Clinical Relevance of the Tonic Stretch Reflex Threshold and μ as Measures of Upper Limb Spasticity and Motor Impairment After Stroke

Background After a central nervous system lesion, the ability to control muscle activation and relaxation in specific joint ranges may be impaired. The underlying mechanism of this sensorimotor impairment is related to a decreased ability to regulate the tonic stretch reflex threshold (TSRT) through descending and peripheral control processes. In dynamics, the reflex threshold and its velocity-sensitivity (mu) describe how movement in specific upper limb (UL) joint ranges is impaired after stroke.Objective To examine the relationships between measures of elbow flexor impairment using TSRT and mu, and clinical scores of spasticity and motor function. We hypothesized that TSRT and mu would be related to clinical spasticity and motor impairment scores in patients with acute and chronic stroke.Methods TSRT, mu, and clinical data of the resistance to passive movement (Modified Ashworth Scale) and UL motor function (Fugl-Meyer Assessment [FMA]) were collected from 120 patients. Relationships between variables were determined using simple correlations and multiple regression analysis.Results TSRT and mu explained 72.0% of the variance in the FMA of the Upper Extremity [FMA-UE] describing only in-synergy and out-of-synergy movements and reflex function. TSRT explained 68.7% of the variance in the total score of the FMA-UE.Conclusions This study shows for the first time, a significant relationship between deficits in TSRT regulation and mu with UL motor impairment after stroke. TSRT and mu may be valuable clinical biomarkers of sensorimotor impairment for monitoring spontaneous or treatment-induced motor recovery.