Elsevier

Journal of Biomechanics

Volume 47, Issue 1, 3 January 2014, Pages 74-80
Journal of Biomechanics

Local dynamic stability of treadmill walking: Intrasession and week-to-week repeatability

https://doi.org/10.1016/j.jbiomech.2013.10.011Get rights and content

Abstract

Repetitive falls degrade the quality of life of elderly people and of patients suffering of various neurological disorders. In order to prevent falls while walking, one should rely on relevant early indicators of impaired dynamic balance. The local dynamic stability (LDS) represents the sensitivity of gait to small perturbations: divergence exponents (maximal Lyapunov exponents) assess how fast a dynamical system diverges from neighbor points. Although numerous findings attest the validity of LDS as a fall risk index, reliability results are still sparse. The present study explores the intrasession and intersession repeatability of gait LDS using intraclass correlation coefficients (ICC) and standard error of measurement (SEM). Ninety-five healthy individuals performed 5 min treadmill walking in two sessions separated by 9 days. Trunk acceleration was measured with a 3D accelerometer. Three time scales were used to estimate LDS: over 4–10 strides (λ4–10), over one stride (λ1) and over one step (λ0.5). The intrasession repeatability was assessed from three repetitions of either 35 strides or 70 strides taken within the 5 min tests. The intersession repeatability compared the two sessions, which totalized 210 strides. The intrasession ICCs (70-strides estimates/35-strides estimates) were 0.52/0.18 for λ4–10 and 0.84/0.77 for λ1 and λ0.5. The intersession ICCs were around 0.60. The SEM results revealed that λ0.5 measured in medio-lateral direction exhibited the best reliability, sufficient to detect moderate changes at individual level (20%). However, due to the low intersession repeatability, one should average several measurements taken on different days in order to better approximate the true LDS.

Introduction

Falls are a major health issue in older adults (Stevens et al., 2006). Moreover, repetitive falls degrade the quality of life of patients suffering of various neurological disorders (Finlayson et al., 2006, Kerr et al., 2010, Ramnemark et al., 2000). It has been shown that stride-to-stride variability of gait kinematics is related to fall risk (Brach et al., 2007, Verghese et al., 2009). This increased variability may be the result of a decreased ability to optimally control gait. Several analytical methods have been developed to better take into account the nonlinear features of gait variability (Hamacher et al., 2011, Hausdorff, 2007, Stergiou and Decker, 2011). Originally developed to detect deterministic chaos in nonlinear dynamical systems, the maximal Lyapunov exponent has been advocated as a relevant method to assess the degree of sensitivity of gait to small perturbations, or in other words the local dynamic stability (LDS). Computed from various continuously measured kinematic parameters (speed, acceleration, and joint angles), the LDS represents the rate of divergence between neighbor trajectories in a reconstructed state space that describes the dynamics of the system (Dingwell, 2006, Dingwell and Cusumano, 2000, Terrier and Dériaz, 2013).

Some recent experimental and clinical findings support the hypothesis that LDS could be used to predict fall risk (Bruijn et al., 2011, Lockhart and Liu, 2008, McAndrew et al., 2011, Roos and Dingwell, 2010, Toebes et al., 2012). The validity of LDS compared to other indicators has been recently discussed and LDS was found to be one of the best stability indices (Bruijn et al., 2013). However, several issues must be solved before LDS can be routinely used as an early fall-risk predictor in clinical settings. In the first place, information regarding the reliability of LDS measurements is still sparse (Bruijn et al., 2010a, Kang and Dingwell, 2006). Recently, a good intrassession repeatability (ICC between 0.79 and 0.92) has been described by analyzing long duration outdoor walking (200 strides) in young healthy subjects (N=20) (van Schooten et al., 2013). The same study reports lower intersession (between nonconsecutive days) repeatability lying between 0.38 and 0.63. However, more results are needed about the LDS reliability in short indoor walking tests, which are more adapted to patients with diminished walking capabilities (Hilfiker et al., 2013, Terrier et al., 2013).

Other research questions need to be addressed in order to increase the usability of LDS in clinical settings. Two different methods are used to characterize short-term LDS, which is assumed to be the more relevant time scale for assessing gait stability (Bruijn et al., 2011, Roos and Dingwell, 2010): one method computes divergence over the duration of one stride (Manor et al., 2009, McAndrew Young and Dingwell, 2012), and the other one over the duration of one step (Bruijn et al., 2009, Toebes et al., 2012). It is still not known whether one method offers more precision than the other one. In addition, it is known that a minimal length of continuous signal is necessary to assess the maximal Lyapunov exponents (Kang and Dingwell, 2006, Rosenstein et al., 1993). However, the averaging of several estimates of LDS obtained from distinct short-duration signals produces reliable results, at least at group level (Sloot et al., 2011). Thereby two possibilities should be distinguished: (1) Inherent to the calculation method, a too short-duration signal induces very large error on LDS estimates: long-duration walking tests are therefore mandatory; (2) LDS can be precisely assessed from a short-duration signal, but it substantially varies from strides to strides: as a result, the precision of estimate can be increased by averaging the results obtained from several short-duration walking tests.

To address the aforementioned issues, the LDS of 95 healthy adults walking on a treadmill was assessed from trunk acceleration signals. We aimed to answer the following research questions: (1) what is the intrasession (within a 5 min continuous measurement) and intersession (week-to-week) repeatability (i.e. absolute agreement among repetitions) of LDS? (2) How reliable is an estimate of LDS obtained from short-duration walking tests? (3) Does the repeatability of LDS increase with measurement length as expected? (4) Do the two methods that assess short-term LDS exhibit the same repeatability?

Section snippets

Subjects

One hundred healthy subjects (50 males, 50 females) were recruited to participate in the study. They were selected according to their age and sex. Ten males and 10 females for each decade, between 20 and 69 years old, were included. The data of five subjects had to be discarded due to technical issues (age of the discarded participants (yr): 58, 41, 22, 23, 30; 2 males, 3 females). Therefore, the final sample contained 95 participants (48 males, 47 females) whose characteristics were (mean

Results

Regarding the descriptive statistics of LDS (Fig. 2), the results are normally distributed among individuals (Lilliefors test, p>0.05), with some outliers. A substantial difference exists between the estimates obtained from 35 strides and from 70 strides, with a more marked effect in long-term LDS; the relative differences (70 strides−35 strides/35 strides×100) were on average +40% for λ4–10, +6% for λ1, and +8% for λ0.5.

The results of the intrasession repeatability (ICC and SEM) are shown in

Discussion

Referring to the research questions presented in the introduction, the results can be summarized as follows: (1) the intrasession repeatability of gait LDS (ICC, 70-strides estimates) was around 0.50 for long-term LDS and 0.85 for short-term LDS, the intersession repeatability was around 0.6 for both short- and long-term LDS estimated from 210 strides (3×70). (2) Long-term LDS estimated from short-duration measurements (35 strides) exhibited particularly low repeatability (ICC: 0.20), while

Conclusions

To conclude, the following advices should be given for future gait LDS studies. (1) It is recommended to normalize sample length before computing LDS to thwart the trend to higher LDS estimates with longer measurements. (2) Regarding long-term LDS, its low reliability when few strides are analyzed makes the use of a treadmill highly recommended in order to record long duration walking tests. Furthermore, given the limited reliability, its use should be restricted to group-level assessment with

Conflict of interest statement

There are no known conflicts of interest.

Acknowledgments

The authors would like to thank Olivier Dériaz for his valuable support and thoughtful advice. The study was supported by the Swiss accident insurance company SUVA, which is an independent, non-profit company under public law, and by the clinique romande de réadaptation. The IRR (Institute for Research in Rehabilitation) is supported by the State of Valais and the City of Sion.

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