Flowchart of the optimal FRO stiffness selection procedure. After sorting the different stiffness configurations based on peak knee extension angle in single support (KE-Kx), absolute differences in peak KE will be calculated. KE-K2 and/or KE-K3 will be excluded if this difference is more than five degrees. Otherwise, the remaining configurations will be sorted by net non-dimensional walking energy cost (NNC-Kx) (this can be either two or three remaining configurations). The stiffness that results in the lowest walking energy cost will be selected as the subject’s optimal FRO. K1, K2 and K3 = rigid, stiff, or flexible FRO stiffness configurations; KOPT = subject’s optimal FRO stiffness; KE = knee extension angle; KE-Kx = stiffness sorted by KE; NN_cost%SMC = net non-dimensional walking energy cost relative to speed matched controls; NNC-Kx = stiffness sorted by NN_cost%SMC; SS = single support.