Secular trends in muscular fitness among Finnish adolescents

Aim: To investigate secular change over time in health-related muscular fitness and how leisure time physical activity (LTPA), sport club participation, and body mass index are associated with muscular fitness in adolescents. Methods: Two cross-sectional samples of Finnish 13—16-year-old adolescents were studied in 1976 (n = 643; 312 boys and 331 girls) and in 2001 (n = 579; 308 boys and 271 girls). Muscular fitness index was calculated as the sum of age- and sex-specific z-scores of four tests measuring muscular fitness and agility. Height and weight were also measured. Self-reported weekly frequency of LTPA of at least 30-min duration and regularity of participation in organised sport were obtained by questionnaire. Identical methods were used in 1976 and 2001. Results: Muscular fitness index was higher in 2001 than in 1976 in both boys (by 0.77 points, p = 0.008, effect size d = 0.17, small difference) and girls (1.01 points, p = 0.004, d = 0.20, small difference). Sport club participation, body mass index, and participation in LTPA together explained more of the muscular fitness index variance in 2001 than in 1976 in both boys (coefficient of determination from 0.10 to 0.24) and girls (from 0.25 to 0.40). Conclusions: The muscular fitness of 13—16-year old adolescents slightly improved over time. Organised LTPA showed a stronger association with muscular fitness in 2001 than in 1976. Due to increased polarisation in fitness, in health promotion more attention should be paid on adolescents’ health-related fitness and its association to LTPA.