A theoretical approach for modeling peripheral muscle fatigue and recovery

A three-compartment model is presented to describe muscle activation, fatigue, and recovery under a variety of loading conditions. Muscle is considered to be in one of three states: resting (MR), activated (MA), or fatigued (MF). A bounded proportional controller represents muscle activation–deactivation, the transfer between MR and MA. The fatigue and recovery rates determine the transfer to/from MF state. The model qualitatively demonstrates empirically based fatigue behavior, known as Rohmert's curves, with isometric loading conditions. An expanded version of the model utilizes the properties of three muscle fiber types and a last-in-first-out stack mechanism to represent the known muscle recruitment hierarchy. Additionally, a novel yet practical approach is introduced to quantitatively evaluate task-related muscle fatigue for complex and/or dynamic movements at the joint level, encompassing the nonlinear influences of joint angle and velocity. This approach may have potential for digital human modeling, ergonomics, and other real-time applications due to its computational efficiency.