Ultrasound-Derived Stretch Reflex Threshold Estimation Using Tendon-to-Bone Distance During Tendon Tapping in Post-Stroke Spasticity.

Spasticity assessment after stroke requires objective measures of stretch reflex excitability, yet clinically practical and structurally interpretable approaches for identifying stretch reflex thresholds (SRTs) remain limited. We present an ultrasound-derived method for quantifying SRTs during controlled tendon tapping by tracking tendon-to-bone (T2B) displacement, which reflects local musculotendon deformation. Ten individuals with chronic stroke participated. A LinMot tapper delivered graded mechanical taps to the distal biceps tendon. A B-mode ultrasound probe integrated into the tapper tip tracked T2B displacement, while a force sensor and surface electromyograms (EMG) recorded reflex responses. T2B reflex amplitude was defined as the post-tap peak tendon displacement relative to its pre-tap position. As indentation depth increased, T2B reflex amplitude exhibited a nonlinear-to-linear pattern, enabling clear identification of SRT as the transition point between the two. T2B reflex amplitude showed strong within-subject correlations with reflex force and rectified integrated EMG (RIEMG) (force: mean ${r} =0.86$ ; RIEMG: mean ${r} =0.82$ ), comparable to the force-RIEMG correlation. T2B-based SRT estimates demonstrated within-session repeatability (CV $= 10.48~\pm ~10.06$ %), comparable to force-based SRTs and lower than RIEMG-based SRTs. Baseline T2B preload distance varied across individuals and was positively associated with SRT, suggesting that a structure-informed interpretation may provide physiological context beyond externally imposed indentation depth.