Effects of Carrying a Backpack in an Asymmetrical Manner on the Asymmetries of the Trunk and Parameters Defining Lateral Flexion of the Spine

Abstract

Objective:

The aim of this study was to examine changes in the body posture parameters defining asymmetry of the trunk and lateral flexion of the spine in children while carrying a backpack weighing 10% of a child’s weight.

Background:

Carrying a backpack may negatively affect the posture of schoolchildren and contribute to spinal pain.

Method:

The study involved 162 primary school students ages 11 to 13 years. The parameters describing body posture were assessed with a backpack carried on the right or left shoulder as well as without a load. To assess the predefined parameters, we used the CQ Elektronik System, employing the photogrammetric method.

Results:

Trunk inclination shifted significantly in the opposite direction to the shoulder the backpack was carried on, and an increase in shoulder asymmetry was also found. We also observed a more pronounced right-side lateral flexion of the spine when the backpack was carried on the right shoulder and an analogous relationship for the left side.

Conclusion:

The results of this study show that carrying a backpack in an asymmetrical manner negatively affects spine, even if the backpack weight constitutes 10% of the child’s weight, which has been previously recommended as a safe load for a child’s shoulders.

Application:

We suggest that the issue of safe backpack weight be reassessed and that students be taught basic ergonomic principles on how to carry loads. Changes to the management pattern of carrying textbooks to and from school also should be considered.

Keywords

rucksack trunk asymmetry body posture children

Introduction

Carrying a backpack is common among schoolchildren. Over 90% of all children in developed countries carry a backpack (Sheir-Neiss, 2003).

The spine may be susceptible to injury for a greater length of time, and therefore, proper backpack use should be emphasized during these years. When the backpack load is positioned posterior to the body, the center of gravity shifts posteriorly over the base of the support, the area covered by the feet (Lanes et al., 1995). This shift is accomplished by either leaning forward at the ankle or hip or inclining the head, and the rigidity of postural muscles controlling these adjustments increases to support the load. Children have relatively larger heads and also have higher center of mass at about T12, compared to L5-S1 in adults (Shumway, 2001).

Carrying posterior loads by young people has been linked with spinal pain, and the amount of postural change produced by load carriage has been used as a measure of the potential to cause tissue damage (Lanes et al., 1995; Shumway, 2001; Tulder, Koes, & Bouter, 1997). Back pain in children appears to be more common than was previously thought (Mayank, Upendar, & Nishat, 2006).

According to the recommendations of many researchers, safe backpack weight should not exceed 10% of the child’s weight; however, it often reaches up to 30% to 40% (Goodgold et al., 2002). Carrying a backpack this heavy for many hours could be a factor contributing to spinal pain in children (Cheung, Shum, Tang, Yau, & Chiu, 2010). Moreover, research has shown that such weight has a significant effect on body posture and walking (Brackley, Stevenson, & Selinger, 2009; Chow, Leung, & Holmes, 2007; Hong & Cheung, 2003). However, some studies are contradictory to these results (Cottalorda, Bourelle, & Gautheron, 2004). Carrying a backpack on both shoulders influences body posture and walking less than does an asymmetrical burden (Cottalorda, Bourelle, Gautheron, & Kohler, 2004; Milanese & Grimmer-Somers, 2010; National Back Pain Association, 1997). However, according to research conducted by Pascoe, Pascoe, Wang, Shim, & Kim (1997), most students (72.3%) prefer to carry a backpack on one shoulder.

For that reason, the effect of backpack weight on pain and spinal deformities among children is often covered in literature (Chow, Ou, Wang, & Lai, 2010; Kistner, Fiebert, Roach, & Moore, 2013; Mackenzie, Sampath, Kruse, & Sheir-Neiss, 2003).

The purpose of this study is the assessment of changes in the parameters of body posture defining asymmetry of the trunk and lateral flexion of the spine while carrying a backpack weighing 10% of a child’s weight.

Method

Subjects

This study included 162 primary school students ages 11 to 13 years, 87 girls and 75 boys. The mean weight in the study group was 41 kg ± 5.3 kg. The mean height in the study group was 149 cm ± 6.7 cm. The mean body mass index in the study group was 18.9 kg/m² ± 5.3 kg/m². The study inclusion criteria were as follows: written consent of the subjects and carers, age 11 to 3 years, and no history of diseases significantly affecting the body posture and postural stability, confirmed by an orthopaedic examination.

Experimental Design

The parameters describing body posture of each participating student were assessed three times. The first assessment was conducted in a “habitual” position, without load (Assessment 1). The second assessment was conducted with the backpack carried on the right shoulder (Assessment 2), and the third one with the backpack carried on the left shoulder (Assessment 3). In both Assessments 2 and 3, the weight of the backpack constituted 10% of the total body mass of the child. The value of the load was calculated just before the assessment, after determining the weight of each student using electronic scales (to an accuracy of ±0.1 kg). Each measurement was performed 30 s after positioning the patient.

Apparatus

The study was carried out using MORA 4 Generation, CQ Electronik System, Czernica, Poland. The measuring device uses photogrammetry, that is, performs anthropometric calculations based on the photograph of the studied surface. The photogrammetric method was used to evaluate body posture using the phenomenon of the projection chamber. The tests involved anthropometric measurements based on images of the studied surface.

Procedure

Before the measurement, anthropometric points, used later to calculate the analyzed parameters, were marked on each patient’s body. The anthropometric points were determined by palpation by a physiotherapist with 10 years of experience and extensive practice in photogrammetric measurements. Following the palpation of the points (spinous processes, lower corners of the scapulae, peak of kyphosis, the deepest point of lordosis, transition of kyphosis into lordosis, and posterior iliac spines), they were marked with a dermatograph. These were additional markings, not used in the analysis of computer images. The image was recorded after marking all the essential points and positioning the patient with his or her back to the camera. A computer registered around a dozen images. Then, the image capturing correct positioning of the patient, in the habitual position without twisting of the trunk or pelvis, was selected, and the anthropometric points were transferred onto a photogram on the computer screen. Based on the marked points, the computer defined the parameters describing the body posture by assessing the distance of the selected points from the camera.

The patient is positioned at a distance of 2.6 m from the camera while the device projects lines of strictly defined parameters onto the patient’s back, allowing a spatial image to be obtained. These lines reach the patient’s back at a specific angle and are distorted depending on the distance of a given point from the device. The computer records line image distortions, and numerical algorithms are used to convert them into a contour map of the surface. In optics, the physical basis of this method is called the Moire phenomenon (Drzał-Grabiec, Snela, Rykała, Podgórska, & Banaś, 2013).

Scientific studies have confirmed that the results obtained by the photogrammetric method are very close to X-ray outcomes (Berryman, Pynsent, Fairbank, & Disney, 2008; Scott, Dangerfield, & Dorgan, 1996). According to Saad, Colombo, and João (2009), the photogrammetric measures were found to be reproducible in this study and could be used as supplementary information to decrease the number of radiographs necessary for lateral flexion of the spine monitoring.

Based on X-ray and photogrammetric examinations, Liu, Thometz, Lyon, and McGrady (2002) reported significant differences in the assessment of the deformation angle, especially within the lumbar section. However, they pointed out that the results were more reliable and objective than the clinical trial itself. To date, no scientific study has been published on the reliability of photogrammetry based on detailed data concerning the deformation angle and trunk asymmetry. The only studies in this field involved a comparison of Cobb’s angle from radiographic and photogrammetric images.

Measurement in a habitual position was followed by images recorded with a backpack, first carried on the right and then on the left shoulder. The study participants were not involved in the analysis of photograms (Drzał-Grabiec & Snela, 2012; Drzał-Grabiec & Szczepanowska-Wołowiec, 2011; Świerc, 2011). The parameters used in this study are presented in Table 1. An example of a photogrammetric examination report is presented in Figure 1. The study was approved by the Bioethical Committee. The study was conducted in January 2013.

Table 1:

Parameters Examined in This Study

Variable	Description
SCS (mm)	Shoulder asymmetry coefficient in relation to point S1. The difference of distances from the line crossing through point S1 on the horizontal axis is calculated.
SCC (mm)	Shoulder asymmetry coefficient in relation to point C7. Same as SCS, but the reference point is the spinous process of the 7th cervical vertebra.
KNM (mm)	Inclination of the line connecting the posterior superior iliac spine in relation to the horizontal plane.
ATP (mm)	Angle of torsion of the pelvis; inclination of the line connecting the posterior superior iliac spine at the sagittal plane.
CIT (mm)	Coronal inclination of the trunk.
AS (mm)	Angle of the shoulders; the angle between the line connecting the acromions of the shoulders and the horizontal plane.
UB (degree)	Difference of depth of the lower corners of scapulae, torsion.
UL (degree)	Difference of height of the lower corners of scapulae.
OL (mm)	Difference of distances from the lower corners of scapulae to the spine.
UK (mm)	Maximum deflection of line connecting spinous processes from C7-S1 line.
LpD (mm)	Length of right lateral flexion curve.
LpS (mm)	“Depth” of right lateral flexion curve measured at the coronal plane up to the apical vertebra.
LpK (degree)	Angular value of lateral flexion–right curve; an angle complementary to Cobb’s angle.
LlD (mm)	Length of left lateral flexion curve.
LlS (mm)	“Depth” of left lateral flexion curve measured at the coronal plane up to the apical vertebra.
LlK (degree)	Angular value of lateral flexion–left curve; an angle complementary to Cobb’s angle.

Figure 1.

Example of a photogrammetric examination report: (a) habitual position (Assessment 1), (b) backpack on the right shoulder (Assessment 2), (c) backpack on the left shoulder (Assessment 3).

Figure 2.

Method for determining Cobb’s angle and the angle of lateral flexion of the spine (complementary to Cobb’s angle) in the photogrammetric approach. C7 = spinous process of the seventh cervical vertebra; S1 = transition of lumbar lordosis into the sacrum; sl, sp = deformation peak.

Data Analysis

Statistical analysis of the results was based on the parametric test of one-way ANOVA. This test is used to compare more than two groups of variables. However, a significant result from the F test (ANOVA) does not reveal the specific groups between which the differences are observed. To determine the groups for which statistically significant differences existed, multiple “post hoc” comparisons were performed. Tukey’s test (HSD) was selected as the post hoc test for the analysis of variance. The assumed level of statistical significance was p < .05. The statistical analysis was performed with Statistica 10.0.

Results

The results of the study show significant differences for most of the parameters describing asymmetry of the trunk and lateral flexion curve. The trunk shows significant inclination to the left when assessed while the backpack is carried on the right shoulder and inclination to the right when assessed while the backpack is carried on the left shoulder (coronal inclination of the trunk, p < .0001). Asymmetry of the shoulders is also significantly increased (angle of the shoulders, p < .0001) and scapule (difference of height of the lower corners of scapulae, p < .0001). During assessments with asymmetrical load, the parameters defining asymmetry of the shoulders are also increased (shoulder asymmetry coefficient in relation to point S1, p < .0001; shoulder asymmetry coefficient in relation to point C7, p < .0001). When the backpack is carried on the right shoulder, the distance between spinous processes and the spine also increases (maximum deflection of line connecting spinous processes from C7-S1 line, p = .0002) but is significantly reduced when the left shoulder is loaded (p < .0001).

Significant changes have also been observed for parameters defining lateral deformation. For the left curve, the parameters increase when the backpack is carried on the left shoulder (length of left lateral flexion curve, p = .000; “depth” of left lateral flexion curve measured at the coronal plane up to the apical vertebra, p = .0000; angular value of lateral flexion–left curve, p = .0001), and no significant differences are observed when the weight is carried on the right shoulder. The opposite relationship is true for the right curve; the parameters increase when weight is carried on the right shoulder (length of right lateral flexion curve, p = .0176; “depth” of right lateral flexion curve measured at the coronal plane up to the apical vertebra, p = .0008; angular value of lateral flexion–right curve, p = .0290). Results are presented in Table 2 and Table 3.

Table 2:

Comparison of Analyzed Parameters for Different Assessments

	Assessment 1		Assessment 2		Assessment 3		ANOVA
Variable	M	SD	M	SD	M	SD	F	p
CIT (mm)	−0.69	1.29	−5.26	3.09	3.46	3.34	232.13***	<.0001
AS (mm)	−0.42	5.68	−29.04	22.80	25.59	23.89	181.56***	<.0001
UL (degree)	−0.79	2.83	−5.64	11.50	10.10	13.57	54.67***	<.0001
UB (degree)	−2.56	6.12	−5.45	8.78	−0.14	10.90	8.26	.4030
OL (mm)	−3.38	8.59	−2.96	15.84	−4.12	16.47	0.16	.8549
KNM (mm)	2.45	4.33	2.62	4.57	1.20	4.34	2.79	.0629
ATP (mm)	−2.95	5.78	−3.76	11.70	−2.19	9.01	0.68	.5100
SCS (mm)	3.21	14.62	50.82	34.57	−41.80	35.65	218.46***	<.0001
SCC (mm)	−5.45	9.02	−15.06	15.83	2.31	18.69	30.36***	<.0001
UK (mm)	−3.07	5.70	−9.80	8.96	1.46	7.89	50.15***	<.0001
LpD (mm)	105.49	153.52	65.51	122.01	165.20	164.36	10.50***	<.0001
LpS (mm)	1.46	2.14	1.75	3.11	3.27	4.42	7.69***	.0006
LpK (degree)	158.95	16.69	124.11	27.12	166.37	15.16	8.50***	.0003
LlD (mm)	220.48	161.44	246.15	152.75	112.19	151.83	19.05***	<.0001
LlS (mm)	−4.34	3.85	−9.95	7.64	−3.08	4.39	39.52***	<.0001
LlK (degree)	161.12	19.82	174.36	13.24	141.73	29.81	16.61***	<.0001

Note. See Table 1 for variable descriptions.

p < .05. **p < .01. ***p < .001.

Table 3:

Multiple Comparisons Between Measurements

	ANOVA		Post Hoc Tukey’s Test (HSD)
Variable	F	p	Assessment 1 vs. Assessment 2	Assessment 1 vs. Assessment 3	Assessment 2 vs. Assessment 3
CIT (mm)	232.13	<.0001	.0000	.0000	.0000
AS (mm)	181.56	<.0001	.0000	.0000	.0000
UL (degree)	54.67	<.0001	.0047	.0000	.0000
UB (degree)	8.26	.4030	.0695	.1542	.6002
OL (mm)	0.16	.8549	.9780	.9338	.8448
KNM (mm)	2.79	.0629	.9647	.1365	.0777
ATP (mm)	0.68	.5100	.8210	.8395	.4761
SCS (mm)	218.46	<.0001	.0000	.0000	.0000
SCC (mm)	30.36	<.0001	.0001	.0015	.0000
UK (mm)	50.15	<.0001	.0000	.0002	.0000
LpD (mm)	10.50	<.0001	.0176	.1612	.0000
LpS (mm)	7.69	.0006	.0008	.8280	.0061
LpK (degree)	8.50	.0003	.0290	.2769	.0002
LlD (mm)	19.05	<.0001	.5051	.0000	.0000
LlS (mm)	39.52	<.0001	.2734	.0000	.0000
LlK (degree)	16.61	<.0001	.5374	.0001	.0000

Note. See Table 1 for variable definitions.

Discussion

These results show an increase in asymmetry of the shoulders, scapulae, pelvis, and trunk while carrying a backpack in an asymmetrical manner. The increase in trunk asymmetry is a result of its compensatory position due to placing too much burden on the child’s shoulders. Lateral deformation is also subject to advancement; if the weight is carried on the right shoulder, lateral flexion of the spine increases on the right side, and if the backpack is carried on the left shoulder, lateral flexion of the spine increases on the left side. This result is accompanied by a compensatory lateral flexion of the body in the direction opposite to the load.

Lateral and forward bending of the trunk increased the spine bending torque by as much as 30% compared to the sagittal plane. This result was probably due to the fact that the pelvis assumed a smaller portion of the movement (Dolan, Earley, & Adams, 1994). Therefore, an asymmetric load with a backpack results in greater spine overload than a symmetric load and trunk forward flexion do.

In research conducted by Bettany-Saltikov and Cole (2012), the value of asymmetry was measured among students while carrying a backpack constituting 15% of body mass. The ISIS method was used to assess body posture. When the backpack was carried in an asymmetrical manner, an increase in asymmetry of the trunk was observed, and when the weight is distributed evenly on both shoulders, symmetry of the trunk is restored. According to a study by the same author, asymmetrical burden of a backpack constituting 17% of body mass significantly increases Cobb angle in the thoracic segment of the spine (Bettany-Saltikov, Warren, & Stamp, 2008). According to Negrini and Negrini (2007), asymmetry of the shoulders increases when the backpack carried in an asymmetrical manner weighs 8 kg, in relation to symmetrical burden. A study conducted by Korovessis, Koureas, Zacharatos, and Papazisis (2005) determined that carrying a backpack asymmetrically increases asymmetry of the shoulders in comparison to carrying the backpack with weight evenly distributed on both shoulders. The authors recommend carrying the backpack symmetrically. According to other researchers, carrying a backpack on one shoulder leads to significant asymmetry of the shoulders (Akbari & Gannad, 2006). As advocated by Hong, Fong, and Li (2011), children carrying heavy school bags may develop spinal problems. Those authors suggested that when children are using stairs, a symmetrical backpack with a load within 20% of their body weight is acceptable for them. When they are carrying an asymmetrical single-strap athletic bag, the bag’s weight should not exceed 10% of the body weight in order to avoid excessive spinal tilt (Hong et al., 2011).

The results of this research along with the conclusions of other researchers show that carrying a backpack in an asymmetrical manner has a negative effect on children, even if the weight constitutes 10% of their body mass, which was recommended until now as a safe load for a child’s shoulders. The research also provides a description of the impact of carrying a backpack on one shoulder on parameters describing the pelvis, scapulae, and trunk. In regard to scoliosis, apart from the angle of lateral flexion, the researchers also calculated changes of the length of the lateral flexion curve, as well as its depth, which facilitated a detailed assessment of changes that take place when carrying a backpack on one shoulder as compared to the “habitual” position. The additional value of the study stems from the fact that it was conducted with backpacks, which the children carry on a daily basis, and not their imitations, which provide similar biomechanical conditions but are not a perfect reflection of an everyday situation. The researchers chose to use asymmetrical load in the study, as the children declare that they carry their backpacks that way more often.

Carrying a backpack on one shoulder causes lifting of the loaded shoulder and slides the shoulder forward slightly. This action is to balance the weight of a backpack and relieve the muscles. The authors assume a similar phenomenon can be observed when a backpack is carried in a symmetrical manner. However, due to the limitations connected with the Moire projection method, this assumption could not be unequivocally determined. It is expected that a considerable load of a backpack may result in pushing both shoulders forward and an inclination of the trunk forward. Verification of this hypothesis requires further analysis.

Our study is important from a clinical perspective. Prolonged asymmetric positioning of the spine due to everyday, asymmetric loading with a backpack may cause significant biomechanical changes resulting in possible back pain.

Backpack usage habits are suspected to be one of the causes for the rising prevalence of back pain in school children.

Some studies indicate an asymmetrical backpack-carrying method as one of the risks of back pain in these age groups. It is not known whether back pain in childhood predicts back pain in adulthood (Negrini, Carabalona, & Sibilla, 1999; Viry, Creveuil, & Marcelli, 1999). Korovessis et al. (2005) also claimed that carrying a backpack in an asymmetrical manner led to intensified back pain during school and holidays.

According to Macias, Murthy, Chambers, and Hargens (2008), pressures at 10%, 20%, and 30% body weight loads on the right or left shoulder during low-back or high-back conditions are higher than the pressure thresholds (approximately 30 mm Hg) to occlude skin blood flow. Furthermore, asymmetric and high pressures exerted for extended periods of time may help explain the shoulder and back pain attributed to backpacks.

In most Polish schools, the students are not able to leave textbooks and school items at school, requiring them to carry them to school every day, spend an entire day with the load on their back, and then carry the backpacks home. Due to the importance of the matter, children should be taught in school about the issues in ergonomics, including carrying a backpack and the effect of not applying the basic rules to maintain appropriate body posture. An important recommendation for schoolchildren would be to wear a backpack on both shoulders. The backpack should be properly shaped to ensure appropriate distribution of its weight on the child’s back and the possibility to maintain a balance between thoracic kyphosis and lumbar lordosis. The limits for the weight of a backpack should also be chosen more carefully. The current limit is 10% of body mass, and as demonstrated in this study, even such a load results in negative changes in body posture.

Limitations

The parameters assessed in this study were not monitored over time, and further research will include the time variable, which may affect the studied parameters. The authors of this study plan to conduct further research on this topic, including measurement of the selected parameters of body posture while carrying a smaller weight, starting at 10% of body mass, to determine what load is safe for the spine. Future studies will also include assessment conducted on children in different age groups, which will allow researchers to determine at what age students are at the highest risk of the negative impacts of carrying a backpack. The results of the study showed statistically significant differences between habitual backpack carrying and asymmetrical carrying; these results may not translate into clinically significant outcomes. The clinical context will be analyzed in further studies.

Conclusions

Carrying a backpack with a weight constituting 10% of the body mass leads to an increase in asymmetry of the shoulders, scapulae, pelvis, and the entire trunk. If the weight is carried on the right shoulder, lateral flexion increases on the right side, and if the weight is carried on the left shoulder, lateral flexion increases on the left side. The load that is currently considered safe, that is, 10% of the body mass of the student, should be reconsidered. As this study demonstrates, carrying such a weight has negative effects on body posture.

Key Points

Carrying a backpack with a weight constituting 10% of the body mass leads to an increase in asymmetry of the shoulders, scapulae, pelvis, and the entire trunk.

As this study demonstrates, carrying such a weight has negative effects on body posture.

Footnotes

Acknowledgements

The authors declare no conflict of interest. The authors thank Proper Medical Writing for language assistance in the preparation of this paper.

Justyna Drzał-Grabiec is an assistant professor in the Institute of Physiotherapy, Faculty of Medicine, Rzeszów University, Poland. She received her PhD in physiotherapy from University School of Physical Education in Crakow in 2011.

Sławomir Snela, PhD, is head of the Physiotherapy Department, Faculty of Medicine, Rzeszów University, Poland. He has been a university professor since 2011.

Maciej Rachwał is a teaching assistant in the Institute of Physiotherapy, Faculty of Medicine, Rzeszów University, Poland. He received his MSc from Rzeszów University in 2012.

Justyna Podgórska is a teaching assistant in the Institute of Physiotherapy, Faculty of Medicine, Rzeszów University, Poland. She received her MSc from Rzeszów University in 2009.

Justyna Rykała is a teaching assistant in the Institute of Physiotherapy, Faculty of Medicine, Rzeszów University, Poland. She received her MSc from Rzeszów University in 2009.

References

Akbari

Gannad

(2006). Prevalence of shoulder postural impairments in 10-12 year old primary students of Zahedan. Journal of Medical Sciences, 6, 332–337.

Berryman

Pynsent

Fairbank

Disney

(2008). A new system for measuring three-dimensional back shape in scoliosis. European Spine Journal, 17, 663–672.

Bettany-Saltikov

Cole

(2012). The effect of frontpacks, shoulder bags and handheld bags on 3D back shape and posture in young university students: An ISIS2 study. Studies in Health Technology and Informatics, 176, 117–121.

Bettany-Saltikov

Warren

Stamp

(2008). Carrying a rucksack on either shoulder or the back, does it matter? Load induced functional scoliosis in “normal” young subjects. Studies in Health Technology and Informatics, 140, 221–224.

Brackley

H. M.

Stevenson

J. M.

Selinger

J. C.

(2009). Effect of backpack load placement on posture and spinal curvature in prepubescent children. Work, 32, 351–360.

Cheung

C. H.

Shum

S. T.

Tang

S. F.

Yau

P. C.

Chiu

T. T.

(2010). The correlation between craniovertebral angle, backpack weights, and disability due to neck pain in adolescents. Journal of Back and Musculoskeletal Rehabilitation, 23, 129–136.

Chow

D. H.

Leung

K. T.

Holmes

A. D.

(2007). Changes in spinal curvature and proprioception of schoolboys carrying different weights of backpack. Ergonomics, 50, 2148–2156.

Chow

D. H.

Z. Y.

Wang

X. G.

Lai

(2010). Short-term effects of backpack load placement on spine deformation and repositioning error in schoolchildren. Ergonomics, 53, 56–64.

Cottalorda

Bourelle

Gautheron

(2004). Effects of backpack carrying in children. Orthopedics, 27, 1172–1175.

10.

Cottalorda

Bourelle

Gautheron

Kohler

(2004). Backpack and spinal disease: Myth or reality? Revue de Chirurgie Orthopédique et Réparatrice de l’Appareil Moteur, 90, 207–214.

11.

Dolan

Earley

Adams

M. A.

(1994). Bending and compressive stresses acting on the lumbar spine during lifting activities. Journal of Biomechanics, 27, 1237–1248.

12.

Drzał-Grabiec

Snela

(2012). The influence of rural environment on body posture. Annals of Agricultural and Environmental Medicine, 19, 846–850.

13.

Drzał-Grabiec

Snela

Rykała

Podgórska

Banaś

(2013). Changes in the body posture of women occurring with age. BMC Geriatrics, 13, 108.

14.

Drzał-Grabiec

Szczepanowska-Wołowiec

(2011). Weight-height ratios and parameters of body posture in 7-9-year-olds with particular posture types. Ortopedia, Traumatologia, Rehabilitacja, 13, 591–600.

15.

Goodgold

Corcoran

Gamache

Gillis

Guerin

Coyle

J. Q.

(2002). Backpack use in children. Pediatric Physical Therapy, 14, 122–131.

16.

Hong

Cheung

C. K.

(2003). Gait and posture responses to backpack load during level walking in children. Gait Posture, 17, 28–33.

17.

Hong

Fong

D. T.

J. X.

(2011). The effect of school bag design and load on spinal posture during stair use by children. Ergonomics, 54, 1207–1213.

18.

Kistner

Fiebert

Roach

Moore

(2013). Postural compensations and subjective complaints due to backpack loads and wear time in schoolchildren. Pediatric Physical Therapy, 25, 15–24.

19.

Korovessis

Koureas

Zacharatos

Papazisis

(2005). Backpacks, back pain, sagittal spinal curves and trunk alignment in adolescents: A logistic and multinomial logistic analysis. Spine, 30, 247–255.

20.

Lanes

Gauron

Spratt

Wernimon

Found

Weinstein

(1995). Long-term follow-up of patients with low back pain treated in a multidisciplinary rehabilitation program. Spine, 20, 801–806.

21.

Liu

X. C.

Thometz

J. G.

Lyon

R. M.

McGrady

(2002). Effects of trunk position on back surface-contour measured by raster stereophotography. American Journal of Orthopedics, 31, 402–406.

22.

Macias

B. R.

Murthy

Chambers

Hargens

A. R.

(2008). Asymmetric loads and pain associated with backpack carrying by children. Journal of Pediatric Orthopaedics, 28, 512–517.

23.

Mackenzie

W. G.

Sampath

J. S.

Kruse

R. W.

Sheir-Neiss

G. J.

(2003). Backpacks in children. Clinical Orthopaedics and Related Research, 409, 78–84.

24.

Mayank

Upendar

Nishat

(2006). Effect of backpack loading on cervical and shoulder posture in Indian school children. Indian Journal of Physiotherapy & Occupational Therapy, 1, 3–12.

25.

Milanese

Grimmer-Somers

(2010). Backpack weight and postural angles in preadolescent children. Indian Pediatrics, 47, 571–572.

26.

National Back Pain Association. (1997). NBPA School Bag Survey ’97: Findings and recommendations. Teddington, UK: Talkback.

27.

Negrini

Carabalona

Sibilla

(1999). Backpack as daily load for school children. Lancet, 354, 1974.

28.

Negrini

(2007). Postural effects of symmetrical and asymmetrical loads on the spines of schoolchildren. Scoliosis, 2, 8.

29.

Pascoe

D. D.

Pascoe

D. E.

Wang

Y. T.

Shim

D. M.

Kim

C. K.

(1997). Influence of carrying book bags on gait cycle and posture of youths. Ergonomics, 40, 631–641.

30.

Saad

K. R.

Colombo

A. S.

João

S. M.

(2009). Reliability and validity of the photogrammetry for scoliosis evaluation: A cross-sectional prospective study. Journal of Manipulative and Physiological Therapeutics, 32, 423–430.

31.

Scott

N. D.

Dangerfield

P. H.

Dorgan

J. C.

(1996). The relationship between surface and radiological deformity in adolescent idiopathic scoliosis: Effect of change in body position. European Spine Journal, 5, 85–90.

32.

Sheir-Neiss

G. I.

(2003). The association of backpack use and back pain in adolescents. Spine, 28, 922–930.

33.

Shumway

C. A.

(2001). Control of normal mobility. In Shumway

C. A.

Woollacott

M. H.

(Eds.), Motor control: Theory and practical applications (pp. 128–234). Baltimore, MD: Lippincott Williams and Wilkins.

34.

Świerc

(2011). Computer diagnostics of body postures: The manual. Czernica Wrocławska, Poland: CQ Elektronik System.

35.

Tulder

Koes

Bouter

(1997). Conservative treatment of acute and chronic nonspecific low back pain. Spine, 22, 2128–2156.

36.

Viry

Creveuil

Marcelli

(1999). Nonspecific back pain in children. A search for associated factors in 14-year-old school-children. Revue du Rhumatisme (English ed.), 66, 381–388.