Fractal analysis of gait in people with Parkinson's disease: three minutes is not enough

Background: The fractal dynamics of gait variability in people with Parkinson's disease has been studied by applying the detrended fluctuation analysis (DFA) to short time series ( < 200 strides). However, DFA is sensitive to time series length, and it is unclear if DFA results from short time series are reliable and if they reflect the fractal dynamics of longer time series. Research question: Is DFA reliable when applied to short time series? Methods: We applied DFA to stride time series from five 3-min trials and one 15-min trial in 12 people with Parkinson's disease, 14 healthy older adults and 14 healthy young adults walking overground. Within each group, intraclass correlations (ICC 3,1) were performed to assess the reliability of i) the five 3-min trials together, ii) each 3-min trials to the 15-min trial, and iii) the first 150 strides from the 15-min trial to the full 15-min trial. Results: Our three main findings are that 1) stride time alpha-DFA values are not consistent from trial-to-trial for short stride time series, 2) stride time alpha-DFA values from each 3-min trials are not consistent when compared to stride time alpha-DFA values from a 15-min trial, and 3) stride time alpha-DFA values from the first 150 strides of the 15min trial are not consistent when compared to alpha-DFA values from the full 15-min trial. Significance: Our results confirm that alpha-DFA values from 3-min walking trials are not reliable, and that they do not reflect the scale invariant properties of longer time series. This suggests that previous studies assessing the fractal dynamics of gait variability from about 3-min walking must be interpreted with caution. A major clinical implication is that DFA cannot be used to study gait in people unable to perform 500 strides continuously.