Interindividual variability in trunk function across body mass index in young men using exploratory cluster analysis

Abstract Trunk function is important for postural control and spinal loading; however, individuals with similar body mass index (BMI) may differ in trunk endurance and back muscle strength. The extent to which BMI reflects functional trunk capacity therefore remains unclear. This study investigated the relationships between BMI, plank endurance, maximal isometric back extensor strength, and sagittal spinal posture, and explored interindividual variability in trunk function using cluster analysis. A total of 199 male university students were assessed for BMI, maximal isometric back extensor strength, plank-based trunk endurance and sagittal spinal posture. K-means cluster analysis with z-score standardization was used to examine variability in trunk function and to identify functional profiles. Differences between groupings were analyzed in relation to physical activity, plank test termination and sagittal spinal posture. BMI demonstrated a moderate negative association with plank endurance (ρ = − 0.36, p < 0.001) and a weak positive association with maximal isometric back extensor strength (ρ = 0.22, p = 0.002).Cluster analysis identified three exploratory functional profiles, although separation was limited (silhouette = 0.280). The clusters differed in self-reported physical activity frequency ( p = 0.006). Separate comparisons did not detect differences in thoracic kyphosis or lumbar lordosis (Holm-adjusted p = 0.506 and 0.695, respectively), or in reasons for plank test termination ( p = 0.068). Plank endurance and maximal isometric back extensor strength represent distinct but complementary aspects of trunk function that are only partly associated with BMI. Individuals with similar BMI may exhibit different functional capacities, highlighting the importance of multidimensional assessment.