Abstract
Background Disability in adults with low back pain (LBP) is associated with negative back pain beliefs (BPBs). Adult BPBs can be positively influenced with education, resulting in reduced LBP disability. By late adolescence, the prevalence of LBP reaches adult levels. The relationship among LBP experience, LBP impact, and BPBs has not been investigated in late adolescence.
Objective The aim of this study was to document unknown relationships among LBP experience, LBP impact, and BPBs in 17-year-olds.
Design A cross-sectional study design was used.
Methods Adolescents (n=1,126) in the Raine Study provided full information on LBP, LBP impact (sought professional advice or treatment, taken medication, missed school or work, interfered with normal activities, interfered with physical activities), BPBs, and a number of covariates.
Results Back pain beliefs were more positive in participants with experience of LBP (X̄=30.2, SD=5.6) than in those without experience of LBP (X̄=28.5, SD=5.1). Individuals with LBP without activity modification impacts had more positive BPBs than those with activity modification impacts, even after adjustment for mental well-being and sex. The adjusted difference in BPBs between participants with experience of LBP but no activity modification impacts and those reporting all 3 activity modification impacts was 2.9 points (95% confidence interval=1.7 to 4.2). Participants with no activity modification impacts had more positive BPBs than those with no experience of LBP (adjusted difference=2.2 points, 95% confidence interval=1.4 to 2.9). More positive BPBs also were associated with female sex, lower body mass index, higher family income, better 36-Item Short-Form Health Survey (SF-36) Mental Health scale scores, and more positive primary caregiver beliefs.
Limitations Cause and effect cannot be ascertained with the cross-sectional design.
Conclusion Differences in BPBs are associated with different levels of LBP impact at 17 years of age. This finding provides a potential target for intervention early during the life course.
Low back pain (LBP) is one of the most expensive health care disorders,1 with indications that costs are rising.2 For the majority of these disorders, no pathoanatomical diagnosis can be made.3,4 In response to these factors, there is growing acceptance that LBP must be considered from a biopsychosocial perspective.4–6
There is strong evidence that for a small group of patients, LBP becomes disabling, whereas for others, it does not.7 Understanding the basis for LBP disability is a research priority, as this small group of patients results in a disproportionate financial burden.8
Recent research has identified that higher levels of disability in adults with nonspecific chronic LBP are associated with more negative back pain beliefs (BPBs).9–12 In adults, more negative BPBs also have been associated with increased work absenteeism and reduced work productivity13–15 and with poorer long-term outcomes.15,16 These findings suggest that beliefs can be a significant driver of LBP disability in adults.
Back pain beliefs are influenced by culture,10 familial environment,17 professional training,10 pain experience,18 depression,19 education level, perceived general health, work absenteeism history, and LBP activity limitation.20 Back pain beliefs can be positively influenced with education,21–23 and improvements in BPBs are associated with reduced absenteeism and claims for LBP.23,24
Low back pain commonly develops during adolescence, reaching adult levels by the end of adolescence.25,26 For example, a systematic review showed the lifetime prevalence of LBP to be 17% at 12 years of age, escalating to 60% at 18 years of age.25 Adolescent LBP also is a predictor of adult LBP.27,28 Recent research has identified that adolescent LBP often is associated with impairments in physical capacity and activities of daily living, school absenteeism, and care-seeking behaviors.29–31 For example, 70% to 85% of 17-year-olds with current LBP in the Raine longitudinal study have reported care-seeking and activity modification behaviors, such as missing school, related to the experience of LBP.31
To date, no research has investigated whether LBP-related impacts and disability are related to negative BPBs in adolescents. Although various psychosocial factors may be related to development of disability in adolescents with LBP,32 BPBs were of interest in this study, given the positive results from population-based intervention studies targeting BPBs specifically.21,22,24 If a relationship between BPBs and disability can be established in adolescence, targeting BPBs at this time in the life course may be even more advantageous, as beliefs and behaviors related to LBP may not be as entrenched as they are in adulthood. Hence, the objective of this study was to establish the relationships among the experience of LBP, report of LBP impact, and BPBs in 17-year-olds in Australia.
Method
A cross-sectional evaluation was performed using 17-year-old adolescents participating in the West Australian Pregnancy Cohort Raine Study.33
The Raine Study
The Raine Study began as a pregnancy cohort with mothers recruited from May 1989 to November 1991 (Figure). At birth, the Raine cohort was characterized by a higher proportion of high-risk births, a lower proportion of fathers employed in managerial positions, and a higher proportion in professional positions compared with the general Western Australian population.34
Flow diagram of participant inclusion. LBP=low back pain.
Families of participants in the Raine Study who completed the 17-year follow-up were statistically compared with population data of Western Australian families with 15- to 17-year-old adolescents using Australian Bureau of Statistics 2006 census data.35 Overall, the comparisons indicated that the participating 17-year-olds came from families that were broadly representative of basic demographic characteristics of the population, but slightly higher with regard to some indicators of socioeconomic status. In particular, a higher proportion of Raine Study families were urban dwelling (81.6% versus 66.1%), lived in neighborhoods with a high socioeconomic index (23.6% versus 20.6%), and were couple families with both parents employed (58.8% versus 54.6%) and in managerial or professional occupations (14.1% versus 8.0%), and a lower proportion of Raine Study families had a combined family income of less than AUS$25,000 (7.9% versus 10.8%). Ethnicity is predominantly Caucasian (93%).
BPBs and LBP at 17 Years of Age
At the 17-year follow-up, 1,299 adolescents independently completed a paper questionnaire covering LBP prevalence, LBP impacts, BPBs, the 36-Item Short-Form Health Survey (SF-36), and work status (Figure). Of these adolescents, 1,251 underwent a physical examination, including height and weight measurements, and 1,234 completed a computer-assisted questionnaire that included questions about smoking and drinking. At the 17-year follow-up, Raine Study adolescents' primary caregivers also were invited to complete a separate questionnaire, performed independently of any of the adolescent data collections (n=1,402). A single primary caregiver provided information on: (1) family income for 88.9% (n=1,013) and (2) their own (ie, primary caregiver) BPBs for 86.1% (n=982) of those adolescents with self-report data used in this report (n=1,140).
Ethical approval was granted from the appropriate institutional bodies. Participants' guardians provided consent.
LBP Prevalence and Impacts
With regard to prevalence, participants were asked “Have you ever had LBP (anywhere in the shaded area)?,” with the area on the body diagram from the level of the 12th thoracic vertebra to the inferior gluteal fold shaded.36 Low back pain impacts were assessed in 2 domains: care seeking and activity modification. Low back pain impacts related to care seeking were assessed in 2 aspects ascertained from the following questions36–38: (1) “Have you ever sought health professional advice or treatment for LBP?” and (2) “Have you ever taken medication to relieve the LBP?” Low back pain impacts related to activity modification were assessed in 3 aspects36 from the following questions: (1) “Have you ever missed school or work due to the LBP?,” (2) Has the LBP ever interfered with your normal activities?,” and (3) “Has the LBP ever interfered with recreational physical activities (eg, sports, walking, cycling)?” Responses to these questions were either “yes” or “no.” To further explore relationships between BPBs and activity modification impacts, a count variable indicating number of “yes” responses to LBP activity modification impacts reported was derived.
BBQ
The Back Beliefs Questionnaire (BBQ) was used to assess beliefs related to the future course of back pain.15 The questionnaire has 14 items, with only 9 items used to provide a score. The questionnaire has been reported to have good test-retest reliability (intraclass correlation coefficient=.87) and good internal consistency (Cronbach α=.7).15 Scores range from 9 to 45, with lower scores indicating more negative beliefs. If a participant was missing 1 item, it was imputed from the average of the other 8 items, but no score was calculated for those participants missing more than 1 item. Although most extensively used in adults, the BBQ has been used in a cohort with individuals as young as 16 years20 (the adolescent data were merged with adult data and not reported separately) and in another cohort of young adults.10 Item 2 was reworded to “Back trouble will eventually stop you from participating in physical activity,” as opposed to “stop you from working” to suit sample age.
The mean difference in the BBQ scores of more than 2 points can be considered meaningful, given that a change of this magnitude is associated with reduced LBP disability and workers' compensation costs.24 A similar criterion for meaningful change in BBQ scores was been adopted in a clinical trial.19 In the current study, 1,140 of 1,299 participants had valid data for LBP prevalence, LBP impact, and BBQ total score (Figure).
Covariates
Covariates considered for this study (Tab. 1) were selected based on analogous constructs used in 2 previous studies.12,19 The Mental Health scale of the SF-3639 was used to adjust for the possible confounding of negative affect, which potentially could be associated with both poor BPBs and the likelihood to report impact from LBP. The scale was normalized to have a mean of 50 and a standard deviation of 10, based on 1998 general US population scores.40 Smoking and alcohol intake in the previous 12 months were assessed on a dichotomous scale (yes/no) with the question: “Have you used alcohol/cigarettes in the past 12 months?” Confidential self-report of adolescent drug use via a simply worded, closed-ended question is accepted to have adequate reliability and validity.41,42 Family income was assessed by the question: “What is your total family income (before tax) per year now (include income from investments, rent assistance, maintenance, family supplement, and so on)?” Work status was assessed on a dichotomous scale by the question: “Do you currently have a full-time or a part-time job of any kind (excluding home duties)?” Body mass index (BMI) was calculated from height and weight measures.
Univariable Associations of Back Beliefs Questionnaire (BBQ) Scores With Adolescent-Reported and Primary Caregiver–Reported Variablesa
Statistical Methods
Six separate linear regression models were used to estimate differences in BBQ scores among LBP impact groups (ie, no LBP, LBP and no report of specific impact, and LBP and report of specific impact) (Tab. 2) for the 5 impacts separately and among groups defined by the derived variable “number of LBP activity modification impacts” (Tab. 2). Linear regression also was used to quantify associations between BBQ scores and covariates (Tab. 1). Models were checked for linearity of effects and absence of influential outliers by standard methods, including examination of residual plots and influence statistics such as delta-betas and Cook's distance. Final models for BBQ differences in relation to LBP impacts were adjusted for female sex and SF-36 Health Survey Mental Health scale scores (n=1,126) in order to maximize precision of estimates, as the patterns of missing data from the 4 separate sources (computer-assisted and paper questionnaires, physical examination, and primary caregiver questionnaire) resulted in the number for analysis being reduced to 787 when a full model containing all covariates was reported. The absence of confounding influence of other variables considered in this study was checked by confirming that the percentage change in the coefficients of interest (ie, LBP groups) from the unadjusted models compared with the model including the potential confounder was less than 20%.43 Statistical analysis was performed with Stata/IC version 10.1 for Windows (StataCorp LP, College Station, Texas).
Means (SD) of Back Beliefs Questionnaire (BBQ) Scores for (1) Participants Reporting No Low Back Pain (LBP) Ever (n=571) and Those Reporting LBP (n=569) With and Without Each Impact and (2) Count of Activity Modification LBP Impactsa
Results
Table 1 presents the characteristics of the participants for all variables considered in this study and the association of each variable with BBQ score. A higher BBQ total score, indicating more positive BPBs, was associated with female sex (P<.001), lower BMI (P=.05), higher family income (P=.01), better SF-36 Mental Health scale scores (P<.001), and higher primary caregiver BBQ total score (P<.001) (Tab. 1).
The mean BBQ total score for participants who reported experiencing LBP was 30.2 (SD=5.6), which was significantly higher than the mean BBQ total score of 28.5 (SD=5.1) for those who reported never experiencing LBP (mean difference=1.6 points, 95% confidence interval [CI]=1.0 to 2.3, P<.001).
The percentages of all participants reporting an aspect of care-seeking impact and activity modification impact are reported in Table 2. Among those participants with experience of LBP, there were no mean differences in beliefs between those who did or did not report care-seeking impacts, but participants who reported no activity modification impacts had significantly more positive beliefs than those who reported these impacts (mean differences=1.3 to 2.6 points, P<.001 to .005) (Tab. 2).
More positive BPBs were still associated with negative report of activity modification impacts in participants with LBP after adjustment for female sex and SF-36 Mental Health scale scores (n=1,126). Interaction effects between sex and LBP impact variables were tested, but none were found to be close to statistical significance (all P>.10), meaning there was no evidence that the association between LBP activity impacts and BPQ was of a different strength or direction for male versus female participants. Specifically, after accounting for differences in BBQ due to sex and SF-36 Mental Health scale scores, participants reporting LBP with no missed school or work due to LBP were estimated to have scores 2.4 points higher (95% CI=1.3 to 3.5, P<.001) than those of participants reporting LBP with absence from school or work. Participants reporting LBP without interference with normal activities were estimated to have scores 1.7 points higher (95% CI=0.8 to 2.6, P<.001) than those of participants reporting LBP with interference with normal activities. Participants reporting LBP without interference with physical activities were estimated to have scores an average of 1.1 points higher (95% CI=0.2 to 1.9, P=.02) than those of participants reporting LBP with interference with physical activities.
To further explore relationships between BPBs and activity modification impacts, a count variable indicating number of LBP activity modification impacts reported was derived. Of all participants, 571 (50.5%) reported no history of LBP, 261 (23.1%) reported previous LBP with no activity modification impacts, and 98 (8.7%), 111 (9.8%), and 90 (8.0%) reported LBP with 1, 2, or 3 activity modification impacts, respectively. This derived count variable also was significantly associated with BBQ scores (Tab. 2). In participants with experience of LBP, this variable displayed an approximately linear decrease in BBQ scores with increasing count of activity modification impacts (Tab. 2). The unadjusted difference in beliefs (ie, without accounting for sex and SF-36 Mental Health scale scores) between participants with experience of LBP but no activity modification impacts and those reporting all 3 activity modification impacts was 3.4 points (95% CI=1.9 to 4.5, P<.001).
Table 3 presents the multivariable regression model for BBQ scores and count of activity modification impacts, adjusting for sex and SF-36 Mental Health scale scores (n=1,126). Count of LBP activity modification impacts reported remained significantly associated with BBQ scores, with adjusted estimates similar to unadjusted estimates (Tab. 3), meaning that LBP activity modification impacts were uniquely associated with lower BBQ scores regardless of sex or SF-36 Mental Health scale scores. The adjusted difference in beliefs between participants with experience of LBP but no activity modification impacts and those reporting all 3 activity modification impacts was 2.9 points (95% CI=1.7 to 4.2, P<.001). Participants with LBP but no activity modification impacts had more positive BPBs than those with no experience of LBP (adjusted difference=2.2 points, 95% CI=1.4 to 2.9, P<.001). Differences also are reported as standard deviation change in BBQ scores expected between reference group “LBP with 0 impacts” and group of interests (see bStdY, Tab. 3). The adjusted R2 value for the multivariable model was .08, meaning that an estimated 8% of the variation in BBQ scores may be explained by the variables used in the model, namely sex, SF-36 Mental Health scale scores, and LBP impact count.
Multivariable Regression Model for Back Beliefs Questionnaire (BBQ) Scores (n=1,126)a
Discussion
Low back pain was a common experience in this cohort of 17-year-olds, with 50% of the participants having experienced LBP (Tab. 2). This finding is consistent with previous findings that by late adolescence, the prevalence of LBP reaches adult levels.25,27,28 Low back pain also has a significant impact for a substantial proportion of adolescents.31 This impact includes care-seeking behaviors, school or work absenteeism, and activity interference, suggesting LBP in adolescence is not trivial and should be the target of both prevention and management strategies.
Covariates of BPBs
More positive BPBs in this study were associated with female sex, lower BMI, higher family income, higher SF-36 Mental Health scale scores, and higher primary caregiver BBQ scores. The influence of educational level on BPBs was previously reported20 and was related to higher socioeconomic status (similar to the family income relationship in this study). The previously reported small effect of negative psychological factors (fear of pain, depression) with BPBs19 is consistent with our finding of reduced SF-36 Mental Health scale scores, which includes similar constructs such as depressed mood. Primary caregiver BBQ scores as an influencing factor are consistent with familial shaping of beliefs where primary caregiver beliefs and pain behaviors can be transferred to their dependents.10 Other factors may include culture, peers, media, and health care provider interactions. The shaping of BPBs is likely to be complex and multifactorial and to occur over the life course.
BPBs
The BPBs of adolescents were similar to those reported for adult populations13,22 suggesting BPBs are already established in late adolescence and, therefore, are worthy of consideration in understanding the life course development of LBP disability. There are no other reports of the BPBs of adolescents or children with which to compare the beliefs of the current sample.
Overall, adolescents who had never experienced LBP had lower BBQ scores than those who had experienced LBP. Similar results were found in other adult studies,9,10,13 although not in all studies.12,20 This finding highlights the generally more pessimistic BPBs within the adolescent population who do not experience LBP, which may reflect broad societal beliefs regarding the consequences of LBP. This finding potentially highlights a population at risk for developing disability if they experience LBP in the future and will be the focus of our longitudinal research. What also is interesting is the finding of different levels of BPBs for those with LBP, based on the activity modification and school/work absenteeism impacts, which is discussed below.
Care-Seeking Impacts
In participants who had experienced LBP, beliefs were not different between those who responded “no” or “yes” to having specific impacts related to care-seeking behaviors (ie, seeking professional help, taking medication) (Tab. 1). To our knowledge, this is the first study to show this finding in an adolescent or adult cohort. In previous work, we found that LBP-related behaviors changed over time, with nursing students reporting activity modification behaviors in response to LBP and with graduate nurses reporting greater care-seeking behavior (ie, medication use).38 Back pain beliefs were not a variable in that study, but are known to change over time21 and potentially could be linked to changes in care-seeking behaviors over time. Longitudinal data are needed to assess the stability of beliefs from adolescence to adulthood and their relationship to care-seeking behaviors.
Activity Modification Behaviors
For all 3 activity modification behavior impacts, participants who had experienced LBP and who responded “no” to these specific impacts had significantly higher BBQ scores than those who responded “yes” or those who had never experienced LBP (Tab. 2). That these findings stand even when adjusted for sex and SF-36 Mental Health scale scores suggests that BPBs are importantly and independently associated with adolescent LBP activity modification impacts. This finding is consistent with recent research investigating BPBs in working-age individuals that showed LBP impact on activity to be a significant correlate of BPBs.20
Participants with no activity modification impacts, or a low cumulative count of these impacts, had more positive beliefs than those who had never experienced LBP (Tab. 2). Those with the highest cumulative count of impacts (ie, the most disability) had more negative BPBs, which is consistent with the finding of lower BBQ scores being associated with higher levels of disability in adults with LBP, independent of pain intensity levels.9,12 As this study was cross-sectional, the directionality of these relationships could not be ascertained. It is possible that when the experience of LBP is associated with no impact or low impact, it may have a positive influence on beliefs, whereas high disability levels may result in more negative beliefs. However, it also is possible that more positive or more negative beliefs may influence activity-related behavior, and it is certainly plausible that effects may exist in both directions. Future prospective studies within this cohort are planned to investigate this relationship further.
Interestingly, the BBQ scores for participants who had never experienced LBP were on par with those of participants who reported all 3 activity modification behaviors (Tab. 2). It may be that interventions aimed at modifying BPBs should be targeted at these 2 groups for maximum impact and cost-effectiveness.
The impacts related to lower BBQ scores in this study are associated with long-term negative consequences. For example, school absenteeism has been associated with reduced academic achievement, reduced social interaction, and reduced participation in the workforce.44 Development of activity modification behaviors during adolescence may well be a pathway to disabling LBP in adulthood, a premise worthy of further longitudinal investigation.
Limitations
Covariates associated with BBQ scores in this study accounted for a small but meaningful amount of variability in BPB scores45 (see R2, Tab. 2), as the multivariable model R2 was .08. This magnitude of R2 is above the recommended minimum effect size, representing a “practical” effect for social science data.45 It is extremely likely that factors other than those considered in this study also are important correlates of BPB scores and may include culture, peers, media, and health care provider interactions. Assessing covariates as continuous rather than dichotomous variables may increase power in future studies.
Back pain beliefs were assessed with the BBQ, which has been used to assess beliefs related to the future course and consequent inevitability of back pain.15 This is a specific aspect of an individual's belief system. We acknowledge that other aspects of an individual's belief system (eg, fear-avoidance beliefs) will contribute to pain perception, pain behavior, and disability.46,47 Back pain beliefs and fear-avoidance beliefs have recently been shown to independently relate to poor prognosis in people with LBP.11
We also acknowledge that BPBs may be established prior to the age of 17 years, and future investigations of BPBs in early adolescence are warranted. However, most Australian 17-year-olds are completing their last year of high school. Therefore, after this time, other strong influences on beliefs, especially occupational influences, may become more important. Furthermore, at 17 years of age, enough of the cohort have experienced LBP. This age thus represents an ideal time to understand the culmination of preadult influences, with a view to targeting beliefs before adulthood.
In addition to the limitations of a cross-sectional design already described, we acknowledge that the size of a clinically meaningful difference in BBQ scores for adolescents is not yet clear. The minimum clinically important difference is likely to be context specific rather than a fixed number.48 The available evidence from an Australian population-based intervention to change beliefs about back pain22 shows that a difference in adults of 1.9 points on the BBQ was associated with the clinically meaningful differences of a decrease in workers' compensation claims for back pain of approximately 15%, and a reduction in medical costs of 20% per claim. Results from a study of US soldiers19 suggest that a change of 1.3 points on the BBQ, which corresponded to an effect size of 0.18, is possible with an educational intervention and that this same educational intervention was associated with a decrease in the incidence of LBP, resulting in medical care seeking.49 The authors postulated that this positive shift in beliefs in individuals who are pain free may have decreased the likelihood of seeking care when LBP was subsequently experienced, but acknowledged they did not have the data to directly validate this hypothesis.49 The differences between LBP impact groups of 2.2 and 2.9 observed in the current study are similar in magnitude to the differences found in these studies of adults. The differences correspond to effect sizes of 0.39 and 0.54, respectively, which have been interpreted as small and moderate, respectively.50 In adolescents, even smaller changes in BBQ scores may be important at a population level, given the potential for changes over the remaining life course.
Clinical Practice Implications
The results of this study highlight the need for considering BPBs in the clinical context of assessment, management, and prevention of LBP in adolescents. A number of simple, valid, multidimensional screening questionnaires are currently available for use in assessing BPBs and beliefs regarding fear of movement and the future consequence of LBP.51,52 These tools can alert clinicians to question patients with LBP regarding their beliefs about LBP, fear of movement, and concerns for the future, as well as to target management at these factors within a cognitive-behavioral framework. This approach is consistent with recent calls to direct management at psychosocial risk factors known to enhance disability and adopt a more cognitive functional approach to management of LBP disorders.53,54 There is growing evidence of the efficacy of this approach in both the management51 and prevention49 of LBP disorders in adults. Screening tools should augment the clinical examination.55,56 However, it is our experience that beliefs are not always considered in clinical examinations of spinal pain disorders. Although documentation of behaviors and disability is common in the clinical examination, it is frequently conceptualized within a biomedical framework.4,54 Assessment of beliefs inherent to the BBQ can be assessed with direct questioning of future expectations and thoughts on required management. Broader aspects of beliefs also can be ascertained by questioning patients regarding their thoughts about the cause of their disorder and about specific issues such as fear as it may relate to the patient's presentation. In the clinical context, findings from screening tools and clinical questioning must be interpreted in light of an individual's presentation and behaviors.
The finding of negative BPBs in the group without LBP highlights the opportunity for broad early education in schools regarding an evidence-based understanding of LBP and its future consequences to combat pessimistic societal views regarding LBP. However, to date, little research has documented targeting adolescents at a critical time in their life course when the presence of disabling LBP emerges.54
Conclusion
This study demonstrates a relationship between BPBs and the impact of LBP at 17 years of age. Adolescents with LBP who had more negative BPBs had higher LBP impacts (greater activity interference and absenteeism from school or work) than those with more positive BPBs. In addition, adolescents without the experience of LBP had more negative BPBs, highlighting a large group potentially at risk of disability. The differences in BBQ scores observed were of a magnitude associated with reduced LBP disability and workers' compensation costs in a study of adults.24 We recommend that BPBs should be routinely assessed and targeted in both the management and prevention of adolescent LBP disorders.
Footnotes
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Dr Smith, Prof O'Sullivan, and Prof Straker provided concept/idea/research design. All authors provided writing. Dr Smith, Dr Beales, and Prof Straker provided data analysis. The authors acknowledge the Raine Study Team and the Raine Study participants and their families.
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This research, in part, was presented at the Australian Physiotherapy Association Conference; October 27–30, 2011; Brisbane, Australia, and the Melbourne International Forum XI: Primary Care Research on Low Back Pain; March 15–18, 2011; Melbourne, Australia.
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Funding was received from the Australian National Health and Medical Research Council (including project 323200, fellowships 425513 and 373638, and program 003209). Funding also was received from the Raine Medical Research Foundation; the University of Western Australia; Curtin University; the Faculty of Medicine, Dentistry and Health Sciences at the University of Western Australia; the Women and Infants Research Foundation; and the Telethon Institute for Child Health Research.
- Received November 11, 2011.
- Accepted June 20, 2012.
- © 2012 American Physical Therapy Association
References
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