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

Abstract

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 (GRF vert ), 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% GRF vert asymmetry breach occurred in every trial. Limb dominance had minimal influence, except for longer GRF vert 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.