Time-series analysis reveals phase-specific lower-limb asymmetry during single-leg lateral jump landing: limitations of conventional discrete peak metrics

Objective To characterize time-dependent lower-limb asymmetry between the dominant and non-dominant limbs during single-leg lateral jump landing and determine whether discrete peak metrics overlook phase-specific differences. Methods Twenty-nine healthy young men performed single-leg lateral jump landings on both limbs while three-dimensional kinematics and ground reaction forces (GRF) were recorded. The landing deceleration phase was analyzed using discrete peak comparisons, one-dimensional statistical parametric mapping (SPM1D), and a normalized symmetry index based on continuous waveforms (SI_norm). SI_norm thresholds of ±10% and ±20% were used to characterize the direction and duration of individual-level asymmetry. Results Discrete peak analysis showed that the maximum hip joint angle was greater in the dominant limb than in the non-dominant limb, with a paired mean difference of 12.04° (95% CI 7.23°–16.84°, Holm-adjusted p < 0.001, Cohen’s d_z = 0.953). In contrast, SPM1D analysis showed that the dominant limb exhibited significantly greater ankle joint angles during 9.6%–26.5% of the landing deceleration phase (cluster-level p = 0.0052, corrected p = 0.0362), and significantly greater hip joint angles throughout the entire phase (cluster-level p < 0.001, corrected p = 0.000193); no significant differences were found in joint moments or vertical GRF after correction. SI_norm further demonstrated that, even when peak metrics showed no significant inter-limb differences, phase-specific threshold exceedances were common within individual continuous waveforms; at the ±10% threshold, 100% of participants exhibited threshold exceedances for both the hip joint moment and vertical GRF. However, no significant differences were observed between the durations of threshold exceedance in the dominant and non-dominant directions for any variable. Conclusion Lower-limb asymmetry during single-leg lateral jump landing is time-dependent and joint-specific rather than fully captured by peak values. Discrete peak analysis may miss phase-specific differences, and limb dominance does not consistently indicate greater mechanical loading. Combining discrete peaks, SPM1D, and SI_norm provides complementary group- and individual-level descriptions of asymmetry. SI_norm exceedances should not be interpreted directly as absolute load asymmetry or functional abnormality; the utility of these measures for injury-risk screening requires further investigation.