The activation process converts the spiking trains from the neural system into the activation signal on the muscle. This is modelled as a first-order differential equation:6\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${a}_{t+1}=({v}_{t}-{a}_{t}){\rm{\Delta }}t+{a}_{t}$$\end{document}at+1=(vt−at)Δt+atwhere at is the activation signal at time t, vt is the excitation from the motor neuron (Equation 2). Δt is the stepsize.