Heavy schoolbag weight is frequently considered to have negative effects on posture and hypothesized to induce pain.1–3 However, previous research on schoolbag use mainly focused on adolescents,4–6 whereas little information exists with regard to schoolbag problems in young school-age children. Similarly, gender differences related to schoolbag use have not been thoroughly investigated.
As the child grows, the development of the musculoskeletal system passes through different phases. Particularly, skeletal stature, body weight (BW), and height show a constant increase from the age of 5 years onward until approximately the age of 10 to 11 years.7,8 The rate of growth shows a peak (growth spurt) between 11 and 15 years.7,8 However, the changes in BW during growth are not quite proportional to those of body height. BW increases at a slower rate until the age of 9 to 10 years and afterward weight increase shows a rapid acceleration as the child enters puberty.7 Conversely, the rate of height gain remains low up to the adolescent spurt, to be followed later by a rapid acceleration.8,9 Furthermore, growth patterns are not similar in all children, during the prepubertal and pubertal phases.10
Most studies have examined schoolbag weight in pubertal children (Table 1). To our knowledge, only 2 studies12,13 have examined children younger than 9 years. However, many students younger than 6 years report discomfort caused by heavy schoolbags.20 This indicates that the problem of heavy schoolbag use may start at an earlier age, and the effect might continue up to adolescence.21 More importantly, because of interindividual patterns of maturation and lifestyles, it is not clear whether the same (relative) schoolbag weight would have the same effect on the musculoskeletal system of a 5-year-old student compared with that of a 15-year-old student. Furthermore, children attending the same class at school may not have similar anthropometric and physique characteristics. Thus, their responses to schoolbag use and their own perceptions concerning their schoolbag may vary from an early age to adolescence.
Studies have shown that more girls than boys feel that their schoolbags are heavy.14,22 Previous findings on gender differences related to schoolbag weight are conflicting as some data suggest a gender difference23 in the weight and use of schoolbags, whereas others have reported no difference.5,24 However, it is well documented that maturation and growth differ between boys and girls.25 For example, the rate of the growth spurt in stature and weight occurs 2 years earlier in girls than in boys,25 and boys have a longer preadolescent growth spurt.7 Based on this evidence, it could be hypothesized that schoolbag weight effects and students' perceptions may differ between girls and boys even if they are of similar age.
The type of schoolbag and the way of carrying it may depend on student's preferences. On the other hand, students often tend to choose their schoolbag using fashion criteria rather than ergonomic characteristics. Fashion trends might be country and age specific.5,6,16 For example, backpacks are very common, and although some students tend to carry them on both shoulders,6,12,15,16 others prefer to carry their bags on 1 shoulder.26,27 It is possible that younger children may choose a different method of carrying their bags compared with older students. Similarly, boys may prefer different types of schoolbags and ways of carrying them compared with girls. If this is the case, then guidelines regarding schoolbag carriage may be gender and age specific. To our knowledge, age and gender differences in school bag weight have not been previously examined.
Schoolbag weight values reported in literature do vary and may range from 11.7% to 22% of BW (Table 1).4–6 Schoolbag weight values >10% of BW are frequently considered as being harmful for children.3,5,12,13,15,28 If this is the case, then the use of heavy schoolbags is an important issue for school health and needs further examination. In addition, weight and use of schoolbags may vary from country to country (Table 1). Particularly, very high schoolbag weight values have been reported for Italian students6 and some schools in the United States,14,15 whereas low values (<10% of BW) have been reported for North African,3 United Kingdom,11 and some US schools.17 This suggests that the use of heavy schoolbags is an international problem, and provision of safety guidelines for children requires careful examination of intercountry differences.
The purposes of this cross-sectional study were (1) to examine age differences in the weight and use of schoolbags in Greek primary and secondary school students, (2) to examine whether the use of heavy schoolbags differs between boys and girls, and (3) to compare findings across similar studies of schoolbag use in different nations.
Five primary (for children aged 6–11 years) and 3 secondary schools (for adolescents aged 12–14 years) participated in this study. The measurement protocol was approved by the National Department of Education and the University Ethics Committee. The study was performed in the city of Serres, Greece. The schools were randomly selected from the 21 primary and 12 secondary public schools in this area, and they all followed the same curriculum.
An explanation of the procedures of the study was given, and written parental permission was obtained before participation. A total of 980 school students volunteered to participate. Of these, 703 children provided parental consent and were included in this study. Their anthropometric characteristics are presented in Table 2.
The students were divided into 3 age groups (Group 1: ages 6–8 years; Group 2: ages 9–11 years; Group 3: ages 12–14 years) to allow examination of age differences in each variable. This age group formation was based on the following factors: (1) The school curriculum, for example, children attending the first 3 grades of the primary school (6- to 8-year-old children) have a very similar daily program, therefore, minor differences in schoolbag weight between them were expected; (2) the growth rate seems to be relatively similar from 6 to approximately 10 to 11 years of age, and it increases from 11 years onward7,8; (3) practical issues related to the educational system, that is, 6- to 11-year-old children attend primary school, whereas 12- to 14-year-old adolescents attend secondary school. Consequently, mixing 11-year-old primary school students with 12-year-old secondary school students is practically very difficult, and it seems to lack any practical significance because the curriculum and culture markedly differ between primary and secondary schools.
Height was measured to an accuracy of 0.1 cm using a Harpenden Portable Stadiometer (Holtain Ltd, Crosswell, UK). Standing height was measured with the student barefoot. Body weight and schoolbag weight were measured with electronic scales (Wedderburn, Southampton, UK) with an accuracy of ±0.1 kg.
BW, standing height, and schoolbag weight were directly assessed. The average value from 3 measurements was recorded. Subsequently, body mass index (BMI) was estimated (in kilograms per square meters).
The type of schoolbag, the method and time of carrying, the items included, and the subjective perception of the schoolbag load were obtained using a questionnaire (see the online Appendix, http://links.lww.com/PPT/A5). The questionnaire consisted of 11 open-ended and close-ended questions. The participants' perceptions of the load were evaluated by answering the following questions: “During the past 4 weeks, did you feel that carrying your schoolbag made you feel tired?” and “During the past 4 weeks, while carrying your schoolbag to school, did you feel it was heavy?” (never, sometimes, often, or always).
Data were collected on a typical day at school, so that actual schoolbag weight was recorded. A typical day at school started at 8:15 AM and finished at 1:30 PM, with the exception of first-grade students whose school day finished at 12:20 PM. All students and schools were not measured on the same occasion. Based on the school curriculum and after discussion with the school principals, all students of the same grade were tested on the same day of the week to ensure that daily workload was the same for all students of the same class. Neither students nor their parents were aware of the date when schoolbag weight measurements would take place to ensure that schoolbag weight scores represented typical day loads.
Weight measurements were taken on the same day and by the same investigator. The questionnaires were distributed and completed on the same day. Confidentiality was ensured during the experimental session.
In a pilot study, the questionnaire was redistributed to 90 randomly chosen subjects on a separate day. The test-retest reliability was determined using the intraclass correlation coefficient. The intraclass correlation coefficients varied from 0.79 to 0.99, indicating acceptable reliability.
Absolute (in kilograms) and relative (% of BW) schoolbag weight, the type of schoolbag, the method of bag transfer, and student's perceptions of the schoolbag load were further analyzed.
A 2-way analysis of variance was used to examine the effects of age and gender on absolute and relative schoolbag weight. Age and gender effects on the type and method of schoolbag carriage and student's perceptions were examined using χ2 tests.
Although there is no commonly accepted “safe” weight limit, numerous studies have considered that bags heavier than 10% of BW may increase the risk for musculoskeletal pain and related problems.3,5,12,13,15,28 This seems to be supported by biomechanical studies that demonstrated changes in the children's posture and craniovertebral angle, increased frequency of breathing, decreased stride length, and increased stride frequency when carrying bags weighing >10% of BW.26,27 Following the above practice, we divided the sample into those students who carried bags heavier than 10% of BW (heavy bags) and those who carried bags weighing <10% of BW.
Significant correlates of heavy schoolbags were determined using a multinomial logistic regression. In addition, we applied a logistic regression model to examine the associations between perceptions of heavy schoolbag and age, gender, BMI, schoolbag weight, time of carrying schoolbags, relative schoolbag weight, type of schoolbag, and the methods of carrying schoolbag. A separate regression model was used to examine the significant correlates of perception of fatigue while carrying the schoolbag. The level of significance was set at p < 0 0.05.
Schoolbag Weight Measurements
The mean absolute schoolbag weight carried by the whole sample was 5.30 ± 1.25 kg (95% confidence interval = 5.20–5.39 kg). The corresponding relative value was 12.71% ± 4.1% (95% confidence interval = 12.41%–13.01% of BW). The analysis of variance indicated nonsignificant 2-way interaction effects on absolute and relative schoolbag weight (p > 0.05). There was a significant F ratio for age (F2,696 = 115.32, p < 0.0001) on absolute schoolbag weight. Post hoc Scheffé tests indicated that Group 1 (the youngest children) demonstrated significantly lower absolute schoolbag weight compared with Groups 2 and 3. Furthermore, Group 2 displayed significantly lower schoolbag weight compared with Group 3 (Fig 1).
With respect to relative (% of BW) schoolbag weight, there was a significant F ratio for gender (F1,696 = 4.655, p < 0.05) and age (F2,696 = 99.635, p < 0.0001). Girls carried heavier schoolbags than boys. Post hoc Scheffé tests indicated that the relative schoolbag weight was significantly greater in the youngest group compared with the other age groups (Fig 2).
Type of Schoolbag
The most commonly reported type of schoolbag was a 2-strap backpack (88.8%), with the majority of students (89.3%) carrying their bags on both shoulders. Significant gender (χ2 = 12.235, p < 0.05) and age (χ2 = 46.270, p < 0.0001) effects were found on the type of schoolbag (Table 3). More boys carried backpacks compared with girls. For Groups 1 and 2, a higher percentage of girls carried schoolbags with wheels compared with boys. For Group 3, a higher percentage of girls used a shoulder bag compared with boys. With regard to age-group comparisons, a higher percentage of subjects from Groups 1 and 2 used backpack and schoolbags with wheels compared with those of Group 3 (p < 0.05). Furthermore, more subjects from Groups 2 and 3 used single-strap backpacks and sports bags compared with those of Group 1. Furthermore, a significantly higher percentage of Group 3 students used shoulder bags compared with the younger age group pupils.
Method of Schoolbag Carrying
The ways preferred by students to carry their bags are presented in Table 4. The results did not indicate statistically significant gender effects (p > 0.05), and therefore the data were pooled across genders. Most students carried their bags on both shoulders (p < 0.05). Furthermore, there was a significant age effect (χ2 = 39.481, p < 0.0001) because a lower percentage of Group 3 subjects carried their bags on both shoulders compared with the younger age groups (Table 4). In contrast, more subjects from Group 3 carried their bags on 1 shoulder or on the side compared with those of Groups 1 and 2 (p < 0.001).
Mode of Carrying the Schoolbag
Most participants (n = 534, 76%) traveled to and from school on foot. Mean carrying time was 8.7 ± 5.41 minutes. A total of 43.7% of the students reported that their schoolbag was consistently or frequently heavy (Table 5). With regard to this measurement, there was a significant gender effect because more girls than boys complained of heavy schoolbags (χ2 = 21.759, p < 0.0001) (Table 5).
Perceptions of Schoolbag
A total of 28.2% of the students reported that they felt consistently or frequently fatigued because of carrying the schoolbag (Table 5). A significant gender effect was found because more girls felt fatigue compared with boys (χ2 = 12.932, p < 0.05).
After multivariate adjustment, age, gender, and BMI were identified as the characteristics most significantly associated with using a heavy backpack (Table 6). Particularly, compared with students aged 12 to 14 years, those aged 6 to 8 years and 9 to 11 years had a 5-fold and 4-fold likelihood of heavy backpack use, respectively. Compared with female children, male children were 50% less likely to carry heavy loads. Furthermore, students with BMIs >25 had an 11-fold likelihood of carrying backpacks above the 10% of BW limit. No association was found between heavy backpacks and type or method of carrying the schoolbag (Table 6).
The results from the multinomial regression models showed that age and gender were the most significantly associated variables with perceptions of heavy schoolbag use and fatigue when carrying schoolbags (p < 0.05) (Table 7). Specifically, those children aged 6 to 8 years and 9 to 11 years were 2.0 and 1.3 times more likely to experience fatigue when carrying their schoolbag compared with students aged 12 to 14 years (Table 7). Similar, but slightly smaller, age group differences in odds ratios in perceptions of heavy schoolbags were found (Table 7). Compared with female children, male children were 50% less likely to feel fatigue while carrying their schoolbag or feel that their bags are heavy (p < 0.05, Table 7). Finally, there was a significant correlation (p < 0.05) between the duration of schoolbag carriage and the students' sense of fatigue and heaviness of their schoolbags (Table 7).
The results of this study indicated that (1) age and gender had a significant effect on schoolbag weight based on the method of carrying the schoolbag and on the perceptions concerning the weight of the schoolbag and (2) the use of heavy schoolbags varies between countries, depending on the school system, the geographic location, and the national culture.
Age Effects on Schoolbag Use
The results of this study indicated that the relative bag weight was significantly heavier in the younger age group (Fig 2). The regression model showed that younger students were almost 5 times more likely to carry heavier schoolbags than the older ones (Table 6). Two factors may be responsible for our finding: first, younger students tend to carry all their books to school (instead of the essentials), and second, the weight of the books determined by the national curriculum may not be appropriate for the early grades of primary school. This is an interesting finding because the majority of previous studies have considered that the problem of heavy schoolbags relates to adolescents (Table 1). If heavy schoolbags represent a problem that starts from early school age, then it seems that heavy schoolbags could act cumulatively throughout childhood and skeletal maturation. It should be mentioned, however, that Forjuoh et al,13 in a similar study, found results opposite to the present ones. This suggests that our results may hold true only for Greek students. Despite this, the present finding suggests that additional effort should be made so that younger students and their parents are better informed about various ways to reduce schoolbag weight.
Bags are designed to be worn over 2 shoulders to distribute the weight evenly on the spine and the shoulders. Similarly, health professionals have discouraged students from wearing schoolbags on 1 shoulder because that creates a high torque around the spine, and it can also lead to greater local musculoskeletal loads.29 Our results indicate that the majority of students used a safe technique to carry their schoolbags, and the method of carrying the schoolbag did not show a significant correlation with perceptions of fatigue (Table 7). It is worth noting, however, that the percentage of students who carried their bags on both shoulders was lower in older students compared with younger groups (Table 4). It seems that a higher proportion of older students carried their bags on 1 shoulder or on the side of the body compared with Groups 1 and 2 (Table 4). This indicates that older students tend to use techniques that require more energy and may impose greater musculoskeletal loads compared with younger students.18,19,30,31 The need for more intensive education regarding the method of carriage for this age group may assist in reducing this problem.
This study showed that a considerable number of students complained about the weight of their bags (Table 5), which is in agreement with previous findings.2,6,14 Furthermore, the regression analysis showed that age was significantly associated with perceptions of fatigue (Table 7). To our knowledge, this is the first study that shows that children aged younger than 8 years are 2 times more likely to experience fatigue compared with older children (Table 7). This finding may be related to the observation that younger students carried relatively heavier bags than older ones (Fig 1). This might suggest that the symptoms of discomfort or pain due to schoolbags that are observed in adolescents18 may be the result of a long-term process rather than an acute phenomenon. Further research could examine this suggestion using longitudinal experimental designs.
Gender Effects on Schoolbag Use
The relative schoolbag weight was heavier in girls compared with boys (Fig 2). This difference was small (<1% of BW), and its clinical importance is questionable. Despite this, the regression analysis showed that boys were 50% less likely to carry heavy schoolbags compared with girls (Table 6). If the above results are combined together, it seems that schoolbag use may differ among girls and boys. However, it should be mentioned that our results do not agree with those of some previous studies.5,14 This discrepancy indicates that gender differences may depend on the country and the education system under examination. Further studies are necessary to identify potential factors, which may cause different gender responses to the same musculoskeletal load induced by schoolbag weight.
Our results showed that a lower percentage of girls carried backpacks compared with boys. In addition, gender differences in the preferred type of schoolbag by age group were also observed (Table 3). If backpacks cause less discomfort than other types of bag, then it seems that more girls are likely to experience pain and discomfort than boys. However, it should be mentioned that the observed gender differences, although statistically significant, are small, and they cannot fully explain potential differences in pain or discomfort between boys and girls.
More girls felt that the bag was “heavy” and “fatiguing” compared with boys (Table 5). As a consequence, the regression analysis showed that, compared with boys, girls were 50% more likely to feel fatigue while carrying their schoolbag or sense that their bags are heavy (Table 7). This is in agreement with previous studies6,14 and could be attributed to various factors. First, one may suggest that this is a consequence of the heavier schoolbags carried by girls compared with boys (Fig 2). However, the difference, although statistically significant, is small (<1% of BW). Second, it is possible that boys may choose to carry or fill their bag in a different way compared with girls. However, our results showed minor gender differences in the method of carrying or type of schoolbag, and both variables were not significant correlates of fatigue perceptions. Consequently, student preferences related to schoolbag carriage seem similar for boys and girls. The only gender difference found was that a smaller number of girls carried backpacks compared with boys (Table 3). Whether this contributes to less fatigue symptoms in boys is not clear. However, because the method of carrying did not show a significant association with fatigue perceptions, the possibility that gender differences in backpack use lead to different levels of discomfort seems unlikely. Third, it is possible that some variation between girls and boys may be due to differences in skeletal maturation. Our results do not allow safe conclusions regarding this issue, as skeletal maturation was not recorded. Fourth, it has been suggested that girls demonstrate an even lower level of fitness compared with boys and, therefore, are more sensitive to fatigue compared with boys.23 If this is the case, then specific interventions for improving fitness levels in girls are necessary. Further studies are required to confirm this suggestion by recording gender effects on schoolbag weight in combination with the level of physical activity of the students. Finally, it should be mentioned that there may be a whole host of other factors that affect perceptions of fatigue. These factors could be school related (eg, amount of homework assigned, interpersonal issues with peers, test anxiety), nonschool related (eg, family stress), or health related (eg, presence of acute or chronic medical illness). Future research could examine the role of each of these factors for gender differences in fatigue perceptions in children.
International Comparison of the Effects of Schoolbag Use
In many countries, ergonomic regulations determine the maximum lifting weight at work for adults. For children and young adolescents, a similar norm is missing. To our knowledge, there are no specific guidelines for maximum schoolbag weight in Greek schools. Our experience indicates that primary school students tend to carry all their books to school everyday. Furthermore, schoolbag weight and the characteristics of daily carriage are largely dependent on the school curriculum and student and parental culture of the country and area under examination. Therefore, observations made for students in United States,1,12,13,15–17 Italy,6 and New Zealand,5 may not apply to Greece.
The anthropometric characteristics of the present sample are similar to those reported for Greek children.18,32,33 For example, Mamalakis et al32 reported BMI values of 16.4, 17.4, and 20.3 for 6-, 9-, and 12-year-old children, respectively. In this study, the BMI was 17.9, 17.8, and 21.6 for 7-, 10-, and 13-year-old children, respectively (Table 2). Karayiannis et al33 reported that 13.5-year-old Greek boys weighed 57.2 kg, were 166 cm tall, and had a BMI of 20.7. These values are very similar to those of the older age group examined in this study (Table 2). The above suggests that the anthropometric characteristics of the children in this study are representative of children in Greece. Compared with other countries, the children examined in this study were somewhat heavier than Austrians2 and similar to Italians.6 Their anthropometric characteristics were also similar to some US schools,34 but they were lighter than other US children.16 Further, the BMI values were slightly lower compared with those reported by Grimmer et al35 for Australian students.
Table 1 provides a summary of previous findings on schoolbag weight in various countries. It seems that many investigations reported that loads are heavier than the recommended limits.6,14,15 Comparisons of our results with previous studies should take into consideration that the majority of previous studies examined pubertal or postpubertal children (age older than 11 years). The average schoolbag weight value in this study (12.4% of BW) is slightly higher compared with that (10.6% of BW) reported by Korovessis et al18 for Greek adolescent students. Differences in age range and testing methodology may account for such observations. Comparison of our results with those reported for other countries suggests that Greek students had much lower schoolbag weight values compared with Italian students6 and some schools in the United States.14,15 In contrast, the present schoolbag weight values are higher than those observed for North African3 and United Kingdom11 students. Nevertheless, our results are in agreement with those of previous studies2,6,15,16,18 and suggest that heavy schoolbags seem to be a problem worldwide because many students carry bags weighing more than the recommended 10% of BW limit.28
Schoolbag weight characteristics and student's perceptions may be affected by the educational system within which they operate and the socioeconomic level of the community. A number of factors may characterize each system, all of which should be considered when interpreting the results of a study such as the present one. Particularly, the educational policy in Greece is centrally run by the Ministry of Education and is the same for all schools, whether private or public. In this study, only public schools were examined because they represent the majority of schools in Greece. Furthermore, the education department provides students with books for all subjects free of charge, and therefore any effects resulting from poor financial status on schoolbag weight are unlikely. Based on the Greek national curriculum, the students rarely change classrooms throughout the school day. In most cases, on arrival to school, their schoolbags are kept in their classroom until the end of the classes. It has been suggested that schoolbags should be kept in student lockers,5,6,15 whereas in some countries, backpacks are not allowed in school for safety reasons. Neither of the above applies to Greek schools. Our results clearly suggest that the use of lockers in Greek schools is necessary. However, this measure should be applied only after careful consideration of the curriculum and students' needs and preferences. For example, guidelines should be provided for younger students and their parents, so that a reduction of schoolbag weight is achieved. Moreover, older students should be motivated to leave some books in their personal lockers for future use and encouraged to avoid carrying unnecessary objects on a daily basis.
Directions for Future Research
There are additional factors that could influence the effect of schoolbags use that have not been taken into account in this study. For example, upper body strength and the level of physical activity may influence the ability of the child to control posture while walking. This might also be related to body height and the backpack length. For example, even in the case of 2 students carrying the same backpack, the bag may cover the whole back of the shorter child and therefore cause greater discomfort compared with the taller one. The speed of walking may also affect the posture of the child while walking to school, whereas it is not clear whether negative effects of heavy schoolbags appear during walking as opposed to sudden movements such as sudden running or ascending stairs. Future research is needed to examine the role of each of the above factors and their combined effects on musculoskeletal discomfort in children. Monitoring of the daily activity profiles of children while walking to school may provide additional insight into the effects of schoolbag on pain and discomfort in children.
The results of this study are based on a cross-sectional design. It is evident that more information is necessary on age and gender effects on a longitudinal basis. Furthermore, a limitation of this study was that the schoolbag weight was recorded only during 1 day. Consequently, the recorded data cannot account for the variance of schoolbag weight during a whole week. Moreover, 28% of the initial sample did not agree to participate in this study. Our experience from this project suggests that subject participation in screening studies is a difficult task, and even if the sample size were larger, the proportion of students who would refuse to participate would be quite similar to the present one. The reader should be aware of these limitations when interpreting these results.
Relative schoolbag weight is heavier in younger (6–8 years) schoolchildren compared with older ones; similarly, girls carry (relatively) heavier schoolbags compared with boys. Approximately 40% of students, especially girls, report frequent perceptions of fatigue and heavy bags. Future examination of schoolbag weight effects in children should take into consideration that younger children and girls may be more affected by heavier schoolbags than older children and boys. Comparisons of our findings with observations made in other countries suggested that heavy schoolbag use has some common characteristics across countries. However, there are also some potential differences originating from the school system, the geographic location, and the culture of each country. This suggests that the problem of heavy schoolbags should be examined at both international and national levels.
1. Skaggs DL, Early SD, D'Ambra P, et al. Back pain and backpacks in school children. J Pediatr Orthop. 2006;26:358–363.
2. van Gent C, Dols JJ, de Rover CM, et al. The weight of schoolbags and the occurrence of neck, shoulder, and back pain in young adolescents. Spine. 2003;28:916–921.
3. Puckree T, Silal SP, Lin J. School bag carriage and pain in school children. Disabil Rehabil. 2004;26:54–59.
4. Mackenzie WG, Sampath JS, Kruse RW, et al. Backpacks in children. Clin Orthop Relat Res. 2003;409:78–84.
5. Whittfield JK, Legg SJ, Hedderley DI. The weight and use of schoolbags in New Zealand secondary schools. Ergonomics. 2001;44:819–824.
6. Negrini S, Carabalona R. Backpacks on! Schoolchildren's perceptions of load, associations with back pain and factors determining the load. Spine. 2002;27:187–195.
7. Malina RM, Bouchard JP. Somatic growth. In: Malina RM, Bouchard JP, eds. Growth, Maturation and Physical Activity. Champaign, IL: Human Kinetics; 1991.
8. Tanner J. Growth and Adolescence. 2nd ed. Oxford: Blackwell; 1962.
9. Malina R, Little B. Body composition, strength, and motor performance in undernourished boys. In: Binkhorst R, Kemper H, Saris W, eds. Children and Exercise XI. Champaign, IL: Human Kinetics; 1985.
10. Malina RM, Bouchard JP. Maturity-associated variation in growth. In: Malina RM, Bouchard JP, eds. Growth, Maturation and Physical Activity. Champaign, IL: Human Kinetics; 1991.
11. Jones GT, Watson KD, Silman AJ, et al. Predictors of low back pain in British schoolchildren: a population-based prospective cohort study. Pediatrics. 2003;111:822–828.
12. Forjuoh SN, Lane BL, Schuchmann JA. Percentage of body weight carried by students in their school backpacks. Am J Phys Med Rehabil. 2003;82:261–266.
13. Forjuoh SN, Schuchmann JA, Lane BL. Correlates of heavy backpack use by elementary school children. Public Health. 2004;118:532–535.
14. Goodgold S, Concoran M, Gamache D, et al. Backpack use in children. Pediatr Phys Ther. 2002;14:122–131.
15. Siambanes D, Martinez JW, Butler EW, et al. Influence of school backpacks on adolescent back pain. J Pediatr Orthop. 2004;24:211–217.
16. Sheir-Neiss GI, Kruse RW, Rahman T, et al. The association of backpack use and back pain in adolescents. Spine. 2003;28:922–930.
17. Moore MJ, White GL, Moore DL. Association of relative backpack weight with reported pain, pain sites, medical utilization, and lost school time in children and adolescents. J Sch Health. 2007;77:232–239.
18. Korovessis P, Koureas G, Zacharatos S, et al. Backpacks, back pain, sagittal spinal curves and trunk alignment in adolescents: a logistic and multinomial logistic analysis. Spine. 2005;30:247–255.
19. Korovessis P, Koureas G, Papazisis Z. Correlation between backpack weight and way of carrying, sagittal and frontal spinal curvatures, athletic activity, and dorsal and low back pain in schoolchildren and adolescents. J Spinal Disord Tech. 2004;17:33–40.
21. Reneman MF, Poels BJ, Geertzen JH, et al. Back pain and backpacks in children: biomedical or biopsychosocial model? Disabil Rehabil. 2006;28:1293–1297.
22. Mehta TB, Thorpe DE, Freburger JK. Development of a survey to assess backpack use and neck and back pain in seventh and eighth graders. Pediatr Phys Ther. 2002;14:171–184.
23. Navuluri N, Navuluri RB. Study on the relationship between backpack use and back and neck pain among adolescents. Nurs Health Sci. 2006;8:208–215.
24. Cardon G, Balague F. Low back pain prevention's effects in schoolchildren. What is the evidence? Eur Spine J. 2004;13:663–679.
25. Malina RM. Growth and maturation: normal variation and effect of training. In: Gisolfi C, Lamb D, eds. Perspectives in Exercise Science and Sports Medicine, Vol. 2: Youth, Exercise and Sport. Indiana, PA: Benchmark Press; 1989.
26. Li JX, Hong Y, Robinson PD. The effect of load carriage on movement kinematics and respiratory parameters in children during walking. Eur J Appl Physiol. 2003;90:35–43.
27. Chansirinukor W, Wilson D, Grimmer K, et al. Effects of backpacks on students: measurement of cervical and shoulder posture. Aust J Physiother. 2001;47:110–116.
29. Hong Y, Cheung CK. Gait and posture responses to backpack load during level walking in children. Gait Posture. 2003;17:28–33.
30. Pascoe DD, Pascoe DE, Wang YT, et al. Influence of carrying book bags on gait cycle and posture of youths. Ergonomics. 1997;40:631–641.
31. Malhotra MS, Gupta SJ. Carrying of school bags by children. Ergonomics. 1965;8:55–60.
32. Mamalakis G, Kafatos A, Manios Y, et al. Obesity indices in a cohort of primary school children in Crete: a six year prospective study. Int J Obes Relat Metab Disord. 2000;24:765–771.
33. Karayiannis D, Yannakoulia M, Terzidou M, et al. Prevalence of overweight and obesity in Greek school-aged children and adolescents. Eur J Clin Nutr. 2003;57:1189–1192.
34. Forjuoh SN. School backpack weights: a survey of students in Ghana, Guatemala and the USA. Inj Control Saf Promot. 2004;11:287–289.
35. Grimmer K, Williams M. Gender-age environmental associates of adolescent low back pain. Appl Ergon. 2000;31:343–360.
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