title: Are gait variability and stability measures influenced by directional changes?
abstract: BACKGROUND
  Many gait variability and stability measures have been proposed in the literature, with the aim to quantify gait impairment, degree of neuro-motor control and balance disorders in healthy and pathological subjects. These measures are often obtained from lower trunk acceleration data, typically acquired during rectilinear gait, but relevant experimental protocols and data processing techniques lack in standardization. Since directional changes represent an essential aspect of gait, the assessment of their influence on such measures is essential for standardization. In addition, their investigation is needed to evaluate the applicability of these measures in laboratory trials and in daily life activity analysis. A further methodological aspect to be standardized concerns the assessment of the sampling frequency, which could affect stability measures. The aim of the present study was hence to assess if gait variability and stability measures are affected by directional changes, and to evaluate the influence of sampling frequency of trunk acceleration data on the results.
  
  
  METHODS
  Fifty-one healthy young adults performed a 6-minute walk test along a 30 m straight pathway, turning by 180 deg at each end of the pathway. Nine variability and stability measures (Standard deviation, Coefficient of variation, Poincaré plots, maximum Floquet multipliers, short-term Lyapunov exponents, Recurrence quantification analysis, Multiscale entropy, Harmonic ratio and Index of harmonicity) were calculated on stride duration and trunk acceleration data (acquired at 100 Hz and 200 Hz) coming from straight walking windows and from windows including both straight walking and the directional change.
  
  
  RESULTS
  Harmonic ratio was the only measure that resulted to be affected by directional changes and sampling frequency, decreasing with the presence of a directional change task. HR was affected in the AP and V directions for the 200 Hz, but only in AP direction for the 100 Hz group.
  
  
  CONCLUSIONS
  Multiscale entropy, short term Lyapunov exponents and Recurrence quantification analysis were generally not affected by directional changes nor by sampling frequency, and could contribute to the definition of a fall risk index in free-walking conditions.
authors.affiliation: DEI - Department of Electrical, Electronic, and Information Engineering 'Guglielmo Marconi', University of Bologna, Bologna, Italy. f.riva@unibo.it.
authors.forename: Federico
authors.initials: F
authors.lastname: Riva
authors.forename: Eleni
authors.initials: E
authors.lastname: Grimpampi
authors.forename: Claudia
authors.initials: C
authors.lastname: Mazzà
authors.forename: Rita
authors.initials: R
authors.lastname: Stagni
mesh_terms: D000054:Acceleration; D005684:Gait / Q000502:physiology*; D006801:Humans; D004856:Postural Balance / Q000502:physiology; D016138:Walking / Q000502:physiology; D055815:Young Adult