Federico Riva

Estimating fall risk with inertial sensors using gait stability measures that do not require step detection

Falls have major consequences both at societal (health-care and economy) and individual (physical and psychological) levels. Questionnaires to assess fall risk are commonly used in the clinic, but their predictive value is limited. Objective methods, suitable for clinical application, are hence needed to obtain a quantitative assessment of individual fall risk. Falls in older adults often occur during walking and trunk position is known to play a critical role in balance control. Therefore, analysis of trunk kinematics during gait could present a viable approach to the development of such methods. In this study, nonlinear measures such as harmonic ratio (HR), index of harmonicity (IH), multiscale entropy (MSE) and recurrence quantification analysis (RQA) of trunk accelerations were calculated. These measures are not dependent on step detection, a potentially critical source of error. The aim of the present study was to investigate the association between the aforementioned measures and fall history in a large sample of subjects (42 fallers and 89 non - fallers) aged 50 or older. Univariate associations with fall history were found for MSE and RQA parameters in the AP direction; the best classification results were obtained for MSE with scale factor τ = 2 and for maximum length of diagonals in RQA (72.5% and 71% correct classifications, respectively). MSE and RQA were found to be positively associated with fall history and could hence represent useful tools in the identification of subjects for fall prevention programs.

Gait variability and stability measures: Minimum number of strides and within-session reliability

BACKGROUND Several methods are proposed in the literature for the quantification of gait variability/stability from trunk accelerations. Since outputs can be influenced by implementation differences, reliability assessment and standardization of implementation parameters are still an issue. The aim of this study is to assess the minimum number of required strides and the within-session reliability of 11 variability/stability measures. METHOD Ten healthy participants walked in a straight line at self-selected speed wearing two synchronized tri-axial Inertial Measurement Units. Five variability measures were calculated based on stride times namely Standard deviation, Coefficient of variation, Inconsistency of variance, Nonstationary index and Poincaré plot. Six stability measures were calculated based on trunk accelerations namely Maximum Floquet multipliers, Short term/long term Lyapunov exponents, Recurrence quantification analysis, Multiscale entropy, Harmonic ratio and Index of harmonicity. The required minimum number of strides and the within-session reliability for each measure were obtained based on the interquartile range/mean ratio. Measures were classified in five categories (namely excellent, good, average, poor, and very poor) based on their reliability. RESULTS The number of strides required to obtain a reliable measure was generally larger than those conventionally used. Variability measures showed average to poor reliability, while stability measures ranged from excellent to very poor reliability. CONCLUSION Recurrence quantification analysis and multiscale entropy of trunk accelerations showed excellent reliability and a reasonable number of required strides. Based on these results, these measures should be taken into consideration in the assessment of fall risk.

Measures of gait stability: performance on adults and toddlers at the beginning of independent walking

BackgroundQuantifying gait stability is a topic of high relevance and a number of possible measures have been proposed. The problem in validating these methods is the necessity to identify a-priori unstable individuals. Since proposed methods do not make any assumption on the characteristics of the subjects, the aim of the present study was to test the performance of gait stability measures on individuals whose gait is a-priori assumed unstable: toddlers at the onset of independent walking.MethodsTen toddlers, ten adults and ten elderly subjects were included in the study. Data from toddlers were acquired longitudinally over a 6-month period to test if the methods detected the increase in gait stability with experience, and if they could differentiate between toddlers and young adults. Data from elderly subjects were expected to indicate a stability value in between the other two groups. Accelerations and angular velocities of the trunk and of the leg were measured using two tri-axial inertial sensors. The following methods for quantifying gait stability were applied: stride time variability, Poincaré plots, harmonic ratio, short term Lyapunov exponents, maximum Floquet multipliers, recurrence quantification analysis and multiscale entropy. An unpaired t-test (level of significance of 5%) was performed on the toddlers and the young adults for each method and, for toddlers, for each evaluated stage of gait development.ResultsMethods for discerning between the toddler and the adult groups were: stride time variability, Poincaré plots, harmonic ratio, short term Lyapunov exponents (state space composed by the three linear accelerations of the trunk), recurrence quantification analysis and multiscale entropy (when applied on the vertical or on the antero-posterior L5 accelerations).ConclusionsResults suggested that harmonic ratio and recurrence quantification analysis better discern gait stability in the analyzed subjects, differentiating not only between unstable toddlers and stable healthy adults, but also evidencing the expected trend of the toddlers towards a higher stability with walking experience, and indicating elderly subjects as stable as or less stable than young adults.

Orbital stability analysis in biomechanics: a systematic review of a nonlinear technique to detect instability of motor tasks.

Falls represent a heavy economic and clinical burden on society. The identification of individual chronic characteristics associated with falling is of fundamental importance for the clinicians; in particular, the stability of daily motor tasks is one of the main factors that the clinicians look for during assessment procedures. Various methods for the assessment of stability in human movement are present in literature, and methods coming from stability analysis of nonlinear dynamic systems applied to biomechanics recently showed promise. One of these techniques is orbital stability analysis via Floquet multipliers. This method allows to measure orbital stability of periodic nonlinear dynamic systems and it seems a promising approach for the definition of a reliable motor stability index, taking into account for the whole task cycle dynamics. Despite the premises, its use in the assessment of fall risk has been deemed controversial. The aim of this systematic review was therefore to provide a critical evaluation of the literature on the topic of applications of orbital stability analysis in biomechanics, with particular focus to methodologic aspects. Four electronic databases have been searched for articles relative to the topic; 23 articles were selected for review. Quality of the studies present in literature has been assessed with a customised quality assessment tool. Overall quality of the literature in the field was found to be high. The most critical aspect was found to be the lack of uniformity in the implementation of the analysis to biomechanical time series, particularly in the choice of state space and number of cycles to include in the analysis.

Influence of Input Parameters on Dynamic Orbital Stability of Walking: In-Silico and Experimental Evaluation

Many measures aiming to assess the stability of human motion have been proposed in the literature, but still there is no commonly accepted way to define or quantify locomotor stability. Among these measures, orbital stability analysis via Floquet multipliers is still under debate. Some of the controversies concerning the use of this technique could lie in the absence of a standard implementation. The aim of this study was to analyse the influence of i) experimental measurement noise, ii) variables selected for the construction of the state space, and iii) number of analysed cycles on the outputs of orbital stability applied to walking. The analysis was performed on a 2-dimensional 5-link walking model and on a sample of 10 subjects performing long over-ground walks. Noise resulting from stereophotogrammetric and accelerometric measurement systems was simulated in the in-silico analysis. Maximum Floquet multipliers resulted to be affected by both number of analysed strides and state space composition. The effect of experimental noise was found to be slightly more potentially critical when analysing stereophotogrammetric data then when dealing with acceleration data. Experimental and model results were comparable in terms of overall trend, but a difference was found in the influence of the number of analysed cycles.

MEASURES OF GAIT STABILITY: EVALUATION OF THE PROPOSED METHODS COMPARING ADULTS WITH INFANTS AT THE BEGINNING OF INDEPENDENT WALKING

Quantifying stability of locomotion is a more and more interesting question in research. A number of possible measures have been proposed in literature, including methods coming from dynamical systems analysis. The problem in validating these methods is the necessity to identify a-priori instable individuals. Recently Van Schooten et al [2006] used galvanic vestibular stimulation (GVS) to alter the stability of young adults in order to measure this decreased stability, even though the subjects did not fall. The aim of the present study is to test measures of variability, local stability and orbital stability of trunk kinematics on individuals who experience more than one fall every day, that is infants at the onset of independent walking, whose gait is a-priori assumed unstable.

COMPARISON BETWEEN MODEL AND EXPERIMENTAL ORBITAL STABILITY ANALYSIS OF GAIT

Falls in the elderly represent a major community and public health problem [Heinrich, 2010]. Stability of locomotion is one of the more important factors for the clinicians to look for during assessment procedure [Hurmuzlu, 1994]. Many stability indices have been proposed for clinical application; some authors applied orbital stability analysis (via Maximum Floquet multipliers, maxFM) to biomechanics with promising results [Dingwell, 2007], but still the use of this technique in the assessment of fall risk has been deemed controversial [Hamacher, 2011]. The possibility to obtain reliable orbital stability measures from a light portable device such as a single inertial sensor could fasten the acquisition procedure, but still it is not clear how experimental characteristics affect the results. Simulations represent a powerful tool to test reliability of results. The aim of this study was to compare orbital stability results coming from acceleration data of a stable walking model to experimental results obtained with the same implementation.

Is There a Relationship between Clinical Rating Scales and Instrumental Gait Stability Measures

Clinical assessment of walking deficits in post-stroke patients is typically based on clinical rating scales. This approach is however highly dependent on the clinician’s subjective judgement. Objective tools for the quantification of locomotor function are needed, and instrumental measures of gait stability could represent a promising solution. The integration of the two approaches could lead to a more reliable quantification of locomotor deficits in post-stroke patients, and improving the understanding of the physiological correlate of gait stability measures. The aim of the present study was to assess the association between instrumental stability measures based on trunk accelerations during gait and clinical rating scales in a sample of post-acute stroke subjects.