Age- and sex-specific normative references and performance zones for core muscular endurance in adolescents aged 12–16 years: a cross-sectional study

Objective This study aimed to develop age- and sex-specific normative percentile curves and percentile-based performance zones for core muscular endurance using the plank exercise test among adolescents aged 12–16 years in South Punjab, Pakistan, employing the LMS method and a back substitution procedure to assess internal consistency. Method This cross-sectional study included 2, 970 adolescents (49.73% boys, 50.27% girls; mean age 14.00 ± 1.41 years) recruited from 60 higher secondary schools across South Punjab, Pakistan, using stratified random sampling in 2020, representing a pre-pandemic baseline. Height, weight, and BMI were recorded, and core muscular endurance was assessed using the isometric plank test. Age- and sex-specific normative references and smoothed percentile curves were established using the LMS (Lambda, Mu, Sigma) method. Internal consistency of the derived reference values was assessed using a back substitution procedure, in which the LMS model’s P50 (50th percentile) estimate for the full sample was compared against its P50 estimate for a randomly drawn 20% subsample of the same cohort, with the Mean Absolute Percentage Error (MAPE) used to quantify agreement. Sex differences were examined using the Mann-Whitney U test, with statistical significance set at p < 0.05. Results Using the LMS method, the study established age- and sex-specific normative references and smoothed percentile curves for the plank test. Boys were significantly taller, heavier, and had higher BMI than girls (p < 0.001). Boys showed higher mean plank times (80.72 ± 73.48 s vs. 69.07 ± 58.16 s; p < 0.01), effect sizes ranged from negligible to small across all variables (rank-biserial r = 0.00–0.26), with r = 0.06 for plank performance. Median plank performance increased with age in both sexes, from 50 to 61 seconds in boys and from 44 to 66 seconds in girls between ages 12 and 16, with a steeper rise in the upper percentiles among boys. Age- and sex-specific percentile curves and performance cut-points across five zones; Very Poor, Poor, Medium, Good, and Excellent were derived. The majority of participants were classified as “Medium” (32.7%) or “Good” (24.4%), with 9.9% reaching “Excellent”. Boys’ “Excellent” threshold ranged from ≥133 seconds at age 12 to ≥227 seconds at age 16, while in girls it ranged from ≥127 to ≥126 seconds, peaking at ≥146 seconds at age 14. The back substitution procedure produced an average MAPE of 6.69% for boys and 6.04% for girls, supporting the internal consistency of the derived references within this population. Conclusion This study provides the first age- and sex-specific normative references and percentile-based performance zones for the plank exercise test in Pakistani adolescents, with internal consistency supported by a back substitution procedure. These references offer simple, low-resource, population-based benchmarks for monitoring core muscular endurance, with practical value for physical education teachers, coaches, and health professionals working in school-based health initiatives. However, external validation using an independent sample, and replication in other regions of Pakistan, are needed before these references are applied beyond South Punjab.