Application of a force-velocity-endurance model to in situ muscle evaluation in mouse model.

Introduction. The muscular contractile capacity is essential for human and animal movement and locomotion.Owing to their molecular structure, striated skeletal muscle cells produce a force that is a function oftheir rate of shortening. When the force production capacity of the neuromuscular system on an isolated muscleis explored, this relationship can be formulated mathematically by a rational function F(V ) [1]. Moreover, theintensity of this force decreases as a function of the duration F(t), and converges towards a characteristic criticalintensity [2]. The interaction between these two fundamental relationships has been studied only throughindependent comparisons. Considering them as two projections of a single force-velocity-time relationship [3]would make it possible to describe the force production capacities and their interactions in their entirety. Theaim of this conceptual framework is to use an integrative model to unify a new Force-Velocity-Endurance (FVE)relationship that can define muscle properties and fatigability.Methods. This new theoretical framework proposes a model that is a function of two variables (time t andvelocity V ), and seven major parameters to describe muscle properties: initial fatigue-free capacities (initialforce (F0i ), initial velocity (V0i ), initial curvature (Ci) coefficients), critical capacities (F0c , V0c , Cc) and acharacteristic time τ corresponding to the rate of capacity decline. To measure these parameters, a new 3 minall-out test with velocity variation was developed on an isokinetic ergometer (Aurora Scientific 300C) to scanthe maximum capacities of the FVE surface, on both the velocity and time dimensions. The model was testedon 12 wild-type mice (six males and sixfemales) for the tibialis anterior (TA) and gastrocnemius (GA) muscles.Results and discussion. The goodness of fit of the model from the experimental data was excellent for allmuscles (r² > 0.97). The proposed model revealed significant differences between the TA and GA muscle groups.For males, F0i , F0c and τ were higher for GA compared to TA. Considering females, F0i was significantly higherfor GA but relative F0c was higher for TA. This new model also revealed differences in the muscle capacity asa function of sexual dimorphism. For instance, F0i was sgnificanlty higher for males compared to female onlyfor TA but not GA muscles.Conclusions and perspectives. These results demonstrate that it is possible to determine the individualparameters of the proposed model (F0i , V0i , Ci, F0c , V0c , Cc and τ ) from the experimental data obtained fromthe proposed all-out test. Validating the existence of a universal FVE relationship and its theoretical foundationswould open a new conceptual framework for improving our understanding of muscle function. Although thisproject is fundamental, the practical applications resulting from this new framework could be numerous, suchas functional analysis of gene therapy in myology or the impairment of neuromuscular function in patients