Menarche, somatic maturity, and physical fitness in Portuguese girls: an intergenerational analysis based on the Mirwald equation

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Purpose This study aimed to assess age at menarche in Portuguese girls and compare it with their mothers, examine its association with somatic maturity estimated by the Mirwald equation, and characterise physical fitness in pre- and post-pubertal girls. Material and methods 71 Portuguese girls (10–13 years, post-menarche) completed a questionnaire on menarcheal age (self and mothers), anthropometric assessments (height, sitting height, body mass, waist circumference), and physical fitness tests (handgrip, isometric mid-thigh pull, 3-min Kasch Pulse Recovery Test). Somatic maturity was estimated with the Mirwald equation. Analyses included paired and independent t-tests, effect sizes, correlations, and agreement (Bland–Altman, Lin’s CCC). Results Pre-PHV girls had negative maturity offset values (–0.58 ± 0.62 years), while post-PHV girls were beyond PHV (1.28 ± 0.91 years). Mean age at menarche was 10.87 ± 0.93 years, significantly earlier than their mothers (12.00 ± 1.44 years; mean difference = –1.09, p < 0.001). Post-PHV girls showed higher absolute strength (handgrip right: 18.86 ± 3.68 vs. 12.74 ± 3.10 kg, d = 1.72; IMTP: 63.46 ± 12.54 vs. 45.58 ± 11.93 kg, d = 1.44), while relative strength differences were smaller (handgrip: 0.35 ± 0.08 vs. 0.30 ± 0.09 kg/kg, d = 0.67). Endurance did not differ. The Mirwald equation underestimated years since menarche by 0.94 ± 1.10, with weak agreement (Lin’s CCC = 0.37). Conclusions Biological maturation strongly influenced absolute strength, largely explained by body size. Relative strength and maturity offset-based estimates of years since menarche showed weaker validity, highlighting the need for refined prediction models.