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

Musculoskeletal complaints and non-contact injuries remain poorly understood. The pushing maximum strength usually examined does not represent the muscle action at which injuries occur: when muscles hold or brake under load. This study focuses on the new approach of muscle stability (Adaptive Force; AF) - the capacity to adapt to increasing external loads. To obtain a comprehensive picture of this unique muscle function, its special features and to demonstrate practical applications, the available AF data are synthesized and re-evaluated supplemented by single cases. AF data from 481 stable and 307 unstable trials of 112 muscles (elbow/hip flexors, pectoralis major) from six studies were included. All studies used an objectified manual muscle test to assess AF in both conditions (stable/unstable) in identical muscles. The respective conditions were elicited by various interventions. The maximum AF (AFmax; peak force), the maximum isometric AF (AFisomax; holding capacity), AF at onset of oscillations (AFosc) and their ratios were analyzed by a mixed ANOVA between stable and unstable muscles including the factors muscle, tester, participant's sex and experiment. Unstable muscles had highly significant reduced AFisomax, but higher AFmax and AFosc. The relative holding capacity was ~56% in unstable and ~99% in stable state. The absolute parameters showed an interaction effect for "tester" - probably a design artifact. The relative AF parameters were robust to all factors. The results indicate muscles are not weak in response to impairing stimuli (higher AFmax during lengthening), but unstable: they are unable to generate their full force in static position and yield at clearly submaximal forces. This most likely destabilizes joints under load which could be a key factor in injury and complaint development. Muscles can immediately switch between stability and instability in response to various stimuli. This makes the AF a valuable tool for personalized diagnostics and therapy derivation. We suggest shifting the focus from strength to stability and from load to resilience. The robustness of the relative holding capacity makes it independent of reference values, thus offers the properties of a gold standard. Further studies are required to validate the approach.