Figure 7.

Effect of a bi-articular spring on kneed walking. (a) A bi-articular spring speeds the step, causing both knee stop and heelstrike to occur earlier (parameters: $\hat{P}=0.1$, r_kh=0.35). (b) Dimensionless gait parameters (step length, step frequency and walking speed) as a function of bi-articular spring stiffness. Increasing the spring stiffness results in higher step frequencies, shorter steps and greater walking speeds. (c) Contour plot of speed as a function of spring knee-to-hip moment arm ratio r_kh and stiffness k_kh. There is a minimum ratio needed to avoid stumbling (ST). Above that minimum, there are two regions with successful gaits that all walk faster with increasing stiffness. High ratios lead to foot scuffing, whereas intermediate ratios do not. These gaits are separated by a narrow range of ratios that yield no gaits due to the NKS failure. The gaits between the ST and NKS failures are most human-like, with knee stop occurring late in swing (parameter: $\hat{P}=0.1$). (d) Contour plot as a function of stiffness and push-off impulse, keeping moment arm ratio constant (r_kh=0.35). Here, walking speed can be increased by either stronger push-off impulses or stiffer springs, without causing foot scuffing. At higher speeds, the model stumbles with insufficient spring stiffness.