Lower-limb muscle activation patterns during the taekwondo roundhouse kick in elite and youth athletes: a functional principal component analysis

Objective The taekwondo roundhouse kick is a fundamental and performance-critical technique that requires precise lower-limb muscle coordination. However, previous studies have mainly focused on discrete electromyographic variables, providing limited insight into waveform-level variations across the kicking cycle, particularly between athletes of different age and training levels. Therefore, this study aimed to investigate lower-limb surface electromyographic (sEMG) waveform differences during the roundhouse kick between elite and youth taekwondo athletes using functional principal component analysis (FPCA). Methods Thirty-eight male taekwondo athletes were included (19 elite, 19 youth). Surface electromyography was recorded bilaterally from seven trunk and lower-limb muscles, including the rectus abdominis, gluteus maximus, rectus femoris, biceps femoris, semitendinosus, lateral gastrocnemius, and tibialis anterior, and three-dimensional kinematic data were collected synchronously during standardized roundhouse kicks. Time-normalized sEMG waveforms were analyzed using FPCA to identify dominant and secondary modes of variation. Group differences in principal component scores were tested using independent-samples comparisons, with Benjamini–Hochberg false discovery rate correction applied across muscle × component tests. Results After FDR correction, significant between-group differences were identified in four muscle–component combinations. In the support leg, significant differences were observed in PC1 of the left tibialis anterior (p_FDR = 0.001, Cohen’s d = 1.726) and left rectus femoris (p_FDR = 0.029, Cohen’s d = 1.058), whereas the left lateral gastrocnemius differed in PC2 (p_FDR = 0.040, Cohen’s d = 0.943). In the swing leg, the right biceps femoris differed in PC1 (p_FDR = 0.015, Cohen’s d = 1.206). Conclusion Elite and youth taekwondo athletes showed distinct muscle-specific and phase-dependent neuromuscular organization during the roundhouse kick, particularly in support-leg control and swing-leg posterior-chain regulation. FPCA revealed whole-waveform differences that were not captured by conventional discrete measures, providing useful evidence for technique analysis, youth athlete development, and more targeted taekwondo training.