The effect of run duration, gait variable and Lyapunov exponent algorithm on the inter-session reliability of local dynamic stability in healthy young people

Local dynamic stability (LDS) of gait has been used to differentiate between healthy and injured populations, establishing its potential as an indicator of healthy gait and a new objective measure to assess gait function following injury. For LDS to be a reliable assessment tool of healthy gait progression during rehabilitation, it must provide consistent and sensitive inter-session measures. Methodological factors such as trial duration, gait variable, and Lyapunov Exponent (LyE) algorithm can influence LDS estimation and its reliability. Young people are a high-risk population for sport-related injuries, and running is a key activity during rehabilitation and is regularly assessed. Therefore, the effects of run duration, gait variable, and LyE algorithm choice on the reliability and sensitivity of inter-session LDS measures in young people were investigated. Sixteen healthy participants ran on a treadmill on two separate sessions (difference of 7 ± 5 days). LDS was calculated using both the Rosenstein and Wolf algorithm for durations of 1-, 2-, 3-, 4- and 5-min of knee flexion angle and medio-lateral acceleration of the pelvis and thorax from each session. The relative and absolute reliability between sessions was calculated using the intraclass correlation coefficient and standard error of measurement. The sensitivity of inter-session LDS change was quantified by the minimal detectable change. Results showed that longer run durations produced higher relative reliability and a minimum run duration of 4 min is recommended to achieve moderate-to-good inter-session reliability across all gait variables and LyE algorithms. However, shorter durations of 2-3 min may still be sufficient when using medio-lateral pelvis acceleration or knee flexion angle, particularly with the Rosenstein algorithm, which also improves sensitivity to change. These findings provide practical guidance for methodological choices when calculating LDS in young people during running and support their potential use as reliable tools for monitoring gait function and tracking rehabilitation progress in young people following injury.