Figure 4.

Effect of a knee spring on dynamic walking gait. (a) A uni-articular knee spring results in earlier knee stop, less knee flexion and more hip flexion (parameter: $\hat{P}=0.10$). (b) Dimensionless gait parameters as a function of knee spring stiffness. Increasing knee spring stiffness causes small changes in gait characteristics; step length increases, step frequency decreases and walking speed decreases very slightly. As knee stiffness continues to increase, gaits asymptotically approach those of the straight-legged model (filled circles). (c) Contour plot of walking speed (labelled at top) as a function of push-off impulse and spring stiffness. There are two regions where there are no gaits (darker shaded areas): low push-off impulses result in the WP failure and low knee stiffnesses result in the stumbling (ST) failure. There are successful gaits for moderately high knee stiffness and push-off, with walking speed increasing with push-off. Increasing knee stiffness causes knee stop to occur earlier, eventually leading to foot scuffing, as delineated by the mid-stance knee stop (MSKS) boundary (where knee stop occurs as the two legs pass each other). Still faster gaits with high knee stiffness become unstable. Stable gaits without foot scuffing (unshaded areas) are nonetheless possible across a range of speeds, as long as knee stiffness is chosen appropriately. For reference, the limit cycles in (b) are roughly equivalent to a human walking at a relatively slow speed of 0.65 m s⁻¹, step frequency of 1.0 Hz and step length of 0.63 m.