BACKGROUND: Dual-task walking is used as a treatment, for gait assessment, and as an outcome measure in Parkinson's disease (PD). Movement smoothness, i.e. its non-intermittency, is a movement cardinal feature. This study assesses whether dual-tasking reduces gait smoothness in PD alongside reducing speed, one of its well-known effects. METHODS: The Timed Up and Go (TUG) test, instrumented with an inertial sensor fixed to the back, was administered to 33 PD patients (15 females
age: median = 71 years
IQR = 10) to assess two walking types: straight-walking and turning-while-walking. The TUG test was completed in single-task and two dual-task modalities: cognitive (doing successive subtractions) and motor (holding a water glass). The angular speed spectral arc length metric (SPARC) and the Ln-DimensionLess Jerk (LDLJ), two smoothness measures quantifying the peaks and dips in the speed profile, were measured, along with the trunk angular velocity and the foot strikes number. ANOVA was used for hypothesis testing and estimated marginal means for post-hoc tests and effect sizes (ES). RESULTS: In straight-walking and turning, cognitive and motor dual tasks decreased gait speed (ES range = [0.476, 1.379]
p <
0.01) and increased the step number (ES = [0.402, 0.927]
p <
0.05). SPARC (ES = [0.221, 0.493]
p <
0.05) and angular LDLJ (ES = [0.451, 0.929]
p <
0.01) were lower in the two dual-task conditions in both phases, indicating reduced gait smoothness than in single-task. This worsening of gait smoothness was partially confirmed after ruling out the dual-task effect on speed and step number. In particular, anterior-posterior SPARC during turning was still low in cognitive (ES = 0.351
p <
0.01) and motor (ES = 0.283
p <
0.05) dual tasks. CONCLUSIONS: In PD, dual-tasking decreases gait speed and increases the step number when walking straight and turning while walking. Independently of these effects, dual-tasking also reduces gait smoothness. As an independent feature of movement, when dual-task walking is the outcome measure, improving smoothness may represent a novel treatment aim in PD. As long as it is instrumented with an inertial sensor, the TUG test is valuable for studying different walking types. TRIAL REGISTRATION: NCT05904171 (ClinicalTrials.gov
date registration: 2023-06-06).