  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 5min. 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 to 10 strides ({\lambda}4-10), over
one stride ({\lambda}1) and over one step ({\lambda}0.5). The intrasession
repeatability was assessed from three repetitions of either 35 strides or 70
strides taken within the 5min 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 {\lambda}4-10
and 0.84/0.77 for {\lambda}1 and {\lambda}0.5. The intersession ICCs were
around 0.60. The SEM results revealed that {\lambda}0.5 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.
