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Spasticity is a frequent neuromuscular impairment associated with cerebral palsy, stroke, and spinal cord injury, commonly assessed using subjective clinical scales. This exploratory pilot study aimed to develop and preliminarily validate a multimodal instrument for the objective quantification and stratification of spasticity in nine individuals (3 female, 6 male) with upper-limb spasticity due to cerebral palsy (n = 5) or stroke (n = 4). A wearable system integrating surface electromyography, inertial measurement units, and force sensing resistors was designed to simultaneously capture muscle activation, joint kinematics, and generalized resistance force during standardized passive mobilizations. Simple indicators six area under the curve-based indicators were derived: force, sEMG, and angular velocity under two conditions (R1, R2) and given distinct weights depending on their contribution. Principal component analysis revealed that three latent components accounted for 83.86% of the total variance observed across participants. Based on these indicators, a Composite Index was constructed using min-max normalization and weighted linear aggregation. Within the pilot study, the Composite Index could differentiate between spasticity severity levels (F = 6.38, p = 0.0327, η² = 0.68), with sEMG activity during slow stretch (AUC sEMG R2) the most influential contributor indicators. The proposed multimodal instrument demonstrates preliminary feasibility as a non-invasive and portable approach for objective spasticity quantification, warranting further validation in larger cohorts.