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

Objective

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.

What is already known on this topic

What this study adds

• Stability deficits (impaired holding capacity) were strongly associated with musculoskeletal complaints (OR=54.0) with preliminary prospective support, whereas maximal pushing strength and H:Q ratio did not discriminate.

• Hip abductors showed the strongest association with complaints across all body regions—a muscle group rarely included in conventional screening.

How this study might affect research, practice or policy

• AF assessment may complement or refine current screening and prevention strategies by identifying neuromuscular impairments not detected by conventional strength methods, supporting a shift towards stability-based screening, pending prospective validation.