Electrical Somatosensory and Motor Thresholds across Pulse Widths: Characterizing Strength-Duration Properties and Nerve Excitability

Quantitative assessment of peripheral sensorimotor function via surface electrical stimulation provides essential insights into axonal excitability; however, standardized stimulation parameters remain poorly defined. This study aimed to characterize the strength–duration (S–D) relationship of electrical sensory, motor, and pain thresholds (EST, EMT, and EPT) across a wide range of pulse widths to identify physiological stabilization plateaus and determine durations that optimize selective fiber recruitment. Thirty healthy volunteers underwent electrical threshold testing (ETT) on the forearm using a symmetrical biphasic current (100 Hz). Eleven pulse widths (20–650 µs) were evaluated in randomized order. Threshold transitions were modelled using a power function ( y = ax ^b ). Stabilization points were determined through consecutive pairwise comparisons, and discriminative capacity was assessed using effect-size analysis. All thresholds followed a characteristic hyperbolic decay consistent with fundamental axonal excitability properties. A clear stabilization toward the rheobase was identified: EST reached a physiological plateau at ∼150 µs, while EMT and EPT stabilized at ∼200 µs. Extreme pulse widths (20 and 650 µs) demonstrated the highest discriminative capacity between thresholds (η²ₚ = 0.936 and 0.921). These findings demonstrate that electrical thresholds follow a predictable neurophysiological pattern across pulse widths, and suggest that pulse width selection is critical for achieving target fiber selectivity, providing a reference framework for standardizing ETT protocols in both research and clinical diagnosis.