Physical Fitness Level in 9–11-Year-Old Italian Children Is Affected by Body Mass Index and Frequency of Sport Practice but Not by Peak Height Velocity and Relative Age Effect

This study was aimed at analyzing physical fitness in 9–11-year-old children and verifying whether it is affected by body mass index (BMI), peak height velocity (PHV), quartile distribution (QD), and sport practice (SP), also considering any potential effects of sex. One thousand one hundred forty-three Italian primary school children (50.7% males) underwent anthropometric measurements (body mass, height, and BMI) and physical tests for measuring coordination (Plate Tapping, PT), handgrip strength (HandGrip, HG), lower-limb power (standing long jump, SLJ), low-back flexibility (sit-and-reach, SR), and sprint (20 m sprint, 20 m) skills. A series of analyses of covariance (ANCOVAs) were conducted using age as a covariate to examine differences among subgroups for BMI, PHV, QD, and SP in relation to the different physical tests (i.e., PT, HG, SLJ, SL, 20 m). Sex was included in each model as fixed independent variable. Principally, participants with higher SP and BMI reported higher and lower performance (p < 0.001) in SLJ, SR, and 20 m tests, respectively. Differently, for higher BMI levels, higher HG performance was reported (p < 0.001). PHV and QD had isolated effects, whereas no effect emerged for PT. Sex interactions were found only for SP subcategories in SR (p ≤ 0.001, ES range = 0.74–1.30). Although physical performance in 9–11-year-old (non-competitive, pre-puberty) Italian students does not seem to be characterized by involuntary factors (such as PHV and QD), substantial opposite trends seem to exist for voluntary factors (such as BMI and SP), thus suggesting how an adequate lifestyle and physical activity could crucially lead to valuable fitness benefits.