What biological mechanisms are necessary to generate this fractal gait rhythm? To further investigate this question, we asked subjects to walk in time to a metronome that was set to each subject's normal stride interval. The purpose of this test was to help characterize the biological ``clock'' that controls locomotion. A breakdown of long-range correlations during metronomic walking would suggest that some locomotor pacesetter above the level of the spinal cord (supra-spinal mechanism) is essential in generating this scale-free behavior or, at least, that centrally-mediated entrainment of the clock can ``overcome'' long-range correlations generated peripherally. Alternatively, persistence of the long-range correlations during metronomic walking might imply that the scaling property is unrelated to central influences and that it results either from neural circuits at or below the level of the spinal cord, or from peripheral feedback influences. The results during metronomic walking were consistently different from those obtained when the walking rhythm was unconstrained. During metronomically-paced walking, fluctuations in the stride interval were always random and failed to exhibit long-range, fractal correlations.
Metronomic walking and normal, unconstrained walking both utilize the same mechanical systems, the same force generators, and the same feedback networks. The breakdown of fractal, long-range correlations during metronomically-paced walking demonstrates that influences above the spinal cord (a metronome) can override the normally present long-range correlations. This finding is of interest because it demonstrates that supra-spinal nervous system control is critical in generating the robust, fractal pattern in normal human gait.