Dynamic Field Assessment of Hip Adductor Function Using a Smartphone-Based Copenhagen Test: Reliability and Concurrent Associations with Isometric Strength in Amateur Football Players

Assessing hip adductor muscle strength is important for identifying weakness or side-to-side imbalances associated with groin injury risk. Although the Copenhagen adductor test is widely used to evaluate adductor function, the quantification of strength-related variables using inertial sensors during this task has not been systematically examined. This study aimed to evaluate the reliability of a smartphone-based Copenhagen adductor field test and its associations with established isometric hip adductor strength assessments. Twenty amateur male football players (21.1 ± 3.2 years) completed two laboratory sessions separated by one week. Reliability of the smartphone-based test was assessed for endurance (repetition count) and strength-related outcomes (mean repetition time and peak velocity) using intraclass correlation coefficients (ICC), standard error of measurement (SEM), and minimum detectable change (MDC). Participants also performed unilateral and bilateral isometric hip adductor tests using load cells and force platforms to obtain isometric peak force (IPF) and rate of force development at 150 ms (RFD150). Associations were examined using Pearson correlation coefficients. The smartphone-based test demonstrated moderate-to-good reliability (ICC = 0.63–0.83; SEM = 6.7–18.5%). Endurance-related outcomes were not significantly associated with IPF or RFD150. In contrast, peak velocity showed moderate correlations with IPF (r = 0.50–0.64) and RFD150 (r = 0.48–0.63; p < 0.05). These findings indicate that velocity-derived metrics provide a reliable and practical field-based complement to conventional isometric assessments, although further longitudinal research is required to determine their predictive or preventive value.