Guidelines for schoolbag carriage: An appraisal of safe load limits for schoolbag weight and duration of carriage

1 Introduction

Schoolbag weight is the main factor that has been analyzed in an effort to develop guidelines for the safe carriage of schoolbags [1–6]. However, there is no global agreement on the cut-off point for a schoolbag weight limit. The current guideline in Europe and Australia is that children can carry a schoolbag that weighs up to 10% of their own bodyweight [7–11]. In the United States, the American Occupational Therapy Association recommends 10% bodyweight as the upper limit [12], the American Physical Therapy Association recommends 10–15% bodyweight [13] and the American Academy of Pediatrics recommends 10–20% bodyweight [14]. Despite these guidelines, there is inconsistent evidence for their use [5, 15–18] and little assessment of their utility.

The motivation for the development of schoolbag weight guidelines has been a proposed link between the bag weight and musculoskeletal discomfort. However, the relationship between schoolbag weight and musculoskeletal discomfort is not always clear as it has been found that the percent bodyweight (% BW) carried was both associated [5, 19] and not associated [20, 21] with musculoskeletal discomfort. Previous studies are mostly cross-sectional [10, 29] and therefore have not been able to establish a dose–response relationship between schoolbag carriage and musculoskeletal discomfort. Additionally, they have not evaluated the discriminative ability of the proposed schoolbag weight cut-off values for identifying children with schoolbag-related discomfort. A recent study of the dose-response relationship between schoolbag weight and schoolbag-related discomfort concluded that the weight limit guidelines should be reviewed [22]. Additionally the focus of earlier studies has primarily been on back pain, but evidence indicates that schoolbag-related shoulder pain may be a greater problem than back pain[4, 22].

The length of time that a child spends carrying a schoolbag has had scant attention, and it is not a specified factor in the guidance on schoolbag carriage. Although the duration of carriage has been reported [1, 23–28], the systematic assessment of the utility of cut-off values for duration of carriage in identifying schoolbag-related discomfort has not been undertaken.

Schoolbag weight and duration of carriage have been investigated within the same studies, but independent of each other [1, 26–28]. However fundamental biomechanical principles of load and energy consumption suggest that schoolbag weight, and the exposure to that weight, should be investigated in combination with each other in order to acquire a measure of the mechanical burden that results from carrying a schoolbag. Different loads carried for a fixed [29] or pre-determined [30] length of time have been examined, and the issue has been discussed in general terms [25, 31]. However the effects of schoolbag carriage that simultaneously take these two variables into account, as they increase incrementally, have not been investigated.

Based on a gap found within the evidence literature the aim of this study therefore was to explore the concept of a maximum safe limit for schoolbag weight and duration of schoolbag carriage. The objectives were to establish if there was a threshold beyond which musculoskeletal discomfort was significantly increased that could be based on (i) % BW of a schoolbag; or (ii) duration of carriage; or (iii) the combined % BW and duration i.e. the combined mechanical burden effect of carrying a schoolbag.

2 Method

3 Results

The data analysed in this paper was from a discrete subgroup of the original participants in our prior report [22]. Eligibility for inclusion here was limited to children who carried only a schoolbag on the assessment day. The findings therefore are quite distinct from those reported elsewhere [22].

A total of 462 children were included in the analysis of child demographics and schoolbag characteristics. The mean age of the children was 10.6 years±6.9 months, and the sample comprised 53% males and 47% females. The majority (94.6%) had a backpack, 4.5% had a shoulder bag and 0.9% had a schoolbag on wheels. The majority (85%) carried their schoolbag over two shoulders. Not all children completed the BDC and VAS before and after schoolbag carriage therefore fewer children were included in the analysis of schoolbag-related back discomfort (n = 409) and shoulder discomfort (n = 407).

4 Discussion

The current study tested existing guidelines for schoolbag carriage using a novel approach applied to a subset of the dose-response schoolbag carriage data reported in [22]. Similar to previous studies [10, 29] a cross-sectional study design was used for the weight and duration of carriage measurements. However in contrast to previous cross-sectional studies, the prevalence of schoolbag-related musculoskeletal discomfort was established using a quasi-experimental pretest-posttest study design. It was therefore possible in this study to establish the dose-response relationship between schoolbag carriage and musculoskeletal discomfort. The utility of conventional discrete schoolbag weight limits (10, 15, 20% BW) and the weight limit as a continuous variable were examined from different statistical viewpoints. Schoolbag carriage is a global activity that is undertaken by 668 million primary school children [40], therefore the potential impact of the findings is wide-reaching. The findings challenge the suitability of using % bodyweight as a guideline for children’s’ schoolbags and are in keeping with others [5, 41]. Additionally, given the increasing level of obesity in children [42] the calculation of a guideline based on % BW may not be an appropriate approach for individual children because their physical capacity or ability may not correlate with increases in their body weight. In this study, the 10% BW threshold demonstrated a good ability to identify true cases of schoolbag-related back or shoulder discomfort (sensitivity of 0.7–0.72). However, it had poor specificity (0.29) demonstrating a high false positive rate and illustrating its limited ability to be used as a load limit guideline [43]. That is, when the lowestconventional threshold (10% BW) was applied, over 70% of children with schoolbag-related discomfort were correctly identified, but only 29% of those who did not have discomfort were correctly identified. Thus the 10% BW threshold incorrectly classified 71% of those without schoolbag-related discomfort as being in the discomfort group. At the higher thresholds there was poor sensitivity at 15% BW (0.25–0.26) and 20% BW (0.03–0.07). That is, there was little or no ability to correctly identify true cases of children who experienced schoolbag-related discomfort. Furthermore, the probability of discomfort was similar irrespective of the relative schoolbag weight, and there was no evidence of a monotonic relationship between load and risk of back discomfort as the risk ratio (RR) was not higher at each incremental load threshold. These findings are similar to Jones et al. [44] who reported that schoolbag load did not contribute to the risk of developing low back pain in secondary school children. For shoulder discomfort there was a monotonic relationship between load and risk, but the relationship was not linear. The weakness of using % BW as a guideline was further demonstrated in the ROC curve analysis (Figs. 2 and 3) that showed the poor discriminative ability of any load to predict discomfort. In this study the AUC for % BW was close to 0.5 and the curve was close to the 45-degree diagonal of the graph which indicated that no threshold was more accurate at predicting schoolbag-related discomfort than a random guess. Similar analysis has not been undertaken previously, therefore comparisons with other studies could not be made, but these results provide substantial further evidence that relative schoolbag weight alone is not adequate as a guideline.

The duration of schoolbag carrying time was short, and was similar to that reported in the literature on primary school children [4, 27]. Additionally, longer carrying times have been reported in studies that included secondary school children [23–25]. In contrast to others [4, 23–25] the current study did not find a consistent and significant association between duration of carriage and discomfort. It was of interest to note a significant association for carrying time of less than five minutes and back discomfort; however there was no monotonic relationship between duration of carriage and discomfort. An explanation may be that lifting or putting on the schoolbag caused some initial back pain or discomfort and this was reported in the shortest time interval. Although the shortest duration of carriage i.e. <5 minutes, had the highest sensitivity (0.63–0.77) of the three periods of time examined, it had poor specificity (0.37–0.39). Therefore the ‘5 minute’ threshold correctly identified 77% and 63% of children with schoolbag-related back and shoulder discomfort respectively, but incorrectly classified 62% of children without schoolbag-related discomfort as being in the discomfort group. These findings demonstrate that duration of carriage would not be accurate as a guideline for identifying children with schoolbag-related discomfort. Furthermore, the time spent carrying was shown to be a poor indicator of discomfort, as demonstrated by the Area Under the Curve (AUC), but it was close to significance (p = 0.070) for back discomfort as shown in Fig. 2. The different age of the sample population may have contributed to the disparity between this and previous studies of older children [23–25]. It may be the case that duration of carriage is a predictor for schoolbag-related musculoskeletal discomfort in adolescents given that they carry schoolbags for a longer period of time during the day [45]. The discrepancy between this study and another on primary school children [4] could be explained by the difference in type of schoolbag used; very few children in this study used a bag on wheels. The methodological differences between this and previous studies are also likely to account for some discrepancies.

Overall, the level of evidence for duration of carriage as a risk factor for schoolbag-related musculoskeletal discomfort is weak. Previous studies assessed the duration of carriage subjectively by questionnaire. In this study the duration of carriage was also estimated by the children on the researcher-assisted questionnaire. However, the researchers attempted to improve the accuracy with a brief discussion to verify the response with each child as they arrived to school. Furthermore, the response was not influenced by recall bias. An objective measure such as a stopwatch would provide more accurate data as it has been demonstrated that adolescents were not accurate in their estimation of schoolbag carrying time [46]. The accuracy of primary school children in estimating carriage time has not yet been explored, and given the findings of this study further investigation is warranted. If an association between objectively-measured duration of carriage and musculoskeletal discomfort was established, the guidance on schoolbag carriage could be developed to include this as a modifiable risk factor.

A novel approach in this study was the calculation and analysis of the combined concurrent effects of schoolbag weight and duration of carriage. Although the CMB^bag had the best discriminative ability of the three variables, there was still no identifiable threshold with high sensitivity and specificity for CMB^bag that could be used as a predictor of back or shoulder discomfort. The calculation of the CMB^bag was based on the objectively collected data for % BW carried by the children, but the duration of carriage was determined subjectively. Further investigation with objective measures of duration of carriage should be considered in future studies.

For an appropriate schoolbag weight limit, it may be a more reasonable alternative to determine limits based on a risk assessment, similar to the approach used in manual handling for adults in the workplace [47]. For manual handling, the person is informed about the assessment of hazards and risks for musculoskeletal injury associated with manual handling [48]. Each person can then assess the risk and alter the modifiable hazards in order to minimise the risk and make the task as safe as possible. A risk assessment typically focuses on four domains: (i) the task, (ii) the individual person who is doing the task, (iii) the load and (iv) the environment [49]. For schoolbag carriage, the four domains would correspond to: (i) putting on/off and carrying the schoolbag, (ii) the child (characteristics, adequate training or knowledge) (iii) the schoolbag (characteristics, weight, frequency, duration, method of carriage and (iv) the environment (home, travel to/from school, school). In the case of primary school children, the parents and teachers could also be informed of the risk assessment of schoolbag carriage and could assist or advise the children. For example, parents could be instructed to assess for the correct fit of a schoolbag for their child, and advised to buy a schoolbag accordingly. Additionally, they could be advised about the optimum utilization of the schoolbag features, such as the shoulder strap length of a backpack. Previous studies that aimed to improve the knowledge and skills of children with regards to schoolbag carriage [50], posture while carrying a schoolbag [51, 52] and lifting in general [53] reported positive outcomes. Similarly, children who participated in back care education programs had significantly better knowledge [50–52] and practical skills [51, 53] after the program than they had before. In contrast, Feingold and Jacobs [54] reported no significant difference in children’s posture following a back care education session. The positive findings of the majority of these studies on back care education are encouraging and could have implications for the role of risk assessment and preventative strategies for schoolbag-related discomfort in children. The growing evidence for the association of psychosocial factors with musculoskeletal discomfort in children [9, 55] must also be acknowledged, and reflected in future studies.

A limitation of the study was the limited age range of the participants. Children in 4th and 5th class in primary school (age 9–11) were included in the study and the findings therefore may not be extrapolated to all schoolchildren. A further limitation was the subjective estimation of the duration of carriage, although measures were used to ensure that the estimation was as accurate as possible.

The findings of this study would imply that there is no merit in recommending guidelines for safe schoolbag carriage that are based on mechanical factors alone. A threshold of % BW, duration of carriage or mechanical burden that could be used as a reliable cut-off point to detect true cases of schoolbag-related discomfort could not be identified. However, the association between duration of carriage and the combined mechanical burden and back discomfort would be worth further investigation.

Although this study would not support specific weight limit guidelines, it does seem reasonable that advice on best practice for schoolbag carriage should be provided to children, parents and teachers. Carrying a well-designed and correctly-fitted backpack over both shoulders has been accepted as the optimum method of carriage in the real-world setting. It is recommended that future research should focus on factors other than schoolbag weight limit guidelines.