Author(s): GAGLIARDI, M., FRÖHLICH, M., MARCIANTE, E., MALIMPENSA, L., FERRAZZANO, G., VIEIRA, T.M., Institution: POLITECNICO DI TORINO, Country: ITALY, Abstract-ID: 2318

INTRODUCTION:
Lower-limb spasticity, characterized by a pathological, velocity-dependent increase in muscle tone, impairs postural control and standing stability. Intramuscular injections of botulinum neurotoxin (BoNT) are widely used to reduce focal muscle overactivity and support rehabilitation; however, their effects on the efficiency of balance control remain unclear. Traditional stabilometric measures, such as center-of-pressure (CoP), provide limited insight into the neural mechanisms underlying postural regulation. High-density surface electromyography (HD-sEMG) decomposition enables the identification of single motor unit (MU) discharge patterns and offers a powerful framework to investigate neuromuscular control during standing. Efficient postural regulation relies on intermittent modulation of plantar flexor MU activity in relation to CoP dynamics. Therefore, the present study examined whether BoNT administration modifies the neural control of quiet standing.
METHODS:
Twenty-two individuals with unilateral medial gastrocnemius (MG) spasticity were assessed before (T0), 4 weeks (T1), and 12 weeks (T2) after BoNT-A injection. Eighteen participants showing a ≥1-point reduction in Modified Ashworth Scale score were included in the final analyses. During 60 s quiet standing (eyes open and closed), CoP was recorded using a force platform while HD-sEMG signals were acquired bilaterally from the MG and decomposed to identify MU discharge patterns. Traditional CoP descriptors, conventional MU metrics, cumulative spike train (CST) - CoP anterior–posterior coherence (0–0.5 and 0.5–5 Hz), and a novel MU–CoP percentile-based regression (slope = task-related modulation; intercept = baseline tonic activation) were evaluated.
RESULTS:
Closing the eyes increased CoP sway across all descriptors (p<0.001), but no timepoint effect emerged, indicating no global sway change after BoNT. Traditional MU metrics and CST–CoP coherence revealed a significant main effect of limb (p<0.001), with higher values in the control limb, but no treatment effect. In contrast, MU–CoP regression parameters showed time-dependent modulation: at T0, the treated limb exhibited reduced slope and higher intercept, reflecting diminished task-related modulation and elevated tonic activation. After BoNT, slope increased and intercept decreased in the treated limb, whereas the contralateral limb demonstrated complementary adaptations. These changes indicate convergence toward a more symmetrical neuromuscular control strategy despite unchanged sway amplitude.
CONCLUSION:
BoNT did not modify global stabilometric measures during quiet standing but altered the coupling between MUs discharge and CoP dynamics. The percentile-based MU–CoP approach was more sensitive than traditional metrics in detecting treatment-related changes, suggesting that BoNT primarily reshapes neuromuscular modulation. These findings support the use of integrated neuro-mechanical analyses to better understand treatment effects on postural control in spasticity.