The aim of this study is to implement and integrate a novel and versatile muscle fatigue model in an existing multibody formulation with natural coordinates. The equations of motion of the formulation are rearranged to deal with muscle actuators as externally concentrated forces instead of being considered as kinematic muscle actuator drivers. A muscle fatigue model, originally developed to predict the endurance time of the main body joints, is included in the multibody formulation. The model stems from the compartment theory and uses a bounded controller to rule the rate of motor units changing between the three compartments (resting, activated, and fatigued) that made up the model. The resulting formulation is applied to two example cases involving the human upper extremity, with redundant muscles, to evaluate its robustness and accuracy. Results showed that the proposed methodologies are a valid approach for the calculation of the redundant muscle forces in the presence of muscular fatigue.
New dynamic muscle fatigue model to limit musculo-skeletal disorder