Clinical Relationship Between the Tonic Stretch Reflex Threshold and μ as Measures of Upper Limb Spasticity and Motor Impairment Following Stroke

Background: Stroke is a major cause of disability worldwide, affecting sensorimotor function and impacting the quality of life in up to ~70% of survivors. Evidence suggests that following a stroke, the ability of the central nervous system to modulate the Tonic Stretch Reflex Threshold (TSRT) and the velocity-sensitivity (μ) of the stretch reflex is impaired. This impairment results in a reduced joint workspace where normal muscle activation is possible (i.e., the active control range) and a joint workspace where sensorimotor disorders such as spasticity and abnormal agonist-antagonist coactivation occurs. While emerging evidence suggests that sensorimotor training in the active control zone of the elbow of the most affected upper limb improves the quality of movement during reaching, the relationship between TSRT and μ modulation with functional movement impairment is not well understood. Objective: We aimed to determine the clinical meaningfulness of these physiological measures by investigating the relationships between TSRT and μ values of elbow flexor muscles during a functional reaching task, and clinical scores of UL spasticity and motor function. Methods: We designed an observational, cross-sectional study, retrieving individual demographic and clinical data from eight studies conducted between 2010 and 2023. Data on TSRT, μ, the Modified Ashworth Scale (MAS), and the Fugl-Meyer Assessment (FMA-UE) were collected from 120 stroke survivors (mean age: 60.3 ± 13.7 years). Eleven participants underwent two evaluations, while 57 individuals had more than two evaluations. Relationships between variables were analyzed using simple correlations and multiple regression analyses. Analyses were done considering that lower values of TSRT angles and higher values of μ reflect greater impairment. Results: The FMA-UE total scores were 29.9 ± 18.9 (n=156) for the acute/early subacute group, 31.1 ± 12.3 (n=33) for the late subacute group, and 31.9 ± 15.1 (n=33) for the chronic group. TSRT was positively correlated with μ across all subjects (r=0.386, p <0.001), with stronger correlations in the acute/early subacute group (r=0.592, p<0.001) and the chronic group (r=0.410, p=0.002). TSRT was negatively correlated with MAS (r=−0.317, p<0.001) across all subjects and more strongly in the acute/early subacute group (r=−0.469, p<0.001). TSRT and μ explained 72.0% of the variance in the FMA-UE describing shoulder-elbow movements made in- and out-of-synergy and reflex function. TSRT explained 68.7% of the variance in the total score of the FMA-UE Conclusions: We reported for the first time, a significant relationship between impairments in TSRT and μ regulation and upper limb motor impairments measured by the FMA-UE following stroke. TSRT and μ are potential key biomarkers for assessing sensorimotor impairments and monitoring recovery in stroke rehabilitation