- Email: [email protected]
The Association of Self-Reported Backpack Use and Backpack Weight With Low Back Pain Among College Students
THE ASSOCIATION OF SELF-REPORTED BACKPACK USE BACKPACK WEIGHT WITH LOW BACK PAIN AMONG COLLEGE STUDENTS
AND
Zachary Heuscher, MS,a David P. Gilkey, DC, PhD, CPE, b Jennifer L. Peel, PhD, MPH,c and Catherine A. Kennedy, PhDd
ABSTRACT Objective: Back pain has consistently ranked among the top general health complaints among college students, but few studies have examined risk factors for back pain in this age group. This cross-sectional survey evaluated the association between the self-reported annual low back pain with the estimated usual backpack weight among college students. Methods: Data were collected from health education students during the spring semester of 2007 at the Colorado State University using an online survey. Adjusted odds ratios were calculated using logistic regression. Results: Four hundred sixty-five (94.6%) health education students completed the online survey. The annual prevalence of low back pain was 29.2% (n = 136). A 25% increase in the odds of annual low back pain for each 4-kg increase in the estimated usual backpack weight was observed after adjusting for sex, smoking, reporting frequently feeling overwhelmed, and body mass index (adjusted odds ratio per 4-kg increase, 1.25; 95% confidence interval, 1.17-1.32). There was no evidence of an increased association of annual low back pain with carrying a backpack weight greater than 10% of the students body weight compared with those carrying less (adjusted odds ratio, 1.02; 95% confidence interval, 0.63-1.65). Conclusions: The results of this study suggest that increasing reported backpack weight is associated with increased prevalence of annual low back pain. However, these results do not provide evidence to support the recommendation that the backpack weight necessarily be less than 10% of body weight. (J Manipulative Physiol Ther 2010;33:432-437) Key Indexing Terms: Low Back Pain; Prevalence; Students; Backpack
ower back pain is one of the most common medical conditions encountered by the health care providers with 7% of the adult population consulting their general medical practitioner each year.1 Most research on
L a
Masters in Epidemiology, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. b Associate Professor and Director of Undergraduate Education, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. c Assistant Professor, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. d Associate Professor, Department of Health and Exercise Science, Colorado State University, Ft. Collins, Colo. Submit requests for reprints to: David P. Gilkey, DC, PhD, CPE, Department of Environmental and Radiological Health Sciences, Colorado State University, 146 EH Bldg, Fort Collins, CO 80523-1681 (e-mail: [email protected]). Paper submitted November 1, 2009; in revised form February 16, 2010; accepted June 17, 2010. 0161-4754/$36.00 Copyright © 2010 by National University of Health Sciences. doi:10.1016/j.jmpt.2010.06.003
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nonspecific low back pain has taken place within the occupational setting primarily on adults; therefore, there is a lack of understanding about the initial onset of low back pain among adolescents and young adults.1-3 The incidence of low back pain among adolescents and young adults has been shown to be similar to adult rates.2,4-6 The incidence of adolescent low back pain has been found to increase during the course of high school and may be associated with students who work during the school year.2 According to the National College Health Assessment (NCHA) survey, back pain has increased from 44.2% to 47.7% between the years 2003 (n = 19 497) and 2007 (n = 71 860), respectively.7,8 A prospective study among adolescent British school children showed a nonsignificant increase in the risk for future low back pain for carrying a heavy backpack weight.6 Whereas Brattberg9 and Hestbaek10 concluded that there is an increased incidence of back pain among adults who had had back pain as adolescents, studies measuring the relationship between a student's backpack weight as a percentage of body weight and low back pain have been conflicting, 11-13 but the growing weight of evidence suggests that use of backpacks and increasing loads enhance
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Heuscher et al Backpack Weight and Low Back Pain Among College Students
Table 1. Frequencies of low back pain in university men and women during the preceding 12 months, the College Back Survey, 2007 Low back pain measure
Males, n (%) (n = 192)
Females, n (%) (n = 273)
Percent difference (95% CI for the % difference)
Ever had low back injury/problem in lifetime Low back pain in the previous year Low back pain in the previous 2 wk Lives with continuous low back pain, 462 (99%)
70 52 19 21
106 (38.8) 84 (30.8) 30 (11.0) 33 (12.2)
−2.3 −3.7 −1.1 −1.3
(36.5) (27.1) (10.0) (10.9)
(−11.3 to 6.6) (−12.0 to 4.6) (−6.7 to 4.5) (−7.1 to 4.6)
CI is the confidence interval calculated using the Z test for the difference in 2 proportions.
the risk of back pain among children and adolescents.14 Studies evaluating the effect of increasing backpack loads measured as a proportion of body weight among children found alteration in disc height,15 and in adolescents and young adults postural changes including muscle activity alteration using electromyography,16-19 digital inclinometry20 as well as subjective reporting of stress and pain.16-18 In this study, we examined the cross-sectional relationship between the perceived backpack weight and self-reported low back pain in college students while controlling for multiple confounders to gain a better understanding of the relationship between these factors.
METHODS Data for the proposed study were obtained from the 2007 College Back Survey, which was motivated by previous results from a study using the NCHA survey.2 The data for this study were collected by having undergraduate students at Colorado State University enrolled in a freshman-level health education class complete an online survey, entitled the College Back Survey. No personal identifiers were recorded. The study was approved by the Colorado State University Institutional Review Board. We determined the prevalence of low back pain in the previous year using the question, “Have you had an episode of low back pain in the last year that caused you to alter some aspect of your normal living or seek treatment?” A total of 473 (94.6%) of the eligible 500 health education students were given extra credit for completing the online survey. Eight subjects were excluded from the analysis, with 7 students missing values for annual low back pain and 1 student missing a value for sex, resulting in 465 (93.0%) subjects with complete information. An additional 23 subjects who did not use backpacks were excluded from the analytic models comparing backpack weight and backpack weight as a percentage of body weight to annual low back pain. Mantel-Haenszel common odds ratio (OR) estimates and 95% confidence intervals (CIs) were calculated for the association between self-reported low back pain by each exposure variable of interest: backpack use (always vs mostly or never use), backpack weight (per a 4-kg increase in reported weight), and relative backpack weight (backpack weight N10% of body weight vs ≤10%) using SPSS, version 17.0 (SPSS, Inc., Chicago, IL).
Adjusted odds ratios were calculated using the hierarchically well-formulated model approach to multiple logistic regression models.21 We also evaluated interaction by sex on the relationship of the backpack exposure variables and low back pain. Because students were asked to estimate their average daily backpack weight, a validation study was performed in a sample of 238 students (who did not participate in the original survey) at the spring and fall 2008 student health fairs on the Colorado State University campus to determine the accuracy of the students' estimates of their backpack weight. The students in the validation study were asked to estimate their backpack weight, and then the interviewer measured the actual backpack weight with a dynamometer. The Wilcoxon paired t test, calculated using the univariate procedure in SAS version 9.1 (SAS Institute, Cary, NC), was used to determined if the estimated backpack weights were different than the actual weights.
RESULTS The annual prevalence of low back pain (defined as having an episode/injury/or problem causing them to alter some aspect of their normal living or seek treatment) was 29.2% (n = 136), the lifetime prevalence was 37.8% (n = 176), and the 2-week prevalence was 10.6% (n = 49) (Table 1). Females were more likely to self-report low back pain than males for all prevalence measures and for different types of pain (Table 1). Most of the students participating in the survey were non-Hispanic white (86.5%), followed by Hispanic American (8.0%), and Asian or Pacific Islander (2.2%) (Table 2). Students who worked in addition to their school studies were more likely to report annual low back pain (Table 2). Males were more likely to report annual low back pain if they worked greater or equal to 17 hours per week (32.8%) when compared with females who worked the same amount (20.2%). Most low back injuries/problems were attributed to cumulative activity (23.3%) as opposed to a single incident (19.8%) (Table 3). The most common treatment used for the low back injury/problem was self-care (42.4%), followed by chiropractic care (12.3%), physical therapy (7.7%), and visiting a medical doctor (6.5%) (Table 3). Results from the validation study of 238 students showed that the average backpack weight carried by students was 5.2 kg (SD, 1.9) (or 11.5 lb) and that students
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Table 2. Frequencies of variables of interest by self-reported low back pain status among university students, 2007 Characteristic Backpack use, n (%) (N = 465) Always Most of the time to never Reported weight of backpack, mean (SD) (kg) Weight of backpack % of body weight, n (%) (N = 465) Greater than or equal to 10% Less than 10% Excluded a Age, mean (SD) (y) Body mass index, mean (kg/m2) (SD) Sex, n (%) (N = 465) Female Male Smoking, n (%) (n = 464) Current smoker Former smoker Never smoked Feeling overwhelmed, n (%) (n = 465) Always to frequently Sometimes to never Hours worked per week, n (%) (N = 465) Greater than or equal to 17 h Less than 17 h Race/ethnicity, n (%) (N = 465) Non-Hispanic white Hispanic American African American Native American Asian or Pacific Islander Other
Total population (N = 465)
Low back pain in the previous year (n = 136)
No low back pain in the previous year (n = 329)
307 (66.0) 158 (34.0) 6 (3.3)
93 (68.4) 43 (31.6) 7 (3.1)
214 (65.0) 95 (28.9) 6 (3.4)
119 (25.6) 323 (69.5) 23 (4.9) 19.7 (2.7) 23.3 (3.9)
35 (25.7) 92 (67.6) 9 (6.6) 20.0 (2.5) 23.7 (4.7)
84 (25.5) 231 (70.2) 14 (4.2) 19.6 (2.8) 23.1 (3.5)
273 (58.7) 192 (41.3)
84 (61.8) 52 (38.2)
189 (57.4) 140 (42.5)
59 (12.7) 56 (12.1) 349 (75.2)
15 (11.0) 24 (17.6) 97 (71.3)
44 (13.4) 32 (9.8) 252 (76.8)
175 (37.6) 290 (62.4)
54 (39.7) 82 (60.3)
121 (36.8) 208 (63.2)
97 (20.8) 368 (79.2)
34 (25.0) 102 (75.0)
63 (19.2) 266 (80.8)
402 (86.5) 37 (8.0) 8 (1.7) 2 (0.4) 10 (2.2) 6 (1.2)
117 (86.0) 12 (8.8) 2 (0.1) 1 (0.1) 1 (0.0) 1 (0.0)
285 (86.4) 25 (7.6) 6 (1.6) 1 (0.0) 9 (2.7) 5 (1.4)
a Subjects were excluded if they responded that they “never use a backpack to carry books and other school supplies” and still reported an “approximate weight of their backpack on an average day.”
Table 3. Frequencies of various self-reported low back pain measures among university students, 2007 Back pain measure (N = 465) Care sought for back injury/ problem No pain—no care Self treatment only Chiropractor Physical therapist Medical doctor Nurse practitioner Osteopathic doctor Other Low back injury a single incident or cumulative from activity (n = 464) a None—never had a back injury Cumulative (from repeated activity) Single incident (lifting something too heavy) a
n (%) 197 (42.4) 120 (25.8) 60 (12.9) 36 (7.7) 30 (6.5) 7 (1.5) 5 (1.1) 10 (2.2)
239 (51.5) 133 (23.3) 92 (19.8)
Data include all cases: point, annual, lifetime, and chronic.
overestimated their backpack weight by an average of 1 kg (95% confidence interval for the paired t test, 0.6-1.4) (SD, 3.1). A suggestive but not strong association was observed for the odds of annual low back pain using a backpack always compared to mostly to never adjusting for age
(years), sex, smoking (current, former, never [reference]), feeling frequently overwhelmed (mostly to always, sometimes to never [reference]), and body mass index (kg/m2). After adjusting for age (years), sex, smoking (current, former, never), feeling overwhelmed (mostly to always), and body mass index (kg/m2), a 25% increase in the odds of annual self-reported low back pain for each 4-kg increase in the estimated backpack weight was observed (adjusted OR per 4-kg increase, 1.25; 95% CI, 1.17-1.32) (Table 4). After adjusting age (years), sex, smoker (current, former, and never), and feeling overwhelmed frequently (always to mostly), age, body mass index, and sex, there was no evidence of an increased association of annual low back pain with carrying a backpack weight greater than 10% of the student's body weight compared with those carrying less (adjusted OR, 1.02; 95% CI, 0.63-1.65) (Table 4).
DISCUSSION The average age of the population was 19.7 years (SD, 2.7) and the ethnicity was predominately non-Hispanic
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Table 4. Unadjusted and adjusted OR and 95% CI for exposure variables of interest, the College Back Survey, 2007
Exposure variable Backpack use Always Most of the time to never Among backpack users Weight of backpack (kg) 4-kg unit increase Weight of backpack as % of body weight Greater than or equal to 10% Less than 10%
Adjusted OR (n = 463)
Unadjusted OR (n = 472)
OR
OR
95% CI
95% CI
1.23 a 0.79-1.92 1.00 (n = 437)
1.16 0.76-1.78 1.00 (n = 441)
1.25 b
1.17-1.32
1.29
1.22-1.37
1.02 c
0.63-1.65
1.05
0.66-1.66
1.00
1.00
a
Adjusted for age (years), sex, smoking (current, former, never [reference]), feeling frequently overwhelmed (mostly to always, sometimes to never [reference]), and body mass index (kg/m2). b Adjusted for age (years), sex, smoking (current, former, never), and feeling overwhelmed (mostly to always), and body mass index (kg/m2). c Adjusted for age (years), sex, smoker (current, former, and never), and feeling overwhelmed frequently (always to mostly).
white (Table 1). The association between annual low back pain and backpack use or weight found among this population may not be generalizable to a non-college population, a different age group, or a population with a different morphology, racial and ethnic distribution. The 29.2% annual prevalence estimate for low back pain from the present study was lower than the 47.7% estimate from the NCHA survey2 and the 42.8% determined among the 973 undergraduate students attending Colorado State University, 7 although the present study may have represented a more substantial pain/condition. Previous surveys among college students did not qualify the back pain; therefore, the prevalence estimates may have represented a less severe back pain. Our study qualified an episode of back pain as that which interfered with activities of daily living and/or compelled the individual to seek medical care. Because there is no standard method to characterize exposure to backpack use and low back pain, the relationship between these 2 factors often varies between studies.12,14 Our results contrast with a previous cross-sectional study that found a modest association between the heavy use of backpacks (wears backpack between classes and while standing) and self-reported 4-week prevalence of back pain (at least a 2 on a scale from 0 to 10 for severity of pain) when compared to no or low use (does not wear backpack between classes but carries backpack while waiting) among 12- to 18-year-old students (OR, 1.98; 95% CI, 1.422.76).10 Lindstrom-Hazel 14 reviewed 11 epidemiologic studies published between 2003 and 2007 and reported that 10 of 11 investigators did find association of backpack use and back pain. Our study provided similar findings as the
prospective study among British adolescent school children, which determined the relative risk of carrying a heavy backpack weight (6.4-18 kg compared to less than 3.4 kg) and the risk for future low back pain (adjusted RR, 1.2; 95% CI, 0.7-2.1).6 The lack of an established temporality in our study between the self-reported backpack use or relative backpack weight and annual low back pain is a major limitation. Our findings do suggest that increasing backpack weight by increments of 4 kg was associated with increased risk for LBP but did not necessarily support that a 10% body weight threshold reported by others as associated with LBP. The self-reported backpack weight as a percentage of body weight showed no significant association with low back pain, which contrasts with another cross-sectional study that determined that low back pain was associated with a relative backpack weights but only at weights greater than 20% of the subject's body weight and within an much younger population (OR, 3.1; 95% CI, 1.09.2).11 Devroey et al16 evaluated the effects of backpack weight unloaded and loaded to 5%, 10%, and 15% of body weight using electromyography, kinematic sensors, and subjective reports. The investigation team found significant differences in muscle activity, postural alteration, and subjective complaints with increasing loads and concluded that loads should not exceed 10% of body weight. Chow et al20 recently found that backpack loads of 15% of body weight resulted in significant postural alteration using spinal accelerometers and inclinometers and concluded that a load placement with the center of gravity at the 12 spinal segment resulted in the least postural deviation. Al-Khabbaz et al17 investigated effects of incremental backpack loads at 10%, 15%, and 20% of body weight among 19 university students using electromyography and activity sensors to detect changes in posture inclination and muscle activity with increasing loads. The team found the greatest muscle activity increases in the abdominal muscles and postural changes toward a more backward inclination. A possible reason why we did not observe a similar association by percentage of body weight is that the relative weight model did not distinguish body fat from muscle mass, its distribution on the body, and different adaptive responses to loading among individuals. None of the studies reviewed reported body fat content or range of body mass indexes of subjects. Our study included a large number of college students using a wide array of backpack designs, whereas the laboratory studies used one type of backpack loaded to various weights. There is evidence that backpack design and placement may be a factor in the activation of postural muscles and thus the precipitation of pain, not just weight.22 Physiologic factors such as the student's posture, low back strength, and low back flexibility and other biomechanical exposures from physical exercise, work, and sports
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were not included as potential confounders within the analysis given the relationship between these variables and the student's backpack weight or use and low back pain was difficult to establish.23,24 The type of backpack used may have also confounded the relationship between backpack weight or backpack use and low back pain given that the position and design of the backpack may influence how much the weight is distributed to the low back and potentially the potential for pain or injury.18,25 This study did not evaluate whether the type of backpack among heavy backpack users increased or decreased the likelihood of reporting low back pain. Measurement errors from students overestimating their daily backpack weight would likely bias the effect estimates toward the null. The validation study only determined if students could estimate their backpack weight at one time and not throughout the course of a day or even throughout a school year. In addition, the validation study did not determine if a student's ability to estimate his or her actual backpack weight changed with the presence of low back pain. If this was true, then the observed association between backpack weight and low back pain would likely bias the effect away from the null.
Limitations This cross-sectional study is inherently limiting and cannot establish causation between exposures and outcomes and limited to evaluating the strength of association between variables. Future prospective studies may accurately addresses the relative risk of backpack users to develop LBP. Our sample was not randomly selected but rather a volunteer convenient sample. Students enrolled in courses surveyed may have felt an obligation to report more than actually experienced. This would bias away from the null and result in overreporting. We do not believe this was the case because of the lower prevalence rate compared to previous studies. The student population at CSU is a young homogeneous group not well representative of the greater heterogeneous general population. The CSU cohort may also be different in unrecognized ways from other groups of college students seen across the nation and should not be directly compared. Results from our survey should be cautiously externalized to other populations. A self-report survey by nature may be subject to response and recall bias. The survey is dependent on the subject's memory and honesty to self-report and respondents may not have recalled accurately or willfully reported incorrect information. Response and recall bias are possible and may influence results toward or away from the null. We developed a definition of low back pain that could have been very broadly interpreted to capture a perceived problem among college students. Such a broad definition certainly tells nothing about the actual pain generator, type of pathology, or medical history of the subject. This
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definition could have biased the reporting and accuracy toward or away from the null.
CONCLUSION This was the first study to our knowledge to investigate important relationship between low back pain and backpack use or weight using a hierarchically well-formulated model approach to multiple logistic regression.21 This method allowed the researchers to control for potential confounders while considering the effect on the precision and validity of the OR estimates. Our study results found that backpack weight increase by 4-kg increments had an association to LBP. These findings reinforce the need for college health prevention efforts to reduce the burden of low back pain by encouraging students to minimize loads when using backpacks. The high prevalence of a substantial low back pain among college students justifies a need to prevent the pain from worsening and becoming a chronic condition later in life10 with the resulting long-term disability, lost workdays, and loss of productivity. Although before students seek treatment they should consider that 90% of low back pain patients who seek medical treatment from physicians, chiropractors, and other health care providers will improve in 4 to 6 weeks using conservative measures.26
Practical Applications • Back pain is the most frequent physical health complaint among college students. • Twenty-nine percent of college students reported having had low back pain within the previous year. • The use of a heavy backpack is evaluated as a risk factor for low back pain.
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
REFERENCES 1. Jones GT, Macfarlane GJ. Epidemiology of low back pain in children and adolescents. Arch Dis Child 2005;90:312-6. 2. Feldman DE, Shrier I, Rossignol M, Abenhaim L. Risk factors for the development of low back pain in adolescence. Am J Epidemiol 2001;154:30-5. 3. Kennedy C, Kassab O, Gilkey D. Psychosocial factors and low back pain among college students. J Am Coll Health 2008;57:191-4. 4. Biering-Sorensen F. A prospective study of low back pain in a general population. Scand J Rehabil Med 1983;15:71-9.
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5. Troussier B, Davoine P, deGaudemaris R, Fauconnier J, Phelip X. Back pain in schoolchildren: a study among 1178 pupils. Scand J Rehabil Med 1994;26:143-6. 6. Jones GT, Watson KD, Silman AJ, Deborah PM. Predictors of low back pain in British schoolchildren: a population-based prospective cohort study. Pediatrics 2003;111:822-9. 7. American College Health Association (ACHA). National College Health Assessment spring 2007 reference group data report (abridged). J Am Coll Health 2008;56:469-79. 8. American College Health Association (ACHA). National College Health Assessment spring 2003 reference group data report (abridged). J Am Coll Health 2005;53:201-11. 9. Brattberg G. Do pain problems in young school children persist into early adulthood? A 13-year follow-up. Eur J Pain 2004;8:187-99. 10. Hestbaek L, Leboeuf-Yde C, Kyvik KO, Manniche C. The course of low back pain from adolescence to adulthood. Spine 2006;31:468-72. 11. Skaggs DL, Early SD, D'Ambra P, Tolo VT, Kay RM. Back pain and backpacks in school children. J Pediatr Orthop 2006; 26:358-63. 12. Sheir-Neiss GL, Kruse RW, Rahman T. The association of backpack use and back pain in adolescents. Spine 2003;28: 922-30. 13. Viry P, Creveuil C, Marcelli C. Nonspecific back pain in children: a search for associated factors in 14-year-old school children. Rev Rhum England Edition 1999;66:331-8. 14. Lindstrom-Hazel D. The backpack problem is evident but the solution is less obvious. Work 2009;32:329-38. 15. Neuschwander TB, Cutrone J, Macias BR, Cutrone S, Merthy G, Chambers H, Hargens A. The effect of backpacks on the lumbar spine in children. Spine 2009;35:83-8. 16. Devroey C, Jonkers I, Becker A, Lenaerts G, Spaepen A. Evaluation of the effect of backpack load and position during
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17. 18. 19.
20.
21. 22.
23. 24.
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standing and walking using biomechanical and subjective measures. Ergonomics 2007;50:728-42. Al-Khabbaz YS, Shimada T, Haseqawa M. The effect of backpack heaviness on the trunk-lower extremity muscle activities and trunk posture. Gait Posture 2008;28:297-302. Chow DH, Leung KT, Holmes AD. Changes in spinal curvature and proprioception of schoolboys carrying different weight of backpack. Ergonomics 2007;40:2148-56. Kim MH, Ti CH, Kwon OY, Cho SH, Yoo WG. Changes in neck muscles electromyography and forward head posture of children when carrying schoolbags. Ergonomics 2008;51: 890-901. Chow DH, Ou ZY, Wang XG, Lai A. Short-term effects of backpack load placement on spine deformation and repositioning error in schoolchildren. Ergonomics 2010;53: 56-64. Kleinbaum DG, Klein M. Logistic regression: a self-learning text. New York: Springer-Verlag Inc.; 2002. Vitiello AL. The effect of backpack on the electromyographical activity of back muscles. Doctoral thesis. Sydney, Australia: Department of Chiropractic, Macquarie University; 2009. Rothman KJ, Greenland S. Modern epidemiology. 2nd ed. Philadelphia: Lippiincott-Raven; 1998. Battie MC, Videman KT. Epidemiology of the back. In: Nordin M, Andersson BJ, editors. Musculoskeletal disorders in the workplace: principles and practice. 1st ed. St. Louis, MO: Mosby-Year Book Inc; 1997. p. 253-67. Grimmer KA, Dansie B, Milanese S, Pirunsan U, Trott P. Adolescent standing postural response to backpack loads: a randomized controlled experimental study. BMC Musculoskelet Disord 2002;10:189-96. Rathmell JP, Ballantyne JC. Evaluating patients with low back pain. Pain Manag Rounds 2005;2:42-8.
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AND
Zachary Heuscher, MS,a David P. Gilkey, DC, PhD, CPE, b Jennifer L. Peel, PhD, MPH,c and Catherine A. Kennedy, PhDd
ABSTRACT Objective: Back pain has consistently ranked among the top general health complaints among college students, but few studies have examined risk factors for back pain in this age group. This cross-sectional survey evaluated the association between the self-reported annual low back pain with the estimated usual backpack weight among college students. Methods: Data were collected from health education students during the spring semester of 2007 at the Colorado State University using an online survey. Adjusted odds ratios were calculated using logistic regression. Results: Four hundred sixty-five (94.6%) health education students completed the online survey. The annual prevalence of low back pain was 29.2% (n = 136). A 25% increase in the odds of annual low back pain for each 4-kg increase in the estimated usual backpack weight was observed after adjusting for sex, smoking, reporting frequently feeling overwhelmed, and body mass index (adjusted odds ratio per 4-kg increase, 1.25; 95% confidence interval, 1.17-1.32). There was no evidence of an increased association of annual low back pain with carrying a backpack weight greater than 10% of the students body weight compared with those carrying less (adjusted odds ratio, 1.02; 95% confidence interval, 0.63-1.65). Conclusions: The results of this study suggest that increasing reported backpack weight is associated with increased prevalence of annual low back pain. However, these results do not provide evidence to support the recommendation that the backpack weight necessarily be less than 10% of body weight. (J Manipulative Physiol Ther 2010;33:432-437) Key Indexing Terms: Low Back Pain; Prevalence; Students; Backpack
ower back pain is one of the most common medical conditions encountered by the health care providers with 7% of the adult population consulting their general medical practitioner each year.1 Most research on
L a
Masters in Epidemiology, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. b Associate Professor and Director of Undergraduate Education, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. c Assistant Professor, Department of Environmental and Radiological Health Sciences, Colorado State University, Ft. Collins, Colo. d Associate Professor, Department of Health and Exercise Science, Colorado State University, Ft. Collins, Colo. Submit requests for reprints to: David P. Gilkey, DC, PhD, CPE, Department of Environmental and Radiological Health Sciences, Colorado State University, 146 EH Bldg, Fort Collins, CO 80523-1681 (e-mail: [email protected]). Paper submitted November 1, 2009; in revised form February 16, 2010; accepted June 17, 2010. 0161-4754/$36.00 Copyright © 2010 by National University of Health Sciences. doi:10.1016/j.jmpt.2010.06.003
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nonspecific low back pain has taken place within the occupational setting primarily on adults; therefore, there is a lack of understanding about the initial onset of low back pain among adolescents and young adults.1-3 The incidence of low back pain among adolescents and young adults has been shown to be similar to adult rates.2,4-6 The incidence of adolescent low back pain has been found to increase during the course of high school and may be associated with students who work during the school year.2 According to the National College Health Assessment (NCHA) survey, back pain has increased from 44.2% to 47.7% between the years 2003 (n = 19 497) and 2007 (n = 71 860), respectively.7,8 A prospective study among adolescent British school children showed a nonsignificant increase in the risk for future low back pain for carrying a heavy backpack weight.6 Whereas Brattberg9 and Hestbaek10 concluded that there is an increased incidence of back pain among adults who had had back pain as adolescents, studies measuring the relationship between a student's backpack weight as a percentage of body weight and low back pain have been conflicting, 11-13 but the growing weight of evidence suggests that use of backpacks and increasing loads enhance
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Table 1. Frequencies of low back pain in university men and women during the preceding 12 months, the College Back Survey, 2007 Low back pain measure
Males, n (%) (n = 192)
Females, n (%) (n = 273)
Percent difference (95% CI for the % difference)
Ever had low back injury/problem in lifetime Low back pain in the previous year Low back pain in the previous 2 wk Lives with continuous low back pain, 462 (99%)
70 52 19 21
106 (38.8) 84 (30.8) 30 (11.0) 33 (12.2)
−2.3 −3.7 −1.1 −1.3
(36.5) (27.1) (10.0) (10.9)
(−11.3 to 6.6) (−12.0 to 4.6) (−6.7 to 4.5) (−7.1 to 4.6)
CI is the confidence interval calculated using the Z test for the difference in 2 proportions.
the risk of back pain among children and adolescents.14 Studies evaluating the effect of increasing backpack loads measured as a proportion of body weight among children found alteration in disc height,15 and in adolescents and young adults postural changes including muscle activity alteration using electromyography,16-19 digital inclinometry20 as well as subjective reporting of stress and pain.16-18 In this study, we examined the cross-sectional relationship between the perceived backpack weight and self-reported low back pain in college students while controlling for multiple confounders to gain a better understanding of the relationship between these factors.
METHODS Data for the proposed study were obtained from the 2007 College Back Survey, which was motivated by previous results from a study using the NCHA survey.2 The data for this study were collected by having undergraduate students at Colorado State University enrolled in a freshman-level health education class complete an online survey, entitled the College Back Survey. No personal identifiers were recorded. The study was approved by the Colorado State University Institutional Review Board. We determined the prevalence of low back pain in the previous year using the question, “Have you had an episode of low back pain in the last year that caused you to alter some aspect of your normal living or seek treatment?” A total of 473 (94.6%) of the eligible 500 health education students were given extra credit for completing the online survey. Eight subjects were excluded from the analysis, with 7 students missing values for annual low back pain and 1 student missing a value for sex, resulting in 465 (93.0%) subjects with complete information. An additional 23 subjects who did not use backpacks were excluded from the analytic models comparing backpack weight and backpack weight as a percentage of body weight to annual low back pain. Mantel-Haenszel common odds ratio (OR) estimates and 95% confidence intervals (CIs) were calculated for the association between self-reported low back pain by each exposure variable of interest: backpack use (always vs mostly or never use), backpack weight (per a 4-kg increase in reported weight), and relative backpack weight (backpack weight N10% of body weight vs ≤10%) using SPSS, version 17.0 (SPSS, Inc., Chicago, IL).
Adjusted odds ratios were calculated using the hierarchically well-formulated model approach to multiple logistic regression models.21 We also evaluated interaction by sex on the relationship of the backpack exposure variables and low back pain. Because students were asked to estimate their average daily backpack weight, a validation study was performed in a sample of 238 students (who did not participate in the original survey) at the spring and fall 2008 student health fairs on the Colorado State University campus to determine the accuracy of the students' estimates of their backpack weight. The students in the validation study were asked to estimate their backpack weight, and then the interviewer measured the actual backpack weight with a dynamometer. The Wilcoxon paired t test, calculated using the univariate procedure in SAS version 9.1 (SAS Institute, Cary, NC), was used to determined if the estimated backpack weights were different than the actual weights.
RESULTS The annual prevalence of low back pain (defined as having an episode/injury/or problem causing them to alter some aspect of their normal living or seek treatment) was 29.2% (n = 136), the lifetime prevalence was 37.8% (n = 176), and the 2-week prevalence was 10.6% (n = 49) (Table 1). Females were more likely to self-report low back pain than males for all prevalence measures and for different types of pain (Table 1). Most of the students participating in the survey were non-Hispanic white (86.5%), followed by Hispanic American (8.0%), and Asian or Pacific Islander (2.2%) (Table 2). Students who worked in addition to their school studies were more likely to report annual low back pain (Table 2). Males were more likely to report annual low back pain if they worked greater or equal to 17 hours per week (32.8%) when compared with females who worked the same amount (20.2%). Most low back injuries/problems were attributed to cumulative activity (23.3%) as opposed to a single incident (19.8%) (Table 3). The most common treatment used for the low back injury/problem was self-care (42.4%), followed by chiropractic care (12.3%), physical therapy (7.7%), and visiting a medical doctor (6.5%) (Table 3). Results from the validation study of 238 students showed that the average backpack weight carried by students was 5.2 kg (SD, 1.9) (or 11.5 lb) and that students
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Table 2. Frequencies of variables of interest by self-reported low back pain status among university students, 2007 Characteristic Backpack use, n (%) (N = 465) Always Most of the time to never Reported weight of backpack, mean (SD) (kg) Weight of backpack % of body weight, n (%) (N = 465) Greater than or equal to 10% Less than 10% Excluded a Age, mean (SD) (y) Body mass index, mean (kg/m2) (SD) Sex, n (%) (N = 465) Female Male Smoking, n (%) (n = 464) Current smoker Former smoker Never smoked Feeling overwhelmed, n (%) (n = 465) Always to frequently Sometimes to never Hours worked per week, n (%) (N = 465) Greater than or equal to 17 h Less than 17 h Race/ethnicity, n (%) (N = 465) Non-Hispanic white Hispanic American African American Native American Asian or Pacific Islander Other
Total population (N = 465)
Low back pain in the previous year (n = 136)
No low back pain in the previous year (n = 329)
307 (66.0) 158 (34.0) 6 (3.3)
93 (68.4) 43 (31.6) 7 (3.1)
214 (65.0) 95 (28.9) 6 (3.4)
119 (25.6) 323 (69.5) 23 (4.9) 19.7 (2.7) 23.3 (3.9)
35 (25.7) 92 (67.6) 9 (6.6) 20.0 (2.5) 23.7 (4.7)
84 (25.5) 231 (70.2) 14 (4.2) 19.6 (2.8) 23.1 (3.5)
273 (58.7) 192 (41.3)
84 (61.8) 52 (38.2)
189 (57.4) 140 (42.5)
59 (12.7) 56 (12.1) 349 (75.2)
15 (11.0) 24 (17.6) 97 (71.3)
44 (13.4) 32 (9.8) 252 (76.8)
175 (37.6) 290 (62.4)
54 (39.7) 82 (60.3)
121 (36.8) 208 (63.2)
97 (20.8) 368 (79.2)
34 (25.0) 102 (75.0)
63 (19.2) 266 (80.8)
402 (86.5) 37 (8.0) 8 (1.7) 2 (0.4) 10 (2.2) 6 (1.2)
117 (86.0) 12 (8.8) 2 (0.1) 1 (0.1) 1 (0.0) 1 (0.0)
285 (86.4) 25 (7.6) 6 (1.6) 1 (0.0) 9 (2.7) 5 (1.4)
a Subjects were excluded if they responded that they “never use a backpack to carry books and other school supplies” and still reported an “approximate weight of their backpack on an average day.”
Table 3. Frequencies of various self-reported low back pain measures among university students, 2007 Back pain measure (N = 465) Care sought for back injury/ problem No pain—no care Self treatment only Chiropractor Physical therapist Medical doctor Nurse practitioner Osteopathic doctor Other Low back injury a single incident or cumulative from activity (n = 464) a None—never had a back injury Cumulative (from repeated activity) Single incident (lifting something too heavy) a
n (%) 197 (42.4) 120 (25.8) 60 (12.9) 36 (7.7) 30 (6.5) 7 (1.5) 5 (1.1) 10 (2.2)
239 (51.5) 133 (23.3) 92 (19.8)
Data include all cases: point, annual, lifetime, and chronic.
overestimated their backpack weight by an average of 1 kg (95% confidence interval for the paired t test, 0.6-1.4) (SD, 3.1). A suggestive but not strong association was observed for the odds of annual low back pain using a backpack always compared to mostly to never adjusting for age
(years), sex, smoking (current, former, never [reference]), feeling frequently overwhelmed (mostly to always, sometimes to never [reference]), and body mass index (kg/m2). After adjusting for age (years), sex, smoking (current, former, never), feeling overwhelmed (mostly to always), and body mass index (kg/m2), a 25% increase in the odds of annual self-reported low back pain for each 4-kg increase in the estimated backpack weight was observed (adjusted OR per 4-kg increase, 1.25; 95% CI, 1.17-1.32) (Table 4). After adjusting age (years), sex, smoker (current, former, and never), and feeling overwhelmed frequently (always to mostly), age, body mass index, and sex, there was no evidence of an increased association of annual low back pain with carrying a backpack weight greater than 10% of the student's body weight compared with those carrying less (adjusted OR, 1.02; 95% CI, 0.63-1.65) (Table 4).
DISCUSSION The average age of the population was 19.7 years (SD, 2.7) and the ethnicity was predominately non-Hispanic
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Table 4. Unadjusted and adjusted OR and 95% CI for exposure variables of interest, the College Back Survey, 2007
Exposure variable Backpack use Always Most of the time to never Among backpack users Weight of backpack (kg) 4-kg unit increase Weight of backpack as % of body weight Greater than or equal to 10% Less than 10%
Adjusted OR (n = 463)
Unadjusted OR (n = 472)
OR
OR
95% CI
95% CI
1.23 a 0.79-1.92 1.00 (n = 437)
1.16 0.76-1.78 1.00 (n = 441)
1.25 b
1.17-1.32
1.29
1.22-1.37
1.02 c
0.63-1.65
1.05
0.66-1.66
1.00
1.00
a
Adjusted for age (years), sex, smoking (current, former, never [reference]), feeling frequently overwhelmed (mostly to always, sometimes to never [reference]), and body mass index (kg/m2). b Adjusted for age (years), sex, smoking (current, former, never), and feeling overwhelmed (mostly to always), and body mass index (kg/m2). c Adjusted for age (years), sex, smoker (current, former, and never), and feeling overwhelmed frequently (always to mostly).
white (Table 1). The association between annual low back pain and backpack use or weight found among this population may not be generalizable to a non-college population, a different age group, or a population with a different morphology, racial and ethnic distribution. The 29.2% annual prevalence estimate for low back pain from the present study was lower than the 47.7% estimate from the NCHA survey2 and the 42.8% determined among the 973 undergraduate students attending Colorado State University, 7 although the present study may have represented a more substantial pain/condition. Previous surveys among college students did not qualify the back pain; therefore, the prevalence estimates may have represented a less severe back pain. Our study qualified an episode of back pain as that which interfered with activities of daily living and/or compelled the individual to seek medical care. Because there is no standard method to characterize exposure to backpack use and low back pain, the relationship between these 2 factors often varies between studies.12,14 Our results contrast with a previous cross-sectional study that found a modest association between the heavy use of backpacks (wears backpack between classes and while standing) and self-reported 4-week prevalence of back pain (at least a 2 on a scale from 0 to 10 for severity of pain) when compared to no or low use (does not wear backpack between classes but carries backpack while waiting) among 12- to 18-year-old students (OR, 1.98; 95% CI, 1.422.76).10 Lindstrom-Hazel 14 reviewed 11 epidemiologic studies published between 2003 and 2007 and reported that 10 of 11 investigators did find association of backpack use and back pain. Our study provided similar findings as the
prospective study among British adolescent school children, which determined the relative risk of carrying a heavy backpack weight (6.4-18 kg compared to less than 3.4 kg) and the risk for future low back pain (adjusted RR, 1.2; 95% CI, 0.7-2.1).6 The lack of an established temporality in our study between the self-reported backpack use or relative backpack weight and annual low back pain is a major limitation. Our findings do suggest that increasing backpack weight by increments of 4 kg was associated with increased risk for LBP but did not necessarily support that a 10% body weight threshold reported by others as associated with LBP. The self-reported backpack weight as a percentage of body weight showed no significant association with low back pain, which contrasts with another cross-sectional study that determined that low back pain was associated with a relative backpack weights but only at weights greater than 20% of the subject's body weight and within an much younger population (OR, 3.1; 95% CI, 1.09.2).11 Devroey et al16 evaluated the effects of backpack weight unloaded and loaded to 5%, 10%, and 15% of body weight using electromyography, kinematic sensors, and subjective reports. The investigation team found significant differences in muscle activity, postural alteration, and subjective complaints with increasing loads and concluded that loads should not exceed 10% of body weight. Chow et al20 recently found that backpack loads of 15% of body weight resulted in significant postural alteration using spinal accelerometers and inclinometers and concluded that a load placement with the center of gravity at the 12 spinal segment resulted in the least postural deviation. Al-Khabbaz et al17 investigated effects of incremental backpack loads at 10%, 15%, and 20% of body weight among 19 university students using electromyography and activity sensors to detect changes in posture inclination and muscle activity with increasing loads. The team found the greatest muscle activity increases in the abdominal muscles and postural changes toward a more backward inclination. A possible reason why we did not observe a similar association by percentage of body weight is that the relative weight model did not distinguish body fat from muscle mass, its distribution on the body, and different adaptive responses to loading among individuals. None of the studies reviewed reported body fat content or range of body mass indexes of subjects. Our study included a large number of college students using a wide array of backpack designs, whereas the laboratory studies used one type of backpack loaded to various weights. There is evidence that backpack design and placement may be a factor in the activation of postural muscles and thus the precipitation of pain, not just weight.22 Physiologic factors such as the student's posture, low back strength, and low back flexibility and other biomechanical exposures from physical exercise, work, and sports
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were not included as potential confounders within the analysis given the relationship between these variables and the student's backpack weight or use and low back pain was difficult to establish.23,24 The type of backpack used may have also confounded the relationship between backpack weight or backpack use and low back pain given that the position and design of the backpack may influence how much the weight is distributed to the low back and potentially the potential for pain or injury.18,25 This study did not evaluate whether the type of backpack among heavy backpack users increased or decreased the likelihood of reporting low back pain. Measurement errors from students overestimating their daily backpack weight would likely bias the effect estimates toward the null. The validation study only determined if students could estimate their backpack weight at one time and not throughout the course of a day or even throughout a school year. In addition, the validation study did not determine if a student's ability to estimate his or her actual backpack weight changed with the presence of low back pain. If this was true, then the observed association between backpack weight and low back pain would likely bias the effect away from the null.
Limitations This cross-sectional study is inherently limiting and cannot establish causation between exposures and outcomes and limited to evaluating the strength of association between variables. Future prospective studies may accurately addresses the relative risk of backpack users to develop LBP. Our sample was not randomly selected but rather a volunteer convenient sample. Students enrolled in courses surveyed may have felt an obligation to report more than actually experienced. This would bias away from the null and result in overreporting. We do not believe this was the case because of the lower prevalence rate compared to previous studies. The student population at CSU is a young homogeneous group not well representative of the greater heterogeneous general population. The CSU cohort may also be different in unrecognized ways from other groups of college students seen across the nation and should not be directly compared. Results from our survey should be cautiously externalized to other populations. A self-report survey by nature may be subject to response and recall bias. The survey is dependent on the subject's memory and honesty to self-report and respondents may not have recalled accurately or willfully reported incorrect information. Response and recall bias are possible and may influence results toward or away from the null. We developed a definition of low back pain that could have been very broadly interpreted to capture a perceived problem among college students. Such a broad definition certainly tells nothing about the actual pain generator, type of pathology, or medical history of the subject. This
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definition could have biased the reporting and accuracy toward or away from the null.
CONCLUSION This was the first study to our knowledge to investigate important relationship between low back pain and backpack use or weight using a hierarchically well-formulated model approach to multiple logistic regression.21 This method allowed the researchers to control for potential confounders while considering the effect on the precision and validity of the OR estimates. Our study results found that backpack weight increase by 4-kg increments had an association to LBP. These findings reinforce the need for college health prevention efforts to reduce the burden of low back pain by encouraging students to minimize loads when using backpacks. The high prevalence of a substantial low back pain among college students justifies a need to prevent the pain from worsening and becoming a chronic condition later in life10 with the resulting long-term disability, lost workdays, and loss of productivity. Although before students seek treatment they should consider that 90% of low back pain patients who seek medical treatment from physicians, chiropractors, and other health care providers will improve in 4 to 6 weeks using conservative measures.26
Practical Applications • Back pain is the most frequent physical health complaint among college students. • Twenty-nine percent of college students reported having had low back pain within the previous year. • The use of a heavy backpack is evaluated as a risk factor for low back pain.
FUNDING SOURCES AND POTENTIAL CONFLICTS OF INTEREST No funding sources or conflicts of interest were reported for this study.
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