Transient gait instability during the initial phase of the 6-minute walk test in people with stroke: A nonlinear analysis using inertial measurement units

In people with stroke (PwS), reduced gait speed and trunk acceleration changes in the latter half of the 6-minute walk test (6MWT) have been reported as fatigue-related, but temporal changes in motor control remain understudied. Clinicians often observe instability immediately after gait onset, yet quantitative evidence is scarce. This study used inertial measurement units (IMUs) and nonlinear analysis to examine fatigue-related changes and post-onset instability during the 6MWT in PwS.

In this cross-sectional study, 19 PwS and 19 age-matched controls completed the 6MWT wearing IMUs at the L5 vertebra and both ankles. Spatiotemporal parameters and stability indices (root mean square, harmonic ratio, sample entropy, short-term Lyapunov exponent [sLE]) were calculated. Linear mixed-effects models (LMMs) analyzed the 6-minute period both continuously and by comparing an initial phase (first 60 s) with the subsequent phase. Bayesian and sensitivity analyses were also performed.

Continuous-time LMMs showed reduced cadence and increased double-support ratio in PwS. Among stability indices, only mediolateral sLE (sLE-ML) showed a significant group × phase interaction: increasing in the initial phase in PwS but not in controls. Bayesian and sensitivity analyses confirmed this finding was robust and specific to the 60-second cutoff. Initial-phase sLE-ML correlated negatively with Fugl-Meyer lower-extremity scores.

PwS showed fatigue-related gait changes during the 6MWT, while mediolateral local dynamic stability transiently declined during the first 60 s before stabilizing. This instability was associated with motor paralysis, suggesting trunk IMU-based sLE-ML may reveal motor control deficits not captured by walking distance or speed.