Impedance Myography Circle Mapping: A Geometry-Based Multi-Frequency EIM Framework for Quantitative Assessment of Post-Stroke Muscle Impairment.

Post-stroke neuromuscular impairment arises from heterogeneous structural and electrophysiological alterations, including disrupted membrane integrity, fiber atrophy, and increased intramuscular fat. These changes substantially alter the resistive-capacitive behavior of muscle tissue, yet conventional electrical impedance myography (EIM) metrics remain limited in capturing subtle or spatially heterogeneous abnormalities. Building upon the bio-tissue equivalent circuit model, this study proposes a geometry-based framework termed Impedance Myography Circle Mapping (IMCM), which reformulates multi-frequency EIM spectra in the complex plane to extract two physiologically interpretable parameters: the circle center (IMCM-C), reflecting global conductivity balance and the short axis (IMCM-S), associated with membrane capacitance and microstructural integrity. A cross-sectional experiment was conducted on three upper-limb muscles-the biceps brachii, pronator teres, and flexor digitorum profundus-in 33 healthy controls and 50 stroke patients using multi-frequency ( 3 kHz-1 MHz) impedance measurements. Ten supervised machine learning models were employed to compare IMCM features with conventional single-frequency and Cole-model parameters. IMCM consistently achieved superior classification across all algorithms, reaching a maximum accuracy of 88.6% in binary (healthy vs. stroke) and 93.9% in multi-muscle classification tasks. The IMCM-S feature exhibited a significant negative correlation with the Modified Ashworth Scale (MAS) ($p< 0.001$), demonstrating its potential as a quantitative biomarker of spasticity severity. These findings establish IMCM as a physiologically grounded, model-consistent extension of EIM that integrates biophysical interpretability with data-driven robustness. By linking impedance geometry to muscle pathology, IMCM offers an objective and scalable framework for quantitative neuromuscular assessment and spasticity monitoring in stroke rehabilitation.