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 plantar flexor muscles and assesses the spatial distribution of stiffness between the gastrocnemius medialis (GM) and lateralis (GL) during 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 at various angles of dorsiflexion (P0 =0°; P1=10°; P2=20°; P3=-20° and P4=-30°) and during different levels (30%, 50%, 70%, 100%) of maximal voluntary contraction (MVC). Muscle activations of GM, GL, soleus and tibialis anterior were also evaluated.

Results

The results revealed a significant increase in passive stiffness within the paretic plantar flexor muscles under high tension during passive mobilization (p<0.05). Yet, during submaximal and maximal MVC, the paretic plantar flexors exhibited decreased active stiffness levels (p<0.05). A notable discrepancy was found between the stiffness of the GM and GL, with the GM demonstrating greater stiffness from 0° of dorsiflexion in the SSG (p<0.05), and from 10° of dorsiflexion in the CNT (p<0.05). No significant difference in stiffness was observed between the GM and GL muscles during active condition.

Conclusion

Stroke affects the mechanical properties differently depending on the state of muscle activation. Notably, the distribution of stiffness among synergistic plantar flexor muscles varied in passive condition, while remaining consistent in active condition.