Figure 2.

Powering the kneed walking model with push-off. (a) Kinematics are illustrated with the angle of each segment measured anticlockwise with respect to vertical, plotted for one step (heelstrike to opposite heelstrike). Stronger push-off produces longer steps, as heelstrike occurs with a larger angle between the swing and stance legs. The knee angle is equal to the difference between the swing thigh and shank angles, with knee stop occurring when these angles meet. (b) Gait parameters (step length, step frequency and walking speed) as a function of push-off impulse magnitude. A single vertical axis shows dimensionless values for all parameters; a separate dimensional scale for human legs is provided for each parameter. Greater push-off results in longer steps and faster walking speed, and slightly higher step frequency. These effects are roughly similar to those of a straight-legged model (dashed lines; Kuo 2002), but over a smaller range of speeds. All dimensionless quantities are determined using body mass, leg length and gravitational acceleration as base units for normalization. For example, one dimensionless unit in the vertical axis corresponds to a speed of 3.1 m s⁻¹, step length 1 m or step frequency 3.1 Hz for a typical human with 1 m leg length. The fastest speed of the kneed model (solid lines) is equivalent to 0.74 m s⁻¹ with step length 0.62 m and frequency 1.2 Hz. (c) Work per step as a function of push-off impulse. In the straight-legged model (dashed lines), the only energy loss is in heelstrike. In the model with knees (solid lines), energy is lost at knee stop as well as heelstrike, their sum equalling the energy gained at push-off. The overall collision losses of both straight-legged and kneed models are the same for a given push-off.