The choice of asymmetry index affects clinical interpretation of hip adduction and abduction strength in elite soccer players: A season-long investigation.

Abstract

Multiple formulas exist for quantifying inter-limb asymmetry, yet the extent to which formula choice affects clinical decision-making in longitudinal monitoring remains unclear. Using a prospective observational design, this study applied seven asymmetry formulas to a season-long hip adduction and abduction strength dataset in elite male soccer players, examining formula effects on asymmetry magnitude, clinical flagging rates, direction consistency, and position-group representativeness. Seventeen players from a Premier League club performed weekly bilateral hip adduction and abduction strength assessments across a 36-week competitive season (35 testing weeks; 429 valid observations). The seven formulas were: Limb Symmetry Index (LSI-2), Bilateral Asymmetry Index 2 (BAI-2), Bilateral Strength Asymmetry (BSA), Percentage Difference (%Diff), Bilateral Asymmetry Index 1 (BAI-1), Symmetry Index (SI), and Symmetry Angle (SA). Adduction asymmetry magnitude (mean of absolute values ± SD) ranged from 3.0 ± 2.8% (SA) to 9.8 ± 10.1% (LSI-2). Clinical flagging rates at a 10% threshold ranged from 1.9% (SA) to 37.3% (LSI-2). Formulas separated into signed and unsigned clusters; between-cluster correlations were negative for adduction (r = −0.09 to −0.24) but positive for abduction (r = +0.16 to +0.28), reflecting opposite directional dominance profiles between muscles. Both muscles confirmed that formulas produce non-equivalent representations of the same underlying data. The dominant limb changed direction in 36.8% of consecutive weeks for adduction and 30.1% for abduction (adduction: κ = 0.015; abduction: κ = 0.042; both slight agreement, both significantly greater than zero, p < 0.001), a pattern unsigned formula cannot detect. Position-group means would mis-classify approximately 30% of individual clinical decisions. Asymmetry formula selection fundamentally alters clinical interpretation of the same data. Practitioners should use BAI-2 with individualised monitoring thresholds and explicitly report the formula used when comparing results across studies. SA should not be used for longitudinal hip-strength monitoring with this test protocol: typical SA values fall within the measurement-error floor, so apparent stabilisation reflects compressed scale rather than meaningful precision.