Muscle stability deficits are strongly associated with musculoskeletal complaints in football (soccer) players: the AF-Ratio outperforms conventional strength parameters--a cross-sectional study with preliminary follow-up

Objectives: Given the high injury burden in football and the documented limitations of strength-based screening, novel approaches are warranted. Adaptive Force (AF)being closer to injury-prone movements than pushing/pulling strength--offers an alternative. This study examined the association between AF-based muscle stability and musculoskeletal complaints in football players and compared AF-derived and conventional strength parameters in their discriminative capacity, complemented by a preliminary prospective follow-up. Methods: AF and maximal voluntary isometric contraction (MVIC) were measured in 23 male football players across five bilateral muscle groups (knee extensors/flexors; hip flexors/adductors/abductors). AF parameters (maximal isometric AF, maximal AF, AF-Ratio), MVIC and hamstrings-to-quadriceps (H:Q) ratio were compared between players with and without complaints assessed via questionnaire at baseline and six-month follow-up (n=13). Results: Stability deficits were strongly associated with complaints (OR=54.0, 82% side concordance). AF-Ratio discriminated clearly between players with and without complaints (d=-1.47), with hip abductors showing the strongest effect (d=-1.64). Players with subsequent complaints showed lower baseline AF-Ratio (d=-1.45) and more stability deficits (d=1.67). MVIC and H:Q ratio did not discriminate (p>0.430). Conclusion: The findings suggest that muscle stability assessment outperforms conventional strength parameters in discriminating players with and without complaints, with preliminary follow-up data providing tentative support for predictive value. The concept of functional instability syndrome (FIS) provides a mechanistic framework for non-contact injuries and musculoskeletal complaints. AF assessment offers potential for screening, including return-to-sport decisions. Further studies are needed to verify the results, investigate predictive value, and evaluate whether personalised stability-based interventions can reduce injury incidence.