Background: Musculoskeletal conditions impose a large burden of disability in young populations. University students are prone to neck and back pain, with potential negative impact on physical, mental, and academic functioning. In adolescent athletes, musculoskeletal injuries are common and may result in discontinued sports participation as well as impaired physical and mental health. University students and adolescent athletes tend to have unhealthy lifestyle behaviors. University students report unhealthy behaviors such as high sitting time, low physical activity levels and unhealthy dietary habits. Adolescent athletes are physically active but tend to have inadequate dietary habits in relation to their physiological demands, including insufficient intakes of total energy and carbohydrates.

Identifying modifiable risk factors for musculoskeletal conditions is the first step towards prevention, and knowledge about the role of lifestyle behaviors for these conditions is limited. The overarching purpose with this doctoral thesis was to deepen the knowledge about lifestyle behaviors and their role for the risk of musculoskeletal conditions in university students and high school athletes.

Methods, results, and conclusions: Study I and II are based on the Sustainable UNiversity Life (SUN) study, a cohort study including 4262 students at Swedish universities. The participants reported different aspects of health, lifestyle behaviors and attitudes through web-surveys every third month for one year in 2019-2021.

In Study I, the aim was to assess changes in lifestyle behaviors during the COVID-19 pandemic, compared to before, in 1877 university students (73% women, mean age 26.5 ± 6.8 years). Physical activity levels, sitting time, meal frequency and risk use of alcohol, tobacco and illicit drugs were measured at a pre-pandemic baseline and at two periods during the first six months of the pandemic, the follow-up 1 (FU1) and follow-up 2 (FU2). Population means and differences with 95% confidence interval (CI) in lifestyle behaviors were calculated with generalized estimating equations. The study found that weekly exercise decreased by -7.7 minutes (95% CI -12.6, -2.8) whereas weekly daily activities increased by 14.2 minutes (95% CI 7.9, 20.5) between the pre-pandemic baseline and FU2. Furthermore, daily sitting time decreased by -1.4 hours (95% CI -1.7, -1.2) during this period. Small changes were observed for meal frequency, whereas for risk use of alcohol, tobacco and illicit drugs, changes were only minor. The conclusion of Study I is that lifestyle behaviors in Swedish university students slightly improved during the first six months of the COVID-19 pandemic compared to before.

In Study II, the aim was to assess the association between an unhealthy lifestyle and incident activity-limiting neck/back problems (ALNBP) in university students. Physical inactivity, high sitting time, meal skipping, and risky use of alcohol, tobacco and illicit drugs were combined into an unhealthy lifestyle, categorized as ≥3 unhealthy lifestyle behaviors. A total of 3492 university students (60% women, mean age 24.5 ± 6.0 years) without ALNBP the past three months at baseline were included. Cox regression models were built to assess the association between an unhealthy lifestyle and incident ALNBP, reported as a hazard rate ratio (HRR). Being exposed to an unhealthy lifestyle generated a HRR of 1.35 (95% CI 1.12 to 1.63) of ALNBP, compared to not being exposed to an unhealthy lifestyle. The conclusion of Study II is that an unhealthy lifestyle is associated with incident activity-limiting neck/back problems in university students.

Study III and IV are based on the Swedish Handball Cohort (SHC), a cohort study including 1334 adolescent handball players enrolled to a handball-profiled high school in Sweden. Participants responded to a baseline web-survey with questions about current and previous injuries, training, and dietary habits, among others, in the beginning of the handball season in 2020/2021, 2021/2022 and 2022/2023.Participants were monitored weekly throughout the handball season with questions about their amount of handball training and matches the preceding week, sleep habits, injuries, and injury consequences, among others, reported through a mobile application.

In Study III, the aim was to describe dietary habits in 1040 adolescent handball players enrolled in the SHC seasons 2020/2021 or 2021/2022 (51% males, mean age 16.6 ± 0.9 years). Differences with respect to sex and school grade were estimated with generalized linear models, generating a prevalence ratio (PR) and 95% CI. Overall, 70% and 90% of the participants met recommendations for meal frequency and meal timing, respectively, whereas adherence to recommended provision of carbohydrates was met by 17%. Sixteen percent adhered to the Nordic Nutrition Recommendations (NNR) for fruits/vegetables of ≥500 grams per week, whereas 37% adhered to the recommendations for fish/seafood of ≥2 times per week. Use of dietary supplements was reported by 28% of the participants, and 20% reported food exclusions due to other reasons than intolerances. Females reported a lower frequency of all meals, especially morning snacks and evening snacks. Also, a higher prevalence of food exclusions was observed in females, with a PR of 1.66 (95% CI 1.31to 2.01) for exclusions due to intolerances and a PR of 1.36 (95% CI 1.08, 1.64) for exclusions due to other reasons. Females also reported a higher adherence to fruits/vegetables recommendations with a PR of 2.30 (95% CI 1.98, 2.62) and use of micronutrient supplements to a larger extent with a PR of 1.72 (95% CI 1.43, 2.00), compared to their male counterparts. Small differences were observed with respect to school grade. The conclusion of Study III is that Swedish adolescent handball players’ dietary habits are somewhat in accordance with sports nutrition recommendations whereas adherence to provision of carbohydrates during matches and the NNR for fruit/vegetables and fish/seafood is low. Females seem to display more restrictive dietary habits compared to males.

In Study IV, the aim was to assess the association between dietary habits and incident knee/shoulder injuries in 1144 adolescent handball players in season 2020/2021, 2021/2022 and 2022/2023 of the SHC without a knee or shoulder injury at baseline (1703 player seasons, 53% males, mean age 16.7 ± 0.9 years). Cox regression analyses estimated a HRR with the first event of a knee/shoulder injury as the outcome. A moderate-high nutritional quality, in accordance with the NNR, generated a HRR of 1.46 (95% CI 1.08, 1.98) for knee/shoulder injuries compared to low nutritional quality in adolescent female handball players. Also, ≥2 unfavorable dietary habits in relation to sports nutrition recommendations generated a HRR of 1.38 (95% CI 1.02, 1.86) compared to 1 unfavorable dietary habit in females. For poor meal timing, adjusted analyses generated a HRR of 1.20 (95% CI 0.90, 1.61) in females, compared to adequate meal timing. In adolescent male handball players, analyses generated a HRR of 1.23 (95% CI 0.69, 2.17) for low meal frequency compared to high meal frequency and a HRR of 0.83 (95% CI 0.60, 1.15) for poor meal timing. Menstrual dysfunction was assessed in females with no use of hormonal contraceptives, as a proxy for low energy availability, however, no associations were found between menstrual dysfunction and incident injuries in these females. The conclusion of Study IV is that a moderate-high nutritional quality and having more than one unfavorable dietary habit is associated with higher rate of incident knee/shoulder injuries in adolescent female handball players, while there are no or unprecise associations for meal frequency, meal timing and menstrual dysfunction. In adolescent male handball players, there are no clear associations between dietary habits and incident injuries.

Overall conclusion: This doctoral thesis sheds light on aspects of lifestyle behaviors in university students and high school athletes. Population-specific unhealthy lifestyle behaviors are common in university students and high school athletes. These behaviors seem to be of importance for the risk of musculoskeletal conditions.