Quantifying Plantar Flexor Muscles Stiffness During Passive and Active Force Generation Using Shear Wave Elastography in Individuals With Chronic Stroke

Objectives: This study aims to investigate the mechanical properties of paretic and healthy plantarflexor musclesand assesses the spatial distribution of stiffness between the gastrocnemius medialis (GM) and lateralis (GL) dur-ing active force generation. Methods:Shear wave elastography measurements were conducted on a control group (CNT, n=14;age=59.9 +/- 10.6 years; BMI=24.5 +/- 2.5 kg/m2) and a stroke survivor group (SSG, n=14; age=63.2 +/- 9.6 years;BMI=23.2 +/- 2.8 kg/m2). Shear modulus within the GM and GL was obtained during passive ankle mobilization atvarious angles of dorsiflexion (P0 =0 degrees; P1=10 degrees; P2=20 degrees; P3=-20 degrees and P4=-30 degrees) and during different levels(30%, 50%, 70%, 100%) of maximal voluntary contraction (MVC). Muscle activations of GM, GL, soleus and tibia-lis anterior were also evaluated. Results:The results revealed a significant increase in passive stiffness within the paretic plantarflexor musclesunder high tension during passive mobilization (p<0.05). Yet, during submaximal and maximal MVC, the pareticplantarflexors exhibited decreased active stiffness levels (p<0.05). A notable discrepancy was found between thestiffness of the GM and GL, with the GM demonstrating greater stiffness from 0 degrees of dorsiflexion in the SSG(p<0.05), and from 10 degrees of dorsiflexion in the CNT (p<0.05). No significant difference in stiffness was observedbetween the GM and GL muscles during active condition. Conclusion:Stroke affects the mechanical properties differently depending on the state of muscle activation. Nota-bly, the distribution of stiffness among synergistic plantarflexor muscles varied in passive condition, whileremaining consistent in active condition.