Changes in physical fitness across the school year and summer holiday in Danish adolescents: an exploratory longitudinal study

Physical fitness is an important determinant of adolescent health, yet how physical fitness changes over the summer holiday period relative to the school year remains unclear, particularly in adolescent populations. This exploratory study examined changes in physical fitness across three measurement occasions spanning a school year and summer holiday in Danish adolescents. A longitudinal observational study was conducted in two schools, and 140 Danish adolescents (mean age 13.6 years) participated. Measurements were taken at three occasions: t1 (August 2024, start of school year), t2 (June 2025, end of school year, before summer), and t3 (August 2025, after summer holiday). Primary outcomes were cardiorespiratory fitness, measured using the Yo-Yo Intermittent Recovery Level 1 Test for Children (YYIR1C), blood pressure, and resting heart rate. Secondary outcomes included agility, balance, handgrip strength, standing long jump, and body composition. Linear mixed models were used to assess the changes in the outcome scores over the school year and the summer holidays, and latent transition analysis was used to investigate fitness profiles and profile membership over time. Over one year, YYIR1C distance declined (− 194 m, 95% CI − 307 to − 82, school year: −162 m, 95% CI − 267 to − 58; summer holiday: −32 m, 95% CI − 151 to 87). Diastolic blood pressure decreased across the school year (− 2.4 mmHg, 95% CI − 4.1 to − 0.7) and returned towards baseline over the summer, whereas systolic blood pressure and resting heart rate did not change. Across the school year, girls showed greater declines than boys in agility, handgrip strength, and standing long jump. Muscle mass increased over the year. Three fitness profiles were identified and were largely stable; across the school year, 21.8% of girls and 5.2% of boys moved to a lower-fitness profile. In this sample of Danish adolescents, cardiorespiratory fitness declined over one year and most of this decline was already evident at the end of the school year, with no recovery over the summer. Girls and the least-fit adolescents may warrant particular attention in school-based efforts to support fitness. Studies measuring PE exposure, physical activity and maturation are needed to explain these patterns.