Neuromechanical characteristics of breaststroke in adolescent swimmers across different performance levels: a comparative analysis

Background Breaststroke is technically complex and requires high neuromuscular coordination. Because muscle synergies describe how multiple muscles are organized into low-dimensional activation modules, they provide a functional read-out of neuromuscular control development during adolescence. This study investigated neuromechanical characteristics of adolescent breaststroke swimmers across performance levels. Methods Twenty-six male adolescent breaststroke swimmers were divided into elite (n = 13) and amateur (n = 13) groups. During five 25-m maximal breaststroke trials, two-dimensional underwater video and synchronized surface EMG from 16 muscles were collected. Main outcomes included discrete joint angles, the minimum number of synergies, variance accounted for (VAF), spatial weightings, and temporal activation coefficients. Group differences were assessed using independent-samples or Mann-Whitney U tests, FDR-adjusted spatial-weighting comparisons, SPM1D for activation curves, and Cohen’s d effect sizes. Results At v2, the elite group showed greater hip flexion (43.78° ± 6.92° vs. 34.39° ± 11.32°, P = 0.018, d = 1.00), smaller knee flexion (122.93° ± 8.77° vs. 128.93° ± 4.84°, P = 0.041, d = −0.85), and smaller hip abduction (36.28° ± 7.19° vs. 45.22° ± 7.87°, P = 0.006, d = −1.19). VAF was similar between groups (91.27% ± 0.69% vs. 91.20% ± 0.73%, P = 0.583, d = 0.10), whereas the elite group required fewer minimum synergies (5.42 ± 0.72 vs. 6.03 ± 0.66, P 0.001, d = −0.88). FDR-adjusted spatial-weighting and SPM1D analyses indicated that SYN2 and SYN4 were the main modules distinguishing the groups, with temporal differences concentrated from late glide to in-sweep. Conclusion Performance-related differences mainly reflected lower-limb posture during glide and synergy control during propulsion-related phases. Fewer minimum synergies under comparable VAF conditions suggest greater synergy integration in high-level swimmers. These findings link adolescent breaststroke performance to task-specific multi-muscle coordination.