The unexpected observation of fractal variability in human gait raises a number of questions. Does the fractal gait rhythm exist only during walking at one's normal pace, or does it occur at slower and faster walking rates as well? Does the influence of one stride interval on another continue beyond a few hundred strides, or do the long-range correlations eventually break down during an extended walk? To answer these questions, we asked young healthy men to walk for one hour at their usual rate as well as at slow and fast paces around an outdoor track . A representative example of the effect of walking speed on the stride interval fluctuations and long-range correlations is shown in Fig. 10. Remarkably, the locomotor control system maintains the stride interval at an almost constant level throughout the one hour of walking at all three walking speeds. Nevertheless, both the DFA and power spectral analysis indicate that the subtle variations in walking rhythm are not random. Instead, the time series exhibits long-range correlations at all three walking rates. The fractal scaling indices and remained fairly constant despite substantial changes in walking velocity and mean stride interval. For all subjects tested at all three walking rates, the stride interval time series displayed long-range, fractal correlations over thousands of steps. These findings indicate that the fractal dynamics of walking rhythm are normally quite robust and appear to be intrinsic to the locomotor system.
Figure: An example of the effects of walking rate on stride interval dynamics. A: One hour stride interval time series for slow (1.0 m/sec), normal (1.3 m/sec), and fast (1.7 m/sec) walking rates. Note the breakdown of the temporal structure with random re-ordering of the fast walking trial data points, even though this shuffled time series has the same mean and standard deviation as the original, fast time series. B and C: Fluctuation and power spectrum analyses confirm the presence of long-range correlations at all three walking speeds and their absence after random shuffling of the data points. Adapted from .