Discrete Symmetry Metrics Mask Frequent Phase-Specific Lower Limb Asymmetry During Bilateral Countermovement Jumping in Youth Football Players.

Inter-limb asymmetry is frequently evaluated during bilateral tasks such as the countermovement jump (CMJ), typically using discrete peak-based metrics. However, these approaches may inadequately capture the complex, time-varying nature of inter-limb coordination. This study examined inter-limb asymmetry during CMJ in youth football players using both discrete and continuous assessments, with consideration of limb dominance. Forty-three male youth academy football players (14 ± 1.1 years) performed three maximal bilateral CMJs with data collected using dual force platforms and three-dimensional motion capture. Inter-limb asymmetry was quantified using discrete symmetry indices for peak vertical ground reaction force (GRFvert), joint displacements, and joint moments, alongside time-series approaches including statistical parametric mapping (SPM1D) and the normalized symmetry index (SInorm). Asymmetry threshold breaches (±10% and ±20%) and their duration across the movement cycle were assessed, with comparisons between dominant and non-dominant limbs. Discrete metrics indicated low levels of inter-limb asymmetry at the group level, with mean values typically within ±10% thresholds. In contrast, time-series analyses revealed frequent, phase-specific asymmetry within individual trials, particularly for joint moments, despite no significant group-level differences identified using SPM1D. A ±10% GRFvert asymmetry breach occurred in every trial. Limb dominance had minimal influence, except for longer GRFvert breach durations on the non-dominant limb. Inter-limb asymmetry during bilateral CMJ is observed frequently when assessed using time-series methods, despite apparently symmetrical discrete outcomes. These findings demonstrate that reliance on peak-based symmetry metrics may overlook substantial phase-specific asymmetry, with implications for the interpretation of bilateral loading during athlete monitoring and rehabilitation contexts.