Abstract
Objective
The purpose of this narrative review is to summarize the effects of carrying school backpacks on spine and low-back biomechanics as a risk factor for low back pain in young individuals.
Background
Backpacks constitute a considerable daily load for schoolchildren. Consistently, a large number of children attribute their low back pain experience to backpack use.
Method
A literature search was conducted using a combination of keywords related to the impact of carrying backpacks on lower back biomechanics. The references of each identified study were further investigated to identify additional studies.
Results
Twenty-two studies met inclusion criteria. A total of 1,159 people aged 7 to 27 years were included in the studies. The added load of a backpack and the changes in spinal posture when carrying a backpack impose considerable demand on internal tissues and likely result in considerable spinal loads. The findings included results related to the effects of backpack weight and position on trunk kinematics and spine posture as well as trunk muscle activity during upright standing, walking, and ascending and descending stairs.
Conclusion
Backpack-induced changes in trunk kinematics for a given activity reflect alterations in mechanical demand of the activity on the lower back that should be balanced internally by the active and passive responses of lower back tissues. Although the reported alterations in trunk muscle activities and lumbar posture are indications of changes in the active and passive response of the lower back tissues, the resultant effects on spinal load, that is, an important causal factor for low back pain, remains to be investigated in the future. A knowledge of backpack-induced changes in spinal loads can inform design of interventions aimed at reduction of spinal load via improved backpack design or limitation on carrying duration.
Application
This narrative review is intended to serve as an educational article for students and trainees in ergonomics and occupational biomechanics.
Introduction
Low back pain (LBP) is a growing concern for young people with 40% of 9- to 18-year-olds across the world reporting they have had LBP (Calvo-Munoz, Gomez-Conesa, & Sanchez-Meca, 2013; Negrini & Carabalona, 2002). Reported annual prevalence of LBP ranges from 22% to 51% in children aged 12 to 16 years (Watson et al., 2002) and is responsible for missed school days and sleeping problems in 20% and 50% of children, respectively (Roth-Isigkeit, Thyen, Stoven, Schwarzenberger, & Schmucker, 2005).
The weight carried in a backpack has been suggested to play a pathogenic role in the development of LBP in children (Negrini & Carabalona, 2002; Nicolet, Mannion, Heini, Cedraschi, & Balague, 2014). Furthermore, 82% of children aged 11 to 14 years with LBP attribute their pain to backpack use (Shymon et al., 2014). Backpack loads for young people have increased over the past two decades, raising concerns among medical practitioners and parents about the possible detrimental effects (Al-Khabbaz, Shimada, & Hasegawa, 2008) on their health. Recent studies from different countries have shown average backpacks in school children are heavier than the recommended amount of 10% to 22% body weight (BW) (Brzek et al., 2017; Mackenzie, Sampath, Kruse, & Sheir-Neiss, 2003; Negrini & Carabalona, 2002; Sheir-Neiss, Kruse, Rahman, Jacobson, & Pelli, 2003; Whittfield, Legg, & Hedderley, 2001). This is concerning because LBP at a young age has been suggested to play an important role in developing chronic LBP in adulthood (Negrini & Carabalona, 2002). Despite concerns regarding the negative health effects of heavy backpacks, there is limited knowledge about the mechanism(s) linking carrying a heavy backpack with development of LBP in young people to inform suitable interventions.
Repetitive loading of the lumbar spine increases the risk of LBP through cumulative or overuse injuries to spinal tissues (Adams, 2013; Mackie & Legg, 2008). Both the frequency and magnitude of loads acting on the spine contribute to risk of cumulative injuries (Brinckmann, Biggemann, & Hilweg, 1988). Backpacks constitute a considerable daily “occupational” load for schoolchildren (Shymon et al., 2014); backpacks are often carried during repetitive or prolonged activities of daily living such as standing, walking, jogging, and stair climbing. Under such conditions, spinal loads are likely to increase considerably. The added mechanical demands of the backpack load on the lower back alters trunk muscle response and recruitment (e.g., involving coactivation) because of muscle fatigue and/or spinal instability (Cholewicki, Panjabi, & Khachatryan, 1997; Granata & Orishimo, 2001; Potvin & O’Brien, 1998).
Vertebrae ossification is not complete until the mid-20s and the relatively high amount of cartilage in the skeletons of children put them at greater risk of overuse injuries compared with adults (O’Day, 2008). Vulnerability of cartilage to shear stresses and repetitive trauma decrease soft-tissue flexibility and induce muscle imbalances (O’Day, 2008). Given that overuse injuries in spinal tissues are likely to have a role in developing LBP, the objective of this review is to summarize the findings of studies that have investigated the effects of carrying school backpacks on the lower back mechanics of young people. Specifically, how backpacks affect forces and deformations of lower back tissues—also referred to as mechanical environment of the lower back throughout this review—will be examined.
Method
A literature search was conducted to identify all pertinent research studies regarding the effects of backpacks on the spine and lower back biomechanics among young people. InfoKat Discovery, a search engine offered by the University of Kentucky library system, was used to search for peer-reviewed articles using combinations of keywords (Table 1) in the title or abstract. InfoKat Discover searches many scientific and medical databases including PubMed, CINAHL, and Ei Compendex. Initial screening identified studies with at least one keyword from each category in its title or abstract. The reference lists from identified articles were checked for additional sources. The first author conducted the search and provided initial screening of the identified literature. Assistance from the coauthors was provided in secondary screening of articles to assure relevance to this review. Specifically, the inclusion criteria considered for this review were (a) reporting of biomechanical measures related to the lower back and (b) involving backpack use during activities of daily living. Some studies had outcome measures in addition to those related to the lumbar region of the back and were also included in this review. Due to the small number of papers meeting these criteria, no limit on the publication year was set. Information regarding sample size, age, gender, backpack type, loading type and location, load (%BW), measurement method, task and duration, and outcome measures were extracted from the final set of articles and are summarized in Table 2.
Four Groups of Keywords Were Used to Search the Databases
Note. During the initial search, an article would qualify for additional screening if its title or abstract contained at least one keyword from each category.
Summary of the 22 Reviewed Studies Meeting Review Criteria, Sorted Alphabetically by Last Name of First Author
Note. A typical school backpack is referred to as normal backpack. BW = body weight; M = male; F = female; EMG = electromyography; CCD = charged coupled device; MRI = magnetic resonance imaging; 3D = three-dimensional.
All articles were further screened to exclude any studies that did not investigate biomechanical measures in young people related to carrying a backpack. Because most of the identified backpack studies focused on the effects of weight or position of the backpack on the lower back and spine biomechanics during upright standing, walking, and ascending and descending stairs, this review has been organized to present relevant findings according to these variables.
Results
The initial database search identified 42 papers, of which 22 met our criteria and were included in the review (Table 2).
Effects of Backpack Weight
Posture and kinematics
Backpack loading has been reported to affect deformation of lumbar disks (i.e., L1-L2, L4-L5, and L5-S1) with a positive association between loading and deformation (Shymon et al., 2014). Specifically, a backpack load of ~4 kg, relative to no-load condition, was found to cause ~ 13% decrease in the height of anterior region of the L5-S1 intervertebral disk. Backpack-induced alterations in lumbar posture have been suggested to adversely affect repositioning ability of the lumbar spine (Brzek et al., 2017; Chow, Leung, & Holmes, 2007; Pascoe, Pascoe, Wang, Shim, & Kim, 1997; Shymon et al., 2014).
In standing posture, backpack-induced forward trunk inclination, relative to a vertical line, has been reported to range between 3.02° and 6.8° for backpack weights ranging from 10% to 20% of BW (Brackley, Stevenson, & Selinger, 2009; Kistner, Fiebert, Roach, & Moore, 2013; Mackie & Legg, 2008). Backpack-induced forward trunk inclination has also been observed under lighter backpack weights (Ramprasad, Alias, & Raghuveer, 2010). Specifically, Ramprasad et al. (2010) studied 209 males of average age 12.5 years and reported an increase in forward trunk inclination of 3.21° compared with the no-backpack condition when using a backpack weight equal to 5% of BW (Ramprasad et al., 2010).
On the contrary, a study of 19 males with an average age of 21 years found an average trunk backward inclination of 3.43° during standing for backpack weights of up to 20% of BW (Al-Khabbaz et al., 2008). The conflicting results of this study may be due in part to the material used to increase the load. In the study by Al-Khabbaz et al. (2008), backpacks were filled with sand, which is more likely to collect at the bottom of the backpack, compared with weights or books such as in the study by Ramprasad et al. (2010).
In studies investigating the effects of backpack weight during standing, a negative relationship has been reported between lumbar lordosis (T12-L3-S1) and thoracic kyphosis (C7-T2-T5) and backpack weight. Specifically, an average of ~3° flattening in lumbar lordosis and thoracic kyphosis angles with 10% of BW increase in backpack weight has been shown (Chow et al., 2007; Walicka-Cuprys, Skalska-Izdebska, Rachwal, & Truszczynska, 2015). Negative relationships also were drawn between sacrum inclination and weight of backpack in 109 (58 girls and 51 boys) 7-year-old children such that an increase in backpack weight was associated with a decrease in sacrum inclination (backward pelvis tilt). The average difference in sacrum inclination between children used a backpack lighter than 10% of BW versus those who used a backpack heavier than 10% of BW was ~5° (Walicka-Cuprys et al., 2015).
During walking, trunk forward inclination has been reported to increase from 4.84° to 19.80° by increasing the backpack’s weight from 10% to 20% of BW (Goodgold et al., 2002; Hong & Brueggemann, 2000; Hong & Cheung, 2003). Furthermore, backpack-induced forward inclination of the trunk during walking has been reported to increase not only by increases in backpack weight but also by increases in walking pace and distance (Goodgold et al., 2002; Hong & Brueggemann, 2000; Hong & Cheung, 2003). Li et al. investigated backpack-induced changes in trunk kinematics among 15 males with a mean age of 10.36 years and found that walking with a backpack heavier than 10% of BW induced a 4.55° increase in forward trunk inclination compared with the no-backpack condition after only 1 min (Li, Hong, & Robinson, 2003). Goodgold et al. assessed trunk posture for two male subjects during running under various backpack weights. They found the maximum trunk forward inclination not to change in one subject but decrease ~7° in the other subject when increasing backpack weight from 8.5% to 17.5% of BW. The maximum average for the no-backpack condition was 14.2° (Goodgold et al., 2002).
For ascending stairs (33 steps), the lumbar flexion of 13 male children (average age 12.2 years) was investigated. Increasing the backpack load up to 20% of BW was not found to affect range of lumbar flexion during ascending and descending stairs, but there were >6° larger lumbar range of flexion in ascending versus descending stairs (Hong, Fong, & Li, 2011).
Muscle activity
During standing, an increase in rectus abdominis and obliques activity and a decrease in bilateral muscle activity of the erector spinae longissimus have been reported for a backpack load of 15% of BW when compared with no-backpack condition (Devroey, Jonkers, de Becker, Lenaerts, & Spaepen, 2007).
Using 10 males of mean age 19.9 years, Goh et al. investigated the effects of backpack loading on lower back net moment during walking. They observed that carrying a given backpack load resulted in a nonlinear increase in the L5/S1 joint moment (26.67% for a load of 15% of BW; 64% for a load of 30% of BW) (Goh, Thambyah, & Bose, 1998). Such disproportionate increase in L5/S1 moment suggests a substantial demand on trunk muscles to offset the task demand.
Effects of Backpack Position
Posture and kinematics
In addition to the backpack weight, the position (vertical and horizontal) of the backpack relative to the back affects spine kinematics and kinetics. Contrary to the widespread belief that backpacks should be worn high on the back (Brackley et al., 2009), most studies indicated that children experience the least amount of postural deviations when the backpack is placed low on the back (Brzek et al., 2017; Grimmer, Dansie, Milanese, Pirunsan, & Trott, 2002; Singh & Koh, 2009). Apart from changes in spinal posture under symmetric backpack load, studies reported excessive postural deviation, mainly in the coronal plane, under asymmetric load (i.e., backpack on the left or right shoulder) (Brzek et al., 2017; Singh & Koh, 2009).
For standing posture, a study involving 162 children (82 girls and 80 boys) aged 11 to 13 years found that asymmetric backpack loads compared with no backpack resulted in ~11% reduction in thoracic kyphosis (Drzal-Grabiec, Snela, Rachwal, Podgorska, & Rykala, 2015; Drzal-Grabiec, Truszczynska, et al., 2015). However, none of these studies reported the outcome measures for symmetric loading. Thoracic placement (top of the backpack on the shoulder line) compared with lumbar placement (bottom of the backpack carried just above the spina iliac posterior superior) of backpack was found to be associated with larger pelvic forward rotation (~4°) and larger hip flexion (~3°) (Devroey et al., 2007). Although there was no significant change in lumbar flexion or thoracic rotation for either placement compared with the no-backpack condition during standing, there was a trend that included lumbar extension for thoracic placement and lumbar flexion for lumbar placement. Placement of backpack on thorax versus lumbar spine was found to cause changes in thorax and lumbar posture during walking similar to those observed during upright standing, except for an increase in lumbar flexion and trunk range of motion (Devroey et al., 2007).
Both anterior (front of body) and posterior (back of body) placement of backpack on the trunk resulted in changes in spinal posture, changes that were magnified with increasing backpack load (Chow, Ou, Wang, & Lai, 2010). When the backpack was placed anteriorly with its center of mass located at the T7 spinal level, an increase in pelvic backward tilt (5.5°) was observed. When placed posterior on the trunk, with the backpack’s center of mass at the T7, T12, and L3, there were 6.0°, 5.4°, and 3.3° increases in lower lumbar spine flexion, respectively (Chow et al., 2010). Furthermore, for the same order of positions (i.e., T7, T12, and L3), there were significant increases in upper thoracic rotation (4.4°), lower thoracic rotation (2.0°), and upper lumbar flexion (3.0°), respectively (Chow et al., 2010). The smallest change in spinal posture was observed when the backpack’s center of mass was positioned in front and at the T12 level (Chow et al., 2010).
For asymmetric backpack loading when ascending stairs, Hong et al. (2011) reported an increase of ~3° in trunk lateral bending toward the supported side (strap side) and a decrease of ~2.7° in trunk lateral bending of the loaded side compared with stairs ascending with symmetric backpack loading (Hong et al., 2011). A similar pattern of results but with smaller difference between symmetric and asymmetric loading were found during stairs descending (Hong et al., 2011).
Muscle activity
In general, regardless of backpack positioning, there were significant changes, relative to a no-backpack condition, of bilateral trunk muscle activity for walking tasks (Devroey et al., 2007). These included increases in activity of abdominal muscles and decreases in activity of erector spinae without any backpack-induced co-activation (Devroey et al., 2007).
Discussion
The objective of this narrative review was to summarize the findings of studies that have investigated the effects of carrying school backpacks on the lower back of young people. Although narrative reviews serve as useful educational tools, they do not offer a foundation for design of intervention or making clinical decisions (Green, Johnson, & Adams, 2006). When interpreting the results of studies discussed in this review, the readers should keep in mind that the strengths and weaknesses of the reviewed studies were not discussed due to the nature of this narrative review (e.g., as compared with systematic reviews).
Carrying a backpack in upright standing posture was reported to cause a forward inclination of trunk among young individuals that increased with the weight of backpack as well as the height of its center of mass but decreased if backpack was carried in front of the trunk. There was also an exception wherein carrying backpack caused backward inclination of trunk, an exception that likely was due to the type of load used inside the backpack (i.e., sand versus books). A reduction in lumbar lordosis (i.e., flattening) was also reported with increases in the backpack weight and the height of its center of mass. Backpack-induced flattening of lumbar lordosis is consistent with report of decrease in the height of anterior aspect of lumbar disk from an imaging study. It is also consistent with reports of decrease in activity of extensor muscles and increase in activity of abdominals (i.e., reflecting a shift in demand of the task from flexion to extension demand). Carrying backpack in walking compared with upright standing was reported to cause larger backpack-induced changes in trunk kinematics and lumbar posture, changes that further increased with pace and distance of walking. Trunk kinematics and lumbar posture, however, were not found to be affected by backpack when ascending or descending stairs. Finally, carrying backpack on one shoulder was reported to cause asymmetric trunk kinematics mainly due to deviations in the coronal plane.
Abnormal mechanics of the lower back, including excessive forces and deformations, have been shown to directly and indirectly irritate pain-sensitive nerve endings in tissue and cause LBP (Adams, 2004; Marras, 2008; McGill, 2007; White, 1990). The backpack-induced changes in trunk inclination are likely a response to the posterior shift in the upper body center of mass due to addition of backpack mass. Such a posterior shift in the upper body center of mass negatively affects whole body balance that is likely offset by the reported backpack-induced trunk forward inclination (Winter, 2005). The reported changes in the posture of lumbar spine and trunk muscle activity, on the contrary, are likely in response to changes in equilibrium and stability requirements of the spine that are directly related to forces and deformation experienced in the lower back. Specifically, the added load of backpack imposes an extension moment (a flexion moment in the case of anteriorly positioned backpack) on the spine that should be balanced internally by the active and passive responses of lower back tissues to keep the trunk in the desired posture. Alterations in lumbar posture reflect changes in passive contribution of lower back tissues (due to stretching) to spine equilibrium, whereas changes in measured muscle activity indicate alterations in active contribution of lower back tissues (muscles). Furthermore, increasing the height of backpack center of mass, while may not affect the equilibrium demand of the backpack on the spine, increases stability requirement of carrying backpack that should primarily be balanced by the active muscle responses (Granata & Orishimo, 2001). These changes in lumbar spine equilibrium and stability demands due to carrying backpack lead to substantial increases in spinal loads even under activities that are not physically demanding (e.g., walking). However, the actual impact of backpack on spinal loads (i.e., the resultant of internal tissue responses and external mechanical demand of the task) during daily activities remains unclear. In other words, trunk kinematics, lower back posture, and trunk muscle activities, which are affected by carrying backpack, influence spinal loads but should be coupled with biomechanical models to acquire estimates of backpack-induced changes in loads experienced in spinal tissues. Such future studies could also include investigation of the effects of the magnitude and distribution of backpack weight as well as backpack position and its attachments to trunk on spinal loads.
The risk of fatigue failure of spinal tissues under typical repetitions of daily activities (e.g., 10,000 steps walking) is relatively low for the magnitude of spinal loads experienced during most daily activities. However, the risk of fatigue failure substantially increases with even modest increases in spinal loads associated with carrying a backpack (Brinckmann et al., 1988; Gallagher & Heberger, 2013). To better understand the role of carrying a school backpack on the development of LBP among children, it is therefore important to determine backpack-induced changes in spinal loads due to not only the immediate but also the prolonged effects of carrying a backpack on lower back mechanics. For instance, estimates of spinal loads calculated for one cycle of a repetitive task can be used in combination with fatigue failure models of spinal tissues (Motiwale, Subramani, Kraft, & Zhou, 2018) to acquire insight related to risk of fatigue failure for spinal tissues associated with a given number of repetition (e.g., number of steps) of that task. Such studies will offer an important foundation not only for better design of school backpacks via ergonomics principles (e.g., in terms of load distribution and contact with the trunk) but also for the development of recommendations for durations of carrying backpack that could mitigate the prolonged adverse biomechanical effects of current school backpacks.
Key Points
Back pain is an increasing concern in young individuals.
Backpacks have been suggested to play a pathogenic role in developing back pain in adolescents.
Researchers have observed deviations in trunk kinematics, lumbar posture, and trunk muscle activities while wearing backpacks during activities of daily living.
Future research on the immediate and prolonged effects of carrying backpack on spinal loads can inform design of intervention aimed at reduced risk of low back injury.
Footnotes
Acknowledgments
The authors would like to acknowledge and thank Beth Axtell for her contribution as technical editor for this review. C.S. and I.S. were supported in part by an award (5R03HD086512-02) from the National Center for Medical Rehabilitation Research (NIH-NICHD) and an award from the Office of the Assistant Secretary of Defense for Health Affairs, through the Peer Reviewed Orthopaedic Research Program (award #W81XWH-14-2-0144).
Author Biographies
Cazmon Suri earned his MSc in biomedical engineering from the University of Kentucky in 2018. His current research interests include biomechanics of human musculoskeletal systems.
Iman Shojaei earned his PhD in biomedical engineering from the University of Kentucky in 2019. His current research interests include computational methods, biomechanics, and structural engineering.
Babak Bazrgari is an associate professor of biomedical engineering at the University of Kentucky. He received his PhD from École Polytechnique de Montréal in 2008.