  Synchronizing steps with an external auditory stimulus (rhythmic auditory
cueing (RAC) enhances gait recovery in neurological disorders. The activation
of specific sensory-motor processes, which may partially replace impaired
neural pathways, is likely the cause of the observed benefits. Nonlinear
indexes, such as scaling exponents and Lyapunov exponents, have been proposed
to characterize RAC effects. The maximal Lyapunov exponent estimates the degree
of resilience of gait control to small perturbations, i.e. the local dynamic
stability (LDS). The objective of the present study was to assess to what
extent RAC influences gait LDS, and to compare this effect with that on scaling
exponents. Twenty healthy subjects performed 6x5min walking trials on an
instrumented treadmill at three different speeds. Freely chosen walking
cadences were measured during the first three trials and then imposed
accordingly in the last three trials with a metronome. The 2D trajectory of the
center of pressure on the treadmill was recorded. From the antero-posterior and
the medio-lateral signals, both long-term and short-term LDS were computed.
Long-term LDS was strongly enhanced (relative change +47%), with significant
change in every direction and speed. The average change in short-term LDS was
smaller (+3%), with a more marked effect at low speed (+5%). RAC substantially
modified the fluctuation dynamics of the center of the pressure trajectory. We
also observed that both LDS and fractal dynamics (scaling exponents) responded
similarly to RAC. Thus, both scaling exponents and LDS are responsive to
sensory-motor synchronizing processes that RAC activates, and may constitute
relevant indexes for evaluating gait variability in cued walking. Finally, the
more locally stable gait pattern could be an indication of a lower fall risk,
which may be an advantage to patients of RAC therapies.
