Activation of elbow extensors during passive stretch of flexors in patients with post-stroke spasticity

•Triceps activation during biceps stretch occurred in post-stroke patients with spasticity.•Triceps activation timing and duration were not tightly coupled to stretched biceps activation.•Spasticity and coactivation may be related to deficits regulating tonic stretch reflex thresholds. Deficits in regulation of tonic stretch reflex thresholds (TSRTs) after stroke occur in elbow flexors and extensors leading to spasticity in specific joint ranges. Threshold deregulation may also be responsible for other deficits such as abnormal activation of passively shortening muscles. Goals were to characterize activation of shortening elbow extensors during passive elbow flexor stretch in individuals with stroke, and identify its relationship to upper-limb motor impairment. Thirty-three participants with unilateral stroke participated. TSRTs in elbow flexors were measured by stretching passive elbow flexors at different velocities. EMG responses were recorded from stretched agonist (biceps) and shortened antagonist (triceps) muscles. Triceps activation during passive biceps stretch occurred in all but 4 participants simultaneously with, before or after biceps activation onset. Biceps and triceps activation onsets and durations decreased with stretch velocity. Biceps TSRT and triceps activation magnitude did not correlate with sensorimotor impairment but greater stroke chronicity tended to be related to higher biceps TSRTs (r = 0.406, p = 0.041). Stroke may result in both limitations in reciprocal inhibition and excessive agonist-antagonist co-activation, likely from deficits in TSRT modulation in both muscle groups. Since both reciprocal inhibition and co-activation are fundamental to normal motor control, their cooperative action should be considered in designing interventions to increase the ranges of regulation of TSRTs in flexors and extensors to enhance upper limb functional recovery.