Achilles Tendon Acoustic Properties in Post-Stroke Individuals: Impact of Ankle–Knee Angle and Spasticity Grade

Objective This study aimed to precisely quantify the individual and interactive effects of ankle-knee angles and spasticity grades on passive acoustic and biomechanical properties of the Achilles tendon in post-stroke patients, using shear wave elastography (SWE) - an angle-specific, non-invasive biomarker for static passive tendon assessment. Methods A prospective cohort study was carried out. Two - dimensional ultrasound and SWE were employed to measure the structural parameters and material properties of the triceps surae/Achilles tendon (AT) at various ankle/knee positions in 35 post - stroke patients. Spasticity was evaluated using the Modified Ashworth Scale (MAS). Log - transformed mean Young's modulus (YM) and shear - wave velocity (SWV) data were analyzed via a linear mixed - effects (LME) model. Results The injured side had higher: gastrocnemius medialis (GM)/ soleus (SOL) YM and SWV (all P ≤ 0.027) and increased AT thickness in neutral ankle positions ( P ≤ 0.039). LME models explained 68.6% (YM) and 71.9% (SWV) of total variance. Ankle dorsiflexion significantly increased AT stiffness (YM β = 0.270; SWV β = 0.320, all P < .001), whereas knee extention produced smaller but significant increments (YM β = 0.140, SWV β = 0.160, all P < .001). with marginal ankle - knee interaction ( P = 0.060–0.084). Spasticity grade (MAS 0 - 2) and higher - order interactions had no significant effects ( all P > 0.05). Conclusion Joint angle has a greater impact than spasticity grade on prone-measured passive post-stroke Achilles tendon stiffness, with marginally significant ankle-knee interactions. SWE is a non-invasive, angle-specific biomarker for static passive AT mechanical assessment.