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 (λ_S), 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 λ_S. 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, λ_S was calculated by three different methods using: (a) a fixed time interval and expressed as logarithmic divergence per stride-time (λ_S−a), (b) a fixed number of strides and expressed as logarithmic divergence per time (λ_S−b) and (c) a fixed number of strides and expressed as logarithmic divergence per stride-time (λ_S−c). Mean preferred walking speed was 1.16±0.09 m/s. There was only a minor effect of walking speed on λ_S−a. λ_S−b increased with increasing walking speed indicating decreased local dynamic stability at faster walking speeds, whereas λ_S−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 λ_S should be made with careful consideration of the calculation method.