The total dimensions of the parameter space is 2*16 + 16*36 = 608 for Stage II. The values for the weight parameters are constrained as [0, 1]. The optimal values for the muscle parameters in Stage I (standard case of walking forward) are used at this stage. In practice, we found that it is difficult for the solution to converge given the high dimensions of the parameter space. To ensure the successful convergence, we initially sample the candidates from a narrower value range of [−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha {\theta }_{max}^{s}$$\end{document}αθmaxs, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha {\theta }_{max}^{s}$$\end{document}αθmaxs] and [−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha {\theta }_{max}^{t}$$\end{document}αθmaxt, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha {\theta }_{max}^{t}$$\end{document}αθmaxt], where α ∈ . α starts from value of 0 and progressively increase to 1, with a step of 0.2, so that the initial value stays sufficiently close to the optimal value. Figure 5(b) shows the optimization progress for Stage II. The result shows that when the exploration range is small (for cases when α = 0.2 or 0.4), the optimization converges fast and smooth, while for other cases the optimization requires more iterations to converge.