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The aim was to estimate national temporal trends in muscular fitness for French youth between 1999 and 2023. Data were obtained from five cross-sectional studies on 53,314 children and adolescents (age range: 6–16 years). Lower- and upper-body strength were assessed by standing broad jump (SBJ) and handgrip strength (HGS). BMI z-scores (BMIz) were calculated using WHO growth curves. We found a statistically significant negligible decline in SBJ performance (standardised effect size (ES) trend per decade [95%CI]: −0.08 [−0.10, −0.07] or −2.1 cm [95%CI: −2.4, −1.8]) and a statistically significant negligible improvement in HGS (ES trend per decade [95%CI]: 0.11 [0.08, 0.13]) or 0.6 kg [(95%CI: 0.4, 0.8]). We found evidence of significant increases in distributional variability and asymmetry, with poorer trends in the low performers (<25th percentile) and better trends in the high performers (>75th percentile) compared to the average performers. Our data suggest negligible trends in mean SBJ and HGS, coupled with increased distributional variability and asymmetry. Trends were not uniform across the population distribution with data suggesting an increase over time in the gap between low and high performers. These results reinforce the importance of interventions and programmes aiming at improving muscular fitness specifically in children and adolescents with low muscular strength. [ABSTRACT FROM AUTHOR]