The effect of walking speed on local dynamic stability is sensitive to calculation methods

Local dynamic stability has been assessed by the short-term local divergence exponent (λS), which quantifies the average rate of logarithmic divergence of infinitesimally close trajectories in state space. Both increased and decreased local dynamic stability at faster walking speeds have been reported. This might pertain to methodological differences in calculating λS. Therefore, the aim was to test if different calculation methods would induce different effects of walking speed on local dynamic stability. Ten young healthy participants walked on a treadmill at five speeds (60%, 80%, 100%, 120% and 140% of preferred walking speed) for 3 min each, while upper body accelerations in three directions were sampled. From these time-series, λS was calculated by three different methods using: (a) a fixed time interval and expressed as logarithmic divergence per stride-time (λS−a), (b) a fixed number of strides and expressed as logarithmic divergence per time (λS−b) and (c) a fixed number of strides and expressed as logarithmic divergence per stride-time (λS−c). Mean preferred walking speed was 1.16±0.09 m/s. There was only a minor effect of walking speed on λS−a. λS−b increased with increasing walking speed indicating decreased local dynamic stability at faster walking speeds, whereas λS−c decreased with increasing walking speed indicating increased local dynamic stability at faster walking speeds. Thus, the effect of walking speed on calculated local dynamic stability was significantly different between methods used to calculate local dynamic stability. Therefore, inferences and comparisons of studies employing λS should be made with careful consideration of the calculation method.