Effect of School Backpack Weight on Shape of Children’s Feet

Introduction

The school backpack is an essential student accessory in which school equipment is carried, including books, notebooks, and school devices, but also laptops, sportswear, and bottles. A review of the literature indicates that the weight of a children’s backpack is a constantly debated and controversial issue in the field of education and health [1–4]. Perrone et al [5], based on a critical analysis of publications in a narrative review on the impact of backpack weight on the health of school-aged children, concluded that the carrying school backpacks, especially at loads exceeding 10% of body weight, has biomechanical, physiological, and subjectively perceived discomfort effects on the user, in this case the student. Such effects can include abnormal posture and gait, physical discomfort, altered muscle activity, and increased respiratory rate. Some authors, such as Dockrell et al [6], Spiteri et al [7], Brzęk et al [8], Rodríguez-Oviedo et al [9], Barbosa et al [10], and Ramadan and Al-Tayyar [1], have reported that carrying backpacks that are too heavy, in the wrong way, can lead to spinal overload and asymmetry. According to Adeyemi et al [11], in children with obesity, it can further increase the load on the musculoskeletal system, while Arghavani et al [12] found it can be a factor limiting respiratory capacity and reducing the strength of the expiratory muscles during locomotion. Balko et al [13] indicated differences in the loading patterns of the dominant and non-dominant lower limbs under the load of excessive backpack weight, while Hell et al [3] found that carrying a school backpack weighing more than 4 kg causes changes in gait, muscle activity, posture, and stability, resulting in a backward shift of the body’s center of gravity.

There is a lack of research on the effect of backpack weight on the formation of a child’s feet. This is an important issue, as this part of the musculoskeletal system being the load-bearing element located lowest in the kinematic chain, takes on the highest loads. Therefore, we investigated in depth several issues related to the effect of school backpack weight on the delicate structure of children’s feet. While previous studies have examined the general effects of backpack weight on the musculoskeletal system in children, there is a notable lack of research specifically addressing how backpack load affects foot structure. This distinction is important because foot morphology plays a key role in overall posture, gait, and long-term musculoskeletal health. By focusing on this underexplored area, the present study addresses a clear gap in the literature, providing novel insights into gender-specific susceptibility and informing targeted interventions to prevent foot-related complications in children. The selection criteria for the study group included 10-year-old children who were fourth-grade primary school pupils, which is a transitional stage associated with new types of lessons that differ from those in the first to third grades. With the transition to the fourth grade, the number of lessons increases and lessons take place in different classrooms, which entails carrying a heavier backpack due to the increase in the number of subjects and the associated number of books, notebooks, school supplies, and other materials. We sought to fill a specific gap in the literature by examining school backpack weight and foot structure, and to analyze the effect of backpack load on foot shape in 10-year-old girls and boys.

Material and Methods

ETHICS STATEMENT:

The protocol and informed consent documents were reviewed and approved by the Bioethics Review Committee, University of Rzeszów (Ref. 3/12/2015). The examinations were conducted in conformity to the guidelines of Helsinki Declaration. Each parent/legal guardian received information about all aspects of their child’s participation in the study, including a clear and understandable description of the study’s purpose, research procedures, potential benefits and risks, and the researchers’ contact information. They were also assured of the voluntary nature of the child’s participation, the right to ask questions, and the right to refuse or withdraw at any time without any consequences, and the protection of personal data and confidentiality. The information provided a comprehensive overview of the study in a clear and understandable manner, so that each parent/legal guardian and their child, as a participant, could make an informed decision about participation. All parents and/or legal guardians provided written informed consent.

PROCEDURES:

The 2023 study included 119 children (61 girls and 58 boys) aged 10 years attending primary schools in south-eastern Poland. Subjects were selected randomly in 2 stages. In the first stage, schools were selected, while in the second stage, pupils from these schools were chosen. The sample size was estimated assuming a confidence level of 95%, a fraction size of 0.50, an acceptable measurement error of 5%, and a test power of 0.80. Based on these assumptions, it was estimated that 135 children would need to be eligible to participate. Figure 1 shows the flow of participants through the subsequent stages of the research.

The following inclusion criteria were used to qualify children for the study: age 10 years, using a traditional school backpack on a daily basis, designed to be carried symmetrically on both shoulders, right-footedness-lateralization of the lower limbs was determined by 2 tests: rolling a ball in a straight line and kicking a ball into a small goal. The lower limb chosen by the child for these tasks (right or left) was considered as dominant/preferred and the opposite limb as non-dominant [13,14], understanding of the instructions necessary to carry out the testing process, written informed consent of parents or legal guardians for the child’s participation in the study, the child’s consent and willingness to participate in the research.

Children with neurological diseases, musculoskeletal disorders, orthopedic diseases, previous orthopedic surgery, or pathological changes in the lower limbs were excluded from the study.

ANTHROPOMETRIC FEATURES MEASUREMENTS:

Body mass (with accuracy to 0.1 kg) and height (with accuracy to 0.1 cm) were measured using a medical column scale with a telescopic height measuring device with legalization MS 4971 (Charder, Taichung, Taiwan). The body mass index (BMI) was then calculated for each child.

SCHOOL BACKPACK WEIGHT MEASUREMENTS:

The weight of the school backpack (accurate to 0.001 kg) was measured using a precision scale WLC 60/C2/K RADWAG (Uniwag, Cracow, Poland). A single measurement was taken for each student’s backpack, with the result reflecting the actual weight of the backpack carried by the students on the day of the assessment. The percentage ratio of school backpack weight to body mass was then calculated for each tested child. The 10% of body mass limit as an acceptable backpack load was adopted based on studies reported in the literature. Numerous authors investigating external loads, including Avantika et al [15], Chen and Mu [16], Orantes-Gonzalez et al [17], and Tomal et al [18], indicate that 10% of body weight is a safe and acceptable upper limit for backpack load. Therefore, a cut-off value of 10% of body weight was used in the assessment of the backpack, considering values not exceeding 10% of body weight as normal, and values exceeding 10% of body weight indicated that the backpack was too heavy.

FOOT EXAMINATIONS:

Podoscopic foot examinations were performed twice using a CQ-ST podoscope (Electronic System, Czernica, Poland). During these examinations, the child was positioned on the podoscope plate in a relaxed position, with the head in the Frankfurt plane, the upper limbs hanging along the trunk, and the feet hip-width apart. The body mass was evenly distributed on both feet. The first examination was performed before putting on the school backpack, and the second examination was performed under conditions of loading with the weight of the school backpack. Before the second examination, the child put on the backpack. Each time, the examiner checked the correct position of the backpack by adjusting the length of the shoulder straps (braces), then the examiner placed the backpack on the child’s back so that the center of the vertical line marking the length of the backpack was at the level of the spinous process of Th12, in the so-called T12 position [16].

The following indices of foot shape were analyzed:

The foot shape indices were selected based on their frequency of use in the relevant literature [20–24].

All procedures were performed in the morning, with the children wearing gymnastics clothes, without shoes.

STATISTICAL ANALYSIS:

Statistica ver. 13.3 (TIBCO Software, Inc. Palo Alto, California, USA; StatSoft Poland Sp. z o.o., Cracow, Poland) was used for statistical analysis. The Shapiro-Wilk test was used to determine if a dataset is well-modeled by a normal distribution. Comparisons between girls and boys for anthropometric features, backpack weight, and foot shape characteristics were conducted using the t test for independent samples or the Mann-Whitney U test, depending on the normality of the data. The ratio of backpack weight to body mass according to sex was analyzed using the chi-square test. Comparisons of foot shape characteristics between the first and second examinations, performed separately for girls and boys, were conducted using the t test for dependent samples or the Wilcoxon signed-rank test, depending on data distribution. Finally, relationships between backpack weight relative to body mass and changes in individual foot characteristics between the first and second examinations were assessed using Pearson’s or Spearman’s correlation, according to the distribution of the variables. The level of statistical significance was assumed to be P<0.05.

Results

RESULTS OF ANTHROPOMETRIC FEATURES:

The data in Table 1 show that there were no statistically significant intersex differences in terms of body mass (P=0.736), body height (P=0.355) and BMI (P=0.752).

RESULTS OF SCHOOL BACKPACK WEIGHT MEASUREMENTS:

Data in Table 2 indicate statistically significant differences in girls’ and boys’ school backpack weight values (P=0.021). Girls’ backpacks were heavier than boys’ backpacks. There were no statistically significant intersex differences in the percentage values for school backpack weight in relation to body mass (P=0.069).

The data in Table 3 indicate that of the total study population, 73 children (61% of group), including 40 girls (66% of the girls group) and 33 boys (57% of the boys group), had a backpack that was too heavy in relation to the accepted cut-off value of 10% of body weight, and the frequency of having a backpack that was too heavy was not sex-specific (P=0.331).

RESULTS OF FOOT EXAMINATIONS:

In the second examination, girls had significantly higher values than in the first examination for right (P<0.001) and left (P<0.001) foot length, right (P<0.001) and left (P<0.001) foot width, right (P=0.003) and left (P<0.001) heel angle, and right (P<0.001) and left (P<0.001) angle of varus deformity of the fifth toe. By contrast, the values of Clarke’s angle decreased in the second examination compared to the first for both the right and left foot (P<0.001) (Table 4).

The boys in second examination also had statistically significant higher values for right (P=0.001) and left foot length (P<0.001), right (P<0.001) and left (P<0.001) foot width, right (P=0.003) and left (P=0.001) heel angle, right (P<0.001) and left (P=0.007) angle of the varus deformity of the fifth toe. For Clarke’s angle, the values in second examination were lower for both the right (P<0.001), and left foot: P<0.001 (Table 5).

The data in Table 6 show that there was no statistically significant sex-related variation in differences between first and second examination in foot shape characteristics (P>0.05).

Table 7 shows the correlations between the proportion of school backpack weight relative to body mass and the changes in individual foot characteristics measured before and after carrying the backpack (first versus second examination). In girls, the proportion of school backpack weight relative to body mass was positively correlated with the changes in foot width for the right (r=0.34; P=0.008) and left foot (r=0.41; P =0.001). The higher the school backpack weight relative to body mass, the greater the changes in foot width between first and second examination.

In both sexes, the values for school backpack weight in relation to body mass correlated negatively with the differences in Clarke’s angle values. The higher the school backpack weight relative to body mass, the smaller the changes in Clarke’s angle for the right foot. For girls, the correlation coefficients were R=−0.70 (P <0.001) for the right foot and R=−0.75 (P<0.001) for the left foot. For boys, the values of the correlation coefficients were R=−0.56 (P<0.001) for the right foot, and R=−0.62 (P<0.001) for the left foot.

In girls, backpack weight relative to body mass was positively correlated with changes in the heel angle γ for the right (R=0.34; P=0.007) and left foot (R=0.39; P=0.002). The higher the school backpack weight relative to body mass, the greater the changes in heel angle γ between the first and second examination.

In girls, backpack weight relative to body mass was positively correlated with changes in the hallux valgus angle α of the left foot (R=0.30; P=0.017). The higher the school backpack weight relative to body mass, the greater the changes in the hallux valgus angle α between first and second examination.

Discussion

Our results demonstrated that gender differentiated the weight of the school backpack – girls’ backpacks were heavier compared to boys’ backpacks. Similar results were obtained by Malinowska-Borowska and Flajszok [2] in a study of children aged 6–9 years in Poland. The authors noted that the average weight of items placed in girls’ backpacks was greater than that of boys’. In contrast, Brzęk et al [8] came to different conclusions regarding the weight of girls’ and boys’ backpacks, as a tendency for boys aged 7–9 years from primary schools in Poland to carry heavier backpacks was observed compared to their female peers. These discrepancies suggest that differences in backpack weight may result not only from gender factors, but also from the school context, the specific nature of classes, or individual habits related to packing a backpack.

Our findings showed that gender did not differentiate the percentage ratio of school backpack weight to body mass of the studied children. Barbosa et al [10], on the other hand, found that in ninth-grade primary school pupils in Portugal, the percentage ratio of backpack weight to body mass was higher in girls than in boys – girls carried heavier backpacks in relation to their body mass than boys. In Grobler and Kramer’s [4] study in the Northwest Province of South Africa, 13-year-old students, regardless of gender, carried significantly lighter school backpacks compared to children aged 10–12 years.

In the current investigation, approximately 60% of children, regardless of gender, had a backpack that was too heavy in relation to their own body mass. Disturbing results were also recorded by other authors. Among those who used the limit value of a school backpack weight equal to 10% of body mass, Olmedo-Buenrostro et al [24], who found too heavy backpack, irrespective of gender, in 78% of 240 randomly selected pupils aged 5–12, representing 20 primary schools in Mexico; Spiteri et al [7] in 70% of 4005 pupils aged 8–13, from schools in Spain; Dockrell et al [6] in 70% of 529 primary school pupils aged 9–11 in Ireland; and Rodríguez-Oviedo et al [9] in 66–80% of 1668 pupils from schools in the northern area of Lugo, Spain.

In both boys and girls, there were differences in most of the studied foot characteristics recorded before putting on a backpack and under school backpack weight conditions, suggesting that school backpack weight has the effect of increasing the length and width of the feet, and decreasing their longitudinal and transverse arches, as well as increased the fifth-toe varus deformity. This indicates that the weight of the school backpack has the effect of increasing the ground pressure forces of the feet, and therefore there is an increase in their length and width dimensions. The distribution of foot ground pressure forces under loading conditions was examined by Alfageme-García et al [25] in a prospective, longitudinal observational study of children aged 5 to 11 years from Spain. Over a 3-year follow-up period, the authors found a positive association between the use of backpacks and the risk of developing a pronated foot in children whose FPI showed neutral foot alignment on the day of the first examination. Pau et al [26], in a study of Italian children aged 6–13 years, found that wearing a backpack while standing increased the contact area of the forefoot by 7.3%, the metatarsal region by 18.5%, and the hindfoot by 1.3%. In addition, mean and peak plantar pressures increased, being highest in the forefoot, followed by the midfoot, and lowest in the hindfoot.

The increase in the fifth-toe plantarflexion noted in our study may be a result of increased loading on the lateral edges of the feet after wearing a school backpack. This suggests that, to cope with the task of carrying the backpack, the feet change their alignment to supinate and, therefore, there is a sense of deviation inwards. This alignment can result in stretching of the lateral arches of the feet and shortening of the medial arches. It may also be associated with a tendency towards a varus setting of the hallux, as demonstrated in our study, as negative α-angle values were recorded. These changes are certainly due to the weakness of the muscular system in the children studied. Zunkunft-Huber [27] reported that in a situation of abnormal muscle tone, the medial arch of the foot tends to have increased dorsiflexion and inversion, due to shortening of the tibialis anterior muscle. Conversely, increased tension in the adductor muscle of the hallux results in forefoot shortening and its abduction position. This is of concern as, over time, the capsular and ligamentous apparatus on the medial side of the foot may shorten.

The associations between foot shape changes measured before and during backpack load, and the percentage of backpack weight relative to body mass, indicate a lowering of both the longitudinal and transverse arches. These findings suggest that the foot arches have sufficient capacity and respond appropriately to school backpack load. However, upon further reflection and review of the literature, it must also be considered that the apparent responsiveness of the longitudinal and transverse foot arches to load may reflect their limited capacity to cope with this stress. Schulze et al [28] performed a pedobarographic study of soldiers’ feet under military load, and concluded that the stability of the longitudinal and transverse arches depends on the load carried, as the arches tend to flatten when they can no longer compensate for it. Behmaram et al [29], in a study of young school children walking with backpacks weighing 7.5%, 10%, 12.5%, and 15% of body weight, found differing responses to increasing load in normal versus flat feet. They concluded that flat feet undergo greater deformation due to their lower load-bearing capacity.

In girls, both feet became laterally flatter and wider after wearing a backpack, and hallux valgus increased in the left foot as backpack weight increased, suggesting that girls’ feet are more sensitive to school backpack load. These findings are consistent with Dehghan et al [30] and Sung et al [31], who reported that girls are predisposed to increased soft-tissue laxity, which peaks around 15 years of age and corresponds to hormonal changes during puberty. Matsubara et al [32], in a study of 10- to 12-year-old Japanese children, found a greater hallux angle in girls than in boys. In girls, simple regression analysis indicated that weaker foot grip strength, measured with a dynamometer, contributed to hallux valgus development. The authors emphasized the importance of strengthening foot muscles, particularly in girls, to prevent hallux valgus and support proper foot development.

It should be emphasized that the associations observed between school backpack weight and both heel angle and hallux position in this study may result from changes in the distribution of pressure forces in the forefoot and heel due to the additional load. This is supported by the study of Grobler and Kramer [4] in South African children aged 10–13 years, which showed that increases in forefoot and heel contact forces were proportional to the applied load. The authors also examined the ‘force ratio’ (which is the quotient of the forefoot force divided by the value of the heel force) to find a greater forward shift of the body’s center of gravity after loading compared to preload, and consequently a statistically significant increase in the force ratio from preload compared to postload. Kim et al [33] found that in 8-year-old children, lowering the backpack position increased the arm moment generated by upper torso movement, which may make it difficult to maintain an adequate pressure distribution in the forefoot and hindfoot areas. Similar effects were observed with increased backpack weight, suggesting that wearing the backpack higher, with the shoulder straps fastened, may better normalize load distribution on the feet. In the present study, the backpack was positioned by adjusting the shoulder straps so that the center of its vertical length aligned with the spinous process of Th12, which most closely corresponds to position 3 in the study by Kim et al [30]. Therefore, the associations of school backpack loading with heel angle and hallux setting shown in our study may be due to a shift in pressure forces on the forefoot under additional loading. The positive correlation of the toe angle of the left foot with the weight of the backpack in girls suggests that the left foot, which has a supporting function, is more susceptible to loading, and Phillips [34] reported that 70% of the load is carried by the first radius of the foot terminating in the hallux and, among other reasons, this is why the hallux valgus deformity is so often formed. Self-examination is important in the diagnosis and prevention of foot deformities. The results obtained and their interpretation can provide a useful indication for detecting critical situations that could potentially arise as a result of overloading children’s feet with school backpacks.

This study has limitations. First, the backpack weight was measured once to reflect the actual load carried by the children on the day of assessment, and foot examinations were performed immediately before and after wearing the backpack. This approach allowed us to capture changes in foot shape under real-life loading conditions. Consequently, the results represent a snapshot of typical daily backpack load and its immediate effects on foot shape, rather than long-term adaptations or cumulative outcomes. In addition, the study population was carefully selected according to the inclusion criteria, which, on the one hand, narrowed the scope of the study to one age group and may be considered a limitation of the study, while, on the other hand, it ensured the homogeneity of the study group, which fully corresponded to the characteristics of the population of pupils in the fourth grade of primary school, being at a transitional stage, associated with a different types of lessons from those in the first to third grades. Therefore, the results obtained may be relevant to the general population of peers and have great potential for application in social prevention systems.

Conclusions

The girls’ school backpacks tended to be too heavy in relation to their body mass. The associations of the differences in foot shape characteristics measured before putting on the backpack and under school backpack loading conditions with the values determining the percentage ratio of school backpack weight to body mass of the children studied indicate a lowering of the longitudinal and transverse arches of the feet under the influence of the backpack weight, which may be an expression of their weakness in coping with the load. In the girls, both feet were flatter transversely and widened after wearing the backpack, and in the case of the left foot, hallux valgus increased with the backpack load. This suggests that girls’ feet are more sensitive to the load of the school backpack. There is a need to implement countermeasures, such as monitoring the contents of school backpacks and seeking targeted solutions to educate parents about the need to meticulously check the contents of backpacks daily. There is also a need to seek solutions aimed at reducing the number of items and devices that children need to carry in their school backpacks. One way may be to ensure that children leave as many items as possible in their school lockers that do not need to be taken home.