A model of motoneuron behavior and muscle-force generation for sustained isometric contractions

A model for the simulation of motoneuron firing behavior and muscle force during sustained constant-force isometric contractions was developed. It provides a non-linear relationship between the excitation to the motoneuron pool of a muscle and the firing behavior of motor units; it implements muscle mechanical changes induced by fatigue and it comprises a feedback loop to maintain the muscle force at a given target level. We simulated a series of repeated force contractions sustained at 20% MVC with the first dorsal interosseous muscle of the hand and the vastus lateralis muscle of the thigh. The model generates force and firing behaviors which are consistent with experimental findings and underscores the influence of muscle mechanical changes on the control behavior of motor units during sustained contractions. The model predicts the increase of force fluctuation with fatigue in both muscles, likely due to recruitment of high-threshold high-amplitude twitch motor units. Force variability is greater in the first dorsal interosseous muscle than in the vastus lateralis muscle at any time during the contraction series, due to the different electrical and mechanical properties of the muscles.