This prospective cohort study used data from the Fit Futures (FF) study. The Fit Futures study is a population-based cohort study of adolescents aiming at following adolescents’ lifestyle and health status over time. In the first wave of the study (FF1), conducted 2010–2011, all first-year upper-secondary students in the municipalities of Tromsø (urban) and Balsfjord (rural) in Northern Norway were invited (N = 1117) and N = 1038 participated, yielding a response rate of 93%. In the second wave (FF2), conducted in 2012–2013, all third-year upper-secondary students in the same schools and all FF1 participants, irrespective of educational status and school district, were invited (N = 1130). N = 868 participated for a response rate of 77%. All participants completed online questionnaires, a clinical interview, anthropometric measurements and medical examinations at the research unit at the University Hospital of Northern Norway. For further details regarding the study, see [16, 17]. Our sample was restricted to adolescents who had participated in both study waves. We define the “population at-risk” of developing persistent MSK pain as adolescents without persistent MSK pain in FF1; therefore, we excluded those with persistent MSK pain at baseline (n = 211) from the main analysis. Adolescents older than 18 years in FF1 were excluded (n = 52). Three percent (n = 17) of the cohort had missing outcome data and were also excluded. This resulted in a sample of 539 participants in the main analyses. Secondary analyses were conducted on a mixed sample of both participants with and without persistent MSK pain at baseline (n = 692) (Fig. 1).
Participation was voluntary and based on written informed consent. Written permission from a guardian was required for participants under the age of 16 years. The Regional Committee for Medical and Health Research Ethics in Norway (2019/599/REK Nord) and the Norwegian Centre for Research Data (954769) approved the current study. The study protocol for the present analysis has been published at clinicaltrails.gov (NCT04526522). Reporting of this study follows the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement (Additional file 1) .
The primary outcome was collected through the electronic questionnaire in Fit Futures and defined as persistent MSK pain, assessed with the following questions, “Do you have persistent or recurrent pain that has lasted for three months or more?” The responses were “yes” or “no.” If participants answered yes, they were asked, “How often do you have pain?” with four response alternatives: “constantly, without a pause”, “every day, but not all the time”, “every week, but not every day”, and “rarer than every week”. Then, participants were asked, “where does it hurt”, with 14 body regions as response alternatives. This questionnaire was developed specifically for the Fit Futures study. Pain in the shoulders, arm/elbow, hand, hips, thigh/knee/shin, ankle, neck, upper back, and lower back were defined as MSK pain. We defined persistent MSK pain as pain experienced at least once per week over the last 3 months in at least one body site. MSK pain at baseline was assessed with the same questionnaire.
The secondary outcome was severe persistent MSK pain, assessed with the same questions as for the primary outcome, adding information about pain intensity rated on a numeric rating scale from 0 (no pain) to 10 (worst pain imaginable). Severe persistent MSK pain was defined as pain at least at one site once per week over the last 3 months with an intensity of at least 5/10 .
Social acceptance among peers was measured with five questions from the revised Norwegian version of Harter’s Self-perception Profile for Adolescents; scale for social competence [20, 21]. This subscale has proven good reliability and validity among Norwegian adolescents  and consists of five questions concerning the adolescent’s perception of ease of making friends and being socially accepted by peers. Participants were asked whether they “find it hard to make friends”, “have many friends”, “are hard to like”, “feel popular among peers, and “feel accepted among peers”. The responses were scored on a four-point scale ranging from “highly correct” (4 points) to “highly incorrect (1 point)”. The two negative worded items were reversed, and the average item score was calculated by dividing the total score by the number of items (range 1–4), as suggested by the developers . A higher score indicated a higher level of perceived social acceptance among peers. Because there was little variation in social acceptance data, this variable was dichotomized according to normative values identified in a previous large-scale study of Norwegian adolescents with a cut-off for low social acceptance among peers of ≤3.0 .
Potential effect modifier
Psychological distress, including symptoms of anxiety and depression, was measured by the Hopkins Symptoms Checklist-10 , which is validated in Norwegian adolescents . The questionnaire consisted of 10 items measuring whether the adolescents had been bothered with the feelings: “sudden fear for no reason”, “felt afraid or worried”, “felt faintness or dizziness”, “felt tense or upset”, “self-blame”, “sleeplessness”, “depression or sadness”, “felt useless or worthless”, “felt that life was a struggle”, and/or “the feeling of hopelessness”. Each item was answered on a four-point scale ranging from “not at all” (1 point) to “extremely” (4 points). A mean score was calculated (range 1–4), as described by the developers , and a higher score indicates more symptoms of psychological distress. The score was dichotomized using a well-established cut-off (> 1.85 = symptoms of psychological distress) .
Background variables and possible confounders
Age, objectively measured BMI, and persistent MSK pain were measured at baseline and used to describe the study sample. BMI was categorized into age-adjusted cut-offs from Cole and Lobstein as “thinness”, “normal weight”, and “overweight/obese” . Information regarding sex, comorbidities, and parent’s education were collected and used as potential confounding factors based on theory and previous empirical findings [26, 27]. Sex was measured as girls/boys, and chronic diseases were measured with the question: “Do you have any chronic or persistent diseases?” categorized as yes or no. Parent’s education was a presumed confounder , but due to a large number (24–30%) of adolescents not knowing their parent’s education level, it was not included in the analyses.
Descriptive data were presented as means and standard deviations (SDs) when continuous and categorical data were reported as counts and percentages. A two-year incidence rate of new cases with persistent MSK pain at follow-up was calculated. The two-year incidence was calculated by dividing the number of participants who developed a new episode of persistent MSK pain at follow-up by the number of participants at risk at baseline (study sample). Analyses were conducted to assess possible attrition bias by comparing baseline characteristics between participants lost to follow-up and respondents. Independent sample t-test was used to compare normally distributed pairs of continuous data, and categorical variables were compared using the chi-square test.
Univariate logistic regression was used to estimate the crude association between social acceptance and persistent MSK pain. Multiple logistic regression analyses were used to include sex and comorbidities as confounding factors in the model, based on previous studies [26, 27]. The results were presented with odds ratios (ORs) and 95% confidence intervals (Cis). Due to the low number of missing values (0.1–3.5%) on exposures and confounders, we only performed complete-case logistic regression analyses .
To investigate if psychological distress was an effect modifier, a moderation analysis was conducted. In this model, social acceptance was included as the exposure, persistent MSK pain as outcome, and psychological distress as a possible moderator (Fig.2). The moderation analysis was performed according to Hayes using PROCESS macro in SPSS, model 1 . A bias-corrected bootstrap method with 5000 bootstrap samples was used to estimate the effect modifier’s confidence intervals. In addition to the moderation analysis, univariate regression analyses investigating the association between social acceptance among peers and persistent MSK pain were conducted in a sample stratified into low and high level of psychological distress to observe potential differences in the magnitude or direction of the association in these different subsamples. Due to few cases of persistent MSK pain, multiple regression could not be fitted with sufficient precision. Moderation analysis was not possible to conduct for the secondary outcome due to too few cases of MSK pain.
To investigate the potential impact of incidence-prevalence bias on the measures of association, we performed secondary analyses of the whole cohort (n = 692), including all participants with and without persistent MSK pain at baseline. Associations with a significance level of ≤0.05 were considered statistically significant. All analyses were considered exploratory so no correction for multiple testing was done. All statistical analyses were conducted using SPSS statistical software version 27 (SPSS Inc., Chicago, IL, USA).
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