High prevalence of muscle stability deficits in semi-professional football players-Adaptive Force assessment reveals selective impairment of holding capacity with preserved maximal strength: A cross-sectional study.

PURPOSE: Despite considerable research, strength screening has shown limited value for injury prediction. The concept of muscle stability assessed by Adaptive Force (AF) offers a distinct perspective-addressing neuromuscular holding capacity closer to injury-prone motions rather than pushing strength. This study aimed to assess the prevalence and distribution of muscle stability deficits in football players and their discriminative capacity relative to conventional strength parameters.

METHODS

AF and maximal voluntary isometric contraction (MVIC) were measured in 23 male semi-professional football players. Five muscle groups were tested bilaterally (knee extensors/flexors; hip flexors/adductors/abductors). AF was assessed by objectified manual muscle tests with a handheld device recording force and angular velocity. AF parameters, MVIC and the conventional hamstrings-to-quadriceps (H:Q) ratio were compared between stability categories.

RESULTS

Stability deficits were highly prevalent: 83% of players had at least one deficit; 31% of all tested muscle groups were unstable, highest in hip abductors (52%) followed by hamstrings (46%). The maximal holding capacity was selectively impaired in unstable muscles (on average 61% lower than the maximal force output), while MVIC and maximal AF did not differ between stability categories. The ratio of holding capacity to maximal AF (AF-Ratio) was the strongest discriminator (stable vs. unstable: d = 6.62, ω2 = 0.88), largely consistent across muscle groups and uncorrelated with the H:Q ratio. The H:Q ratio failed to discriminate between stable and unstable hamstrings (d = -0.04), whereas the AF-Ratio discriminated strongly (d = 6.43).

CONCLUSION

Muscle instability represents a selective impairment of holding capacity with preserved maximal strength, not detected by MVIC testing. By capturing a distinct neuromuscular function, AF offers a novel mechanistic framework beyond pushing strength. Further studies in larger samples and prospective designs are required to assess the potential utility of stability assessment in clinical practice.

LEVEL OF EVIDENCE: Level III, diagnostic studies.