Motor severity does not affect habituation to unpredictable postural perturbations in Parkinson’s disease

Introduction: Postural control relies on the integration of sensory information and the generation of adequate reactive motor responses to maintain stability during balance-threatening situations (Beretta et al., 2019, 2021; Horak et al., 1989; Nutt et al., 2004). In Parkinson’s disease (PD), impairments in basal ganglia networks disrupt sensorimotor integration, reduce postural flexibility, and impair the modulation of reactive postural responses (Beretta et al., 2022; Horak et al., 1989; Nutt et al., 2004). Theories of postural inflexibility and abnormal sensory feedback gain scaling suggest that people with PD may demonstrate rigid and inefficient balance strategies, particularly during unpredictable perturbations (Beretta et al., 2019). However, existing evidence compares PD impairments in reactive posture with healthy subjects, and one open question is whether PD motor severity affects the habituation process to repeated unpredictable perturbations. Therefore, this study investigated the influence of motor severity on adaptive postural responses (adaptation/habituation) during repeated support-surface perturbations in individuals with PD.Methods: Thirty-seven individuals with PD (70.9 ± 8.4 years; 17 men/20 women; disease duration: 5.0 ± 3.6 years) were exposed to seven posterior support-surface translation perturbations, consisting of one unexpected (i.e., first trial) and six temporally unpredictable perturbations presented across randomized trials (Beretta et al., 2019) (i.e., 7 out of 17 randomized trials). Reactive postural responses were assessed using center of pressure (CoP) and electromyographic parameters, including CoP amplitude, CoP recovery time, muscle coactivation, muscle onset latency, and time-to-peak activation. These parameters were selected because they reflect both the behavioral and neuromuscular components of reactive postural control, including balance recovery performance, temporal organization of muscle responses, and modulation of agonist-antagonist muscle activity following perturbation (Beretta et al., 2019, 2021). Motor severity was quantified using the Unified Parkinson’s Disease Rating Scale motor section (UPDRS-III). To investigate the influence of motor severity on reactive postural adaptation, linear mixed models were performed with trial, motor severity, and their interaction included as fixed effects, while participant was included as a random effect to account for repeated measures across perturbation trials. To improve model interpretability and reduce multicollinearity in interaction analyses, UPDRS-III scores were mean-centered (UPDRS-IIIc) before inclusion in the models. Significant Trial effects were interpreted as evidence of habituation across perturbation exposures, whereas significant Trial × UPDRS-IIIc interactions would indicate that motor severity modifies the habituation trajectory over repeated perturbations.Results: Linear mixed models revealed significant main effects of trial for CoP amplitude (F = 157.56, p < .001), CoP recovery time (F = 29.26, p < .001), biceps femoris onset latency (F = 16.43, p < .001), and gastrocnemius medialis onset latency (F = 36.68, p < .001). Specifically, participants demonstrated fast adaptation already in the second trial and habituation after trial 4 in reductions in CoP amplitude and recovery time, accompanied by shorter muscle activation latencies in both biceps femoris and gastrocnemius medialis over successive perturbation trials, suggesting improved efficiency of reactive balance responses with repeated exposure to unpredictable perturbations. Motor severity, quantified by mean-centered UPDRS-III scores, demonstrated a significant association with only gastrocnemius medialis latency (F = 5.40, p = .022), in which greater motor severity was associated with shorter activation latency. In contrast, motor severity was not significantly associated with CoP amplitude, CoP recovery time, muscle coactivation, biceps femoris latency, or time-to-peak activation (all p > .05). Furthermore, no significant Trial × UPDRS-IIIc interactions were observed for any outcome variable (all p > .05).Conclusions: Regardless of motor severity, people with PD demonstrated habituation to repeated unpredictable perturbations, characterized by progressive changes in CoP and neuromuscular parameters across trials. Although greater motor severity was associated with altered gastrocnemius medialis activation onset latency, these neuromuscular changes were not associated with differences in CoP responses or habituation trajectories. These findings suggest that motor severity may influence specific neuromuscular aspects of reactive postural control without substantially affecting the adaptation process to repeated perturbations in people with mild-to-moderate PD.Acknowledgements: This work was supported by FAPESP [grant number #2016/00503-0] and CAPES [Finance Code 001].