The research team, led by María del Carmen Carcelén-Fraile of the University of Atlántico Medio and Agustín Aibar-Almazán of the University of Jaén, recruited children aged 10 to 12 from two primary schools in Andalusia, Spain. The final analytical sample comprised 176 boys and 210 girls with a mean age of 11.15 years, drawn from grades three through six. Because children of the same chronological age can differ dramatically in biological maturation, this age window is a fascinating one: it captures the transition from late childhood into early adolescence, when strength, power, and body composition begin to diverge between the sexes. Yet in this sample, none of those expected differences had yet materialized.

The researchers assessed five distinct capacities using internationally standardized field tests. Handgrip strength was measured with a digital dynamometer, with the mean of the best attempts from each hand serving as an indicator of overall muscular fitness. Lower-body explosive strength was captured by the standing long jump, a classic test of horizontal power drawn from the ALPHA-Fitness battery. Trunk muscular endurance was quantified by the number of correctly executed sit-ups completed in 30 seconds, following the EUROFIT protocol. Flexibility was assessed with the sit-and-reach test, which primarily reflects hamstring and posterior-chain extensibility. Finally, change-of-direction performance was measured with the 4 x 10-meter shuttle run, in which children sprint back and forth between two lines 10 meters apart, exchanging sponges at each turn, with the fastest of two trials recorded in seconds.

The correlation results were emphatic. Lower-body explosive strength showed the strongest relationship with shuttle-run time, with a correlation coefficient of minus 0.775, meaning children who jumped farther tended to complete the shuttle run dramatically faster. Handgrip strength followed at minus 0.706, flexibility at minus 0.534, and trunk muscular endurance at minus 0.532, all statistically significant. Because lower times indicate better performance, the negative signs all point in the same direction: fitter children turned more quickly. Notably, neither age nor body mass index correlated significantly with shuttle-run performance, suggesting that within this narrow age band, raw fitness capacity outweighed chronological age or body size.

Correlations alone, however, can be misleading when fitness components overlap heavily, as they do in children. The team therefore turned to hierarchical multiple linear regression, first entering age, sex, and body mass index as covariates, and then adding the four fitness measures simultaneously. The covariates alone explained a negligible 1.2 percent of the variance in shuttle-run time. Adding the fitness components raised the explained variance by 61.4 percentage points, bringing the final model to 62.6 percent, an unusually large share for a field-based study of motor performance.

Within that adjusted model, the hierarchy of importance became clear. Lower-body explosive strength retained by far the strongest independent association, with a standardized coefficient of minus 0.566. Handgrip strength followed at minus 0.180 and flexibility at minus 0.111, both still statistically significant but far weaker. Trunk muscular endurance, despite its respectable bivariate correlation, dropped to essentially zero once the other components were accounted for. This pattern illustrates a crucial statistical lesson: because muscular endurance is strongly correlated with both grip strength and leg power, its apparent link with agility was likely just borrowed variance from its stronger cousins.

The handgrip result deserves particular attention. Grip strength plays no direct biomechanical role in propelling or braking the body during a shuttle run, so its association with agility cannot reflect a direct mechanical effect. Instead, the authors suggest it functions as a broader marker of neuromuscular development and general muscular fitness. Importantly, handgrip strength and standing long jump performance were correlated at 0.823, meaning the two measures share most of their statistical variance. The researchers caution that their regression coefficients should therefore be read as adjusted associations conditional on the other predictors, not as fully independent contributions, and they note that collinearity diagnostics in the moderation model showed elevated variance inflation factors for these two measures.

Perhaps the most surprising finding was what did not appear: sex differences. Boys and girls showed no statistically significant differences in any fitness measure or in shuttle-run time, and all between-sex effect sizes were small with confidence intervals crossing zero. When the team formally tested whether sex moderated the fitness-agility relationships by adding interaction terms to the regression, the results were null, explaining only an additional 0.3 percent of variance. This stands in contrast to studies of older adolescents, where sex differences in strength and power widen considerably after pubertal development, and to some prior work in primary schoolchildren that reported boys outperforming girls in jumping and shuttle tasks. The authors suggest the developmental stage of their sample, around age 11, may simply precede the point at which sex-related differences reliably emerge, though they acknowledge that biological maturation was not directly measured, a key limitation.

The flexibility finding also carries nuance. The sit-and-reach test captures hamstring and posterior-chain extensibility but not the multiplanar, joint-specific mobility involved in cutting and pivoting, so the association should not be read as proof that stretching routines will make children faster at changing direction. Previous research in elite youth soccer players has found no significant link between sit-and-reach scores and change-of-direction time, highlighting that the relationship may depend on age, population, and task demands. Similarly, the study’s cross-sectional design means causality cannot be established: stronger children may simply be more agile, or more active children may be both stronger and more agile. The 4 x 10-meter shuttle run also measures pre-planned change of direction, not reactive agility involving decision-making.

Still, the practical message is compelling. Most intervention research on agility has focused on trained adolescent athletes, particularly male soccer players, where plyometric and strength training reliably improve change-of-direction speed. This study extends the evidence to ordinary schoolchildren, showing that the same principle, leg power above all, appears to govern multidirectional movement long before specialized training begins. For physical education teachers and public health planners, the implication is that developing lower-body explosive strength through jumping, hopping, and games may offer the greatest return for children’s motor competence, while the absence of sex differences at this age suggests that boys and girls can be trained and assessed on equal footing during the primary school years. Longitudinal studies incorporating measures of biological maturation will be needed to confirm whether boosting leg power in childhood actually drives improvements in agility over time.

Subject of Research: Associations between muscular strength, flexibility, sex, and change-of-direction performance in primary schoolchildren

Article Title: Physical fitness factors associated with change-of-direction performance in primary schoolchildren: the role of muscular strength, flexibility, and sex

Article References: Carcelén-Fraile, M. D. C., Aibar-Almazán, A., Peris-Reig, L., & Vigil-Torres, N. (2026). Physical fitness factors associated with change-of-direction performance in primary schoolchildren: the role of muscular strength, flexibility, and sex. Frontiers in Sports and Active Living, 8, Article 1972450. https://doi.org/10.3389/fspor.2026.1972450

Image Credits: AI Generated

DOI: 10.3389/fspor.2026.1972450

Keywords: change of direction, physical fitness, children, explosive strength, handgrip strength, flexibility, muscular endurance, shuttle run, sex differences, motor performance, pediatric exercise science, schoolchildren