From strength to stability: Proposal for a changed perspective on the mechanisms of musculoskeletal complaints and non-contact injuries - a synthesizing reconsideration of previously collected data on muscular holding capacity

Abstract

BackgroundMusculoskeletal complaints and non-contact injuries remain poorly understood. Conventionally assessed pushing strength shows limited association and predictive value. As it does not represent the muscle function at which injuries occur--holding or braking under load--novel approaches are needed. This study focuses on muscle stability assessed by Adaptive Force (AF)--the capacity to adapt to increasing external loads while maintaining a static position--and aimed to characterize stable and unstable muscles, evaluate the robustness of key parameters, and derive practical implications. MethodsAF data from 481 stable and 307 unstable trials of 112 muscles (elbow/hip flexors, pectoralis major) from six studies were synthesized and re-evaluated, supplemented by single cases. AF was assessed using objectified manual muscle testing in both states (stable/unstable) with identical muscles; conditions were elicited by various interventions. AF parameters (e.g., AFmax: peak AF; AFisomax: max. isometric AF and their ratio) were compared using paired t-tests and mixed ANOVA with factors muscle, tester, participant sex, and experiment. ResultsUnstable muscles showed significantly reduced AFisomax but higher AFmax. AF-Ratio averaged [~]56% in unstable and [~]99% in stable state (d = 2.7). While absolute parameters were influenced by the tester--likely confounded by the non-balanced tester-participant constellation--relative AF parameters were robust to all factors. Unstable muscles are not weak: they fail to access their full force capacity isometrically but generate it only during lengthening. ConclusionShared neural substrates for motor control, somatosensation, and emotion provide a plausible neuroanatomical basis for the observed immediate and reversible switching between stability and instability. The concept of functional instability syndrome (FIS) is proposed: muscle instability likely destabilizes joints, representing a potential key factor in the development of complaints and injuries. The AF-Ratio is self-referenced and sensitive, suggesting broad applicability for screening, prevention, diagnostics, and individualized therapy derivation. Further studies are required for validation.