltages of shoulder and back muscles, as shown in Figure 2 and Figure 3, are exhibited distinct variation in different postures. From the analysis of the results (table 5 and table 6), it was revealed that the EMG voltage recorded from the back muscle (Latissimus dorsi and External oblique muscle) and the shoulder muscles (Trapezius muscle) of the children were changed with the variation of posture. The analysis variance showed that there was a significant variation (p<0.001) of EMG voltages of shoulder and back muscles with the change of posture. The RMS values also showed similar results. Form the post-hoc analysis it was observed that the EMG voltage the back and shoulder muscles showed significantly lower values (p<0.01) during sitting on the floor with folded knees as well as with stretched legs than that in sitting on the bench.

Figure 2: EMG records (5-5000 Hz) taken from the back muscle (external oblique) of school children in different postures. Panel-I: right side and Panel-II: left side (A- Standing; B- sitting on the bench; C: Sitting on the floor with folded knees; and D: Sitting on the floor with stretched legs).

Figure 3: EMG records (5-5000 Hz) taken from the shoulder muscle (trapezius muscle) of school children in different postures. Panel-I: right side and Panel-II: left side (A- Standing; B- sitting on the bench; C: Sitting on the floor with folded knees; and D: Sitting on the floor with stretched legs).

Table 5: Mean ± SD, F-value of ANOVA and Post hoc analysis of EMG voltage and RMS values of back muscle of right and left side of the body among the primary school children (n=15).

It may be pointed out that the EMG parameters of shoulder had lower values in case of sitting on the floor with folded than that in sitting with stretched legs; however, the results were not statistically significant except for RMS values of the right side of the subjects (p<0.01). The RMS values of EMG also showed similar trends of results.

Table 6: Mean ± SD, F-value of ANOVA and Post hoc analysis of EMG voltage and RMS values of shoulder muscle of Right and Left side of the body among the school children (n=15).

The base contact areas between the body and the seat on floor have been presented in table 7. The results of ANOVA showed that there was a significant variation (p<0.001) in base contact area among different postures. The contact areas in all sitting conditions were significantly greater than that normal standing posture. The contact area in sitting with folded legs was significantly higher than that of the sitting on the bench (p<0.001).

Table 7: Mean ± SD, F-value of ANOVA and Post hoc analysis of base contact area of school children in different postures (n=15).

There was a significant negative correlation between the base contact area and the EMG voltage of the students (Figure 4). There was a significant negative correlation between the base contact area and the EMG voltage of the students (table 8). The inverse correlation between base contact area and the EMG voltage indicated that the electrical response of muscles was increased when the base contact area was decreased. There was also significant negative correlation between the base contact area and the percentage of location of whole body CG of the students (table 8).

Figure 4: Graphical representation of relationship between EMG voltage (mv) of Back and Shoulder muscle, and Base contact area (sq.cm) during different postures adopted by primary school children.

Table 8: Correlation coefficient and t-value between Base contact area (sq.cm) with EMG (mv) and percentage of location of CG.

The primary school students reported musculoskeletal symptoms differentially in different sitting postures during attending classes. It may be pointed out that the occurrence of the segmental discomfort / pain was considerably lower in the students during adopting sitting posture with folded knees or stretched legs than that of sitting in the bench. The ill-fitted furniture forced the school children to do class work in awkward postures (lateral bend, forward bend, twisting, etc.) for long times, which imposes physical and mental strain on the children [14]. Problems with the lower limbs may be due to the less compatibility between the bench height and the popliteal height of the school children. Due to high bench height and the absence of a footrest, the school children are forced to sit on the front edge of the benches, hanging their legs for a long time. A seat that is too high compresses the soft tissues of the underside of the thigh [15], creating pressure that interferes with the return of blood from the lower limbs. It may cause discomfort/problems in their thighs, knees, and feet [16 ]. In addition, there is no backrest for the seats. This may intensify the problem of backache. The mismatches of body dimensions and furniture dimensions may lead to quick changes of posture because of the imbalance and/or discomfort of the children’s bodies while using the classroom furniture. To achieve temporary relief from these discomforts, they frequently change their sitting positions during class. They are also compelled to sit in various inappropriate postures. The greater the mismatch, the greater the frequency of posture change will be. Variations in the frequencies of posture changes in different age groups may be due to the variations in the degree of such mismatches [14].

When the children sat on the floor there was no question of fitting the furniture with body dimension of the user. In other words they were not required to adjust their body with the bench. They can sit freely on the floor. These were the probable reasons for low occurrence of discomfort or pain among the students when they sat on the floor.

The prevalence of musculoskeletal symptoms in lower back and the perceived rate of discomfort in lower back were notably higher during sitting on the bench than that of sitting on the floor. When the children sat on the floor, they could maintain their posture as they desired. A free posture allows higher capability compare to fixed postures in which restrictions are typically imposed on the postures that can be assumed [17]. Dramatically lower incidence of back pain has been found among native Indian jungle dwellers who customarily sit on the floor or squat: a reported zero incidence of back pain [18]. So, the lower back pain was much lesser while sitting on the floor than sitting on the bench. This might cause lesser stress on the spine. The greater prevalence of discomfort in legs and thigh during sitting on the floor with folded knees might be due to compression of leg and thigh muscles as well as imposing body weight on these areas. Extreme flexion of lower limb at knee joints probably another associated factor for this.

The greater deviation of the CG during sitting on the floor with folded knees (16%) from that of the reference location than other sitting posture might be due to adopting slightly forward bending posture during performing their class activities. Sitting slight forward bending position provides a more stable posture and eliminates the impact of lower limb discrepancy, and therefore may be considered a recommended position [19]. The shifting of CG towards upper side of body makes the body unstable and leads to impose postural load [12]. So, due to higher location of CG during sitting on the bench, the children were forced to adopt unstable posture and this might impose static load on certain groups of the musculo-skeletal system. This might be the probable reason for higher prevalence of musculoskeletal discomfort among the children. But when the children sat on the on the floor with folded knees, the location of CG became lowered indicating higher stability of the body. So from the above observation, it appeared that sitting on the floor was better than sitting on the bench in terms of CG.

The lower deviation of neck angle during sitting on floor than that of sitting on the bench indicated that there was a lower degree of neck flexion during performing class activities. Thus it imposed comparatively greater musculoskeletal stress in neck region during sitting on the bench. This might be one of the causes for higher rate of perceived discomfort in the neck while sitting on the bench. It may be stated that the children had lesser shoulder abduction while sitting on the floor than that of sitting on the bench. It indicated that this posture (sitting on the floor) might be helpful for reducing discomforts in the upper arms of the children. The hip joint angle during sitting on the floor indicated better posture than that of sitting on the bench. As the angle was lesser than 90˚ during sitting on the bench it appeared that the children had to adopt forward bending posture. Sitting in the same posture in a forward bending position for a long time puts an extremely undesirable physiological strain on the muscles [7]. Several investigators pointed out that postural load on thigh muscles increases due to progressive increase of knee joint angles [20]. So from the above observation, sitting on the floor is better than sitting on bench in terms of body joint angles.

Significantly lower EMG values during sitting on the floor with folded knees in comparison to sitting on the bench might be due to lesser level of muscular contraction. The peak movement velocity of EMG voltage was faster when subjects were standing normally than when they were seated [9]. Other factors might be related to the reduced myoelectric activity.

The higher base contact area during sitting on the floor might be due to the higher surface area of buttock and thigh regions [21]. The largest base contact area was found in case of the students sitting with folded legs. Larger contact and support helps to sustain the forward tilt of the pelvis, which restores natural spinal curvatures to improve posture and muscle balance, thereby reducing fatigue and increasing endurance [2]. The base contact area of sitting on the floor with folded knees was about 55% higher than that of sitting on the bench. Increased base contact area in sitting on the floor yielded greater stability of the body in comparison to the sitting on the bench.

Sitting on the floor with folded knees is a traditional Indian sitting posture. In many of the day to day activities are performed in this posture by the pupil, particularly in rural areas. It was found that sitting on the floor with folded knees and stretched legs had lower rate of discomfort in different parts of the body than that in sitting on the bench.

During sitting on the floor with folded knees the stability of the body might be increased. The results of the study of base contact area supported this view. Increased base contact area in sitting on the floor yielded greater stability of the body in comparison to the sitting on the bench. The electrical response of the postural muscles would be increased if the base contact area is decreased. On the other hand, the greater base contact area represents the lower value of percentage of location of whole body CG. Lower value of CG denotes downward location of CG, which is helpful for maintaining the higher equilibrium state of body. Hence, sitting in the floor with higher base contact area provide greater body stability.

From the study of the joint angles, the children had lesser shoulder abduction and hip angle of slightly greater than 90° while sitting on the floor than that of sitting on the bench was indicating better posture than sitting on the bench.

From the results of the present study, it revealed that location of CG became lowered during sitting on the floor with folded knees indicating higher stability of the body. Increased base contact area in sitting on the floor yielded greater stability of the body in comparison to the sitting on the bench. The body joint angle study also showed that there was lesser degree of neck flexion as well as lower shoulder abduction during sitting on the floor than sitting on the bench. The lower value of EMG of back and shoulder muscle also support the fact. So it appeared that sitting on the floor with folded knees required lesser effort and was more comfortable than sitting on the bench. Further, the children were not required to adjust with the furniture indicating the greater freedom for maintain the sitting posture. In addition to that the school authority may be relieved from the cost of furniture, which is an important matter for rural schools. Hence, sitting on the floor with folded knee posture may be recommended for the rural school children in the classroom and the cost of the classroom furniture can be minimized.