Marmelat Vivien, Meidinger Ryan L
Department of Biomechanics, University of Nebraska at Omaha, Omaha, Nebraska, 68184, USA.
Gait Posture. 2019 May;70:229-234. doi: 10.1016/j.gaitpost.2019.02.023. Epub 2019 Feb 26.
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.
Is DFA reliable when applied to short time series?
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.
Our three main findings are that 1) stride time α-DFA values are not consistent from trial-to-trial for short stride time series, 2) stride time α-DFA values from each 3-min trials are not consistent when compared to stride time α-DFA values from a 15-min trial, and 3) stride time α-DFA values from the first 150 strides of the 15-min trial are not consistent when compared to α-DFA values from the full 15-min trial.
Our results confirm that α-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.
通过对短时间序列（<200步）应用去趋势波动分析（DFA）来研究帕金森病患者步态变异性的分形动力学。然而，DFA对时间序列长度敏感，尚不清楚短时间序列的DFA结果是否可靠，以及它们是否反映了长时间序列的分形动力学。
将DFA应用于短时间序列时是否可靠？
我们对12名帕金森病患者、14名健康老年人和14名健康年轻人在地面行走时的五步时间序列进行了DFA分析，这些序列来自五次3分钟试验和一次15分钟试验。在每组中，进行组内相关分析（ICC 3,1）以评估以下方面的可靠性：i）五次3分钟试验合在一起；ii）每次3分钟试验与15分钟试验；iii）15分钟试验的前150步与整个15分钟试验。
我们的三个主要发现是：1）对于短步幅时间序列，每次试验的步幅时间α-DFA值不一致；2）与15分钟试验的步幅时间α-DFA值相比，每次3分钟试验的步幅时间α-DFA值不一致；3）与15分钟试验的完整α-DFA值相比，15分钟试验的前150步的步幅时间α-DFA值不一致。
我们的结果证实，3分钟步行试验的α-DFA值不可靠，且不能反映较长时间序列的尺度不变特性。这表明，先前评估约3分钟步行的步态变异性分形动力学的研究必须谨慎解读。一个主要的临床意义是，DFA不能用于研究无法连续完成500步的人的步态。
