Group normative data and arbitrary asymmetry thresholds fail to represent individual players in weekly hip strength monitoring

Abstract Hip and groin injuries account for 10–18% of all time-loss injuries in elite football, yet return-to-play decisions can rely on group normative data or arbitrary asymmetry thresholds (10–15%) with limited empirical support. We characterised the longitudinal behaviour of isometric hip adduction and abduction strength across a full Premier League season at weekly resolution, evaluated whether group normative data can adequately represent individual players, and assessed the clinical utility of standard asymmetry thresholds. Seventeen male Premier League players (age: 25.0 ± 5.7 years, height: 181.7 ± 7.9 cm, body mass: 75.7 ± 7.3 kg) underwent weekly bilateral isometric hip strength testing using a fixed-frame dynamometer across 35 weeks (589 player-week observations). Linear mixed-effects models, intraclass correlation coefficients, and three clinically used reference strategies (pre-season baseline, contralateral limb, 4-week rolling average) were employed. Normalised adduction strength increased significantly across the season ( p < 0.001), but players changed at significantly different rates ( p < 0.001), meaning no single group trajectory could represent all individuals. Between-player variance accounted for 64–78% of total variance in normalised hip strength, causing group normative bands to be too wide to reliably detect meaningful within-player change. Despite all 17 players remaining injury-free, 37% of sessions exceeded the 10% interlimb adduction asymmetry threshold. Ipsilateral adduction: abduction ratios averaged 0.75–0.85, rendering the 10% interlimb threshold inappropriate. The three reference strategies agreed on approximately half of flagging decisions, and weekly monitoring revealed clinically meaningful fluctuations entirely invisible to traditional pre-mid-post designs. Group normative data, static baselines, and arbitrary thresholds may be inadequate for individual clinical decision-making. Individualised longitudinal monitoring is recommended; the three reference strategies answer different clinical questions and are best used in combination, with rolling averages as the primary within-season tracking tool alongside pre-season baselines and contralateral comparisons as complementary reference points.