From strength to stability: muscular holding capacity characterized by Adaptive Force and its potential relevance for musculoskeletal complaints and non-contact injuries

Musculoskeletal complaints and non-contact injuries remain poorly understood; conventional strength assessments show limited predictive value. They omit the function at which injuries occur--holding or braking under load. This is addressed by muscle stability quantified by Adaptive Force (AF)--the capacity to adapt to increasing loads while maintaining position. Available AF data were synthesized to characterize stable and unstable muscle states, evaluate parameter robustness, and explore practical relevance. 481 stable and 307 unstable trials of 112 muscles (elbow/hip flexors, pectoralis major) from six studies were pooled, supplemented by illustrative single cases. Both states were assessed in identical muscles by objectified manual muscle testing. AF parameters were compared by paired t-tests; the difference between states was regressed on five factors (muscle group, tester, sex, age, experiment) in linear mixed-effects models and permutation tests. Robustness was examined with states regrouped by experimental condition, in leave-one-experiment-out, and blinded-tester analyses. Unstable muscles showed reduced maximal isometric AF (-84 N) but higher peak AF (+14 N). Their ratio (AF-Ratio) amounted to ~56% in unstable and ~99% in stable state (dz = 2.7), with hardly any overlap; this was reproduced by condition-based grouping (agreement 97%, {kappa} = 0.94). A pre-defined AF-Ratio threshold (stable [≥]98%) agreed similarly (97%, {kappa} = 0.93). Without pectoralis (n = 3), no factor was associated with AF-Ratio difference; absolute parameters showed partial associations. In most cases, unstable muscles are not weak: they fail to access their full force capacity isometrically but generate it during lengthening. Muscles can switch instantly and reversibly between states in reaction to diverse stimuli, possibly based on shared neural substrates of motor control, somatosensation, and emotion. The functional instability syndrome (FIS) is proposed: muscle instability may destabilize joints and thus be a key factor in complaints and non-contact injuries. AF-Ratio is self-referenced and sensitive, suggesting applicability for screening, prevention, diagnostics, and AF-guided individualized therapy; its clinical value must be evaluated.