Implications of Intra-Individual Variability in Motor Performance on Functional Mobility in Stroke Survivors

<b>Background:</b> Motor impairments following stroke contribute to deficits in functional mobility. Traditionally, these impairments are quantified by mean-level motor performance. However, this mean-level approach neglects the well-established fact that motor performance becomes highly variable in aging and disease. Increased intra-individual variability (IIV) in behavior predicts functional decline in neurological disorders. Despite this, the impact of stroke on IIV in motor performance and its influence on functional mobility has not been investigated. This study aimed to (1) quantify the impact of stroke on IIV in motor performance, and (2) determine the contribution of IIV and mean motor performance to functional mobility. <b>Methods</b>: Twenty stroke survivors and 20 age-matched controls performed a goal-directed ankle movement task over 30 trials. We measured average accuracy (mean endpoint error) and IIV (within-person SD of endpoint error). Functional mobility was assessed with postural control (sway area during quiet standing) and braking response time in a driving simulator. <b>Results</b>: Stroke participants showed a higher mean (<i>p</i> = 0.04) and greater IIV (<i>p</i> = 0.016) in endpoint error than controls. Sway area did not differ between groups (<i>p</i> = 0.24), but stroke survivors had increased braking response time (<i>p</i> = 0.016). In stroke survivors, IIV significantly predicted sway area (R² = 0.33, <i>p</i> = 0.008) and braking response time (R² = 0.27, <i>p</i> = 0.02), and mean error did not account for any additional variance. <b>Conclusions:</b> Stroke reduces the trial-to-trial consistency of executing motor tasks with precision. IIV in motor performance predicts postural balance and braking response time and can potentially serve as an indicator of increased vulnerability and an important target for stroke rehabilitation.