The Correlation Between EMG and Shear Modulus in Two Portions of Medial Gastrocnemius

This study investigated the correlation between muscle activation measured by high-density surface electromyography (HDsEMG) and muscle stiffness assessed via SuperSonic Imaging (SSI) dynamic elastography in two portions (proximal and distal) of the medial gastrocnemius (MG) during submaximal voluntary isometric contractions. Thirteen participants performed four contractions increasing linearly from 0 to 30% MVC while both HDsEMG and SSI were concurrently recorded. EMG envelope amplitude was calculated for the proximal and distal portions and correlated with shear modulus (μ) values measured by SSI elastography. Results indicate a strong correlation between HDsEMG envelope amplitude and μ across all subjects for both muscle portions (proximal: r= 0.85 ± 0.07; distal: r = 0.93 ± 0.03). Moreover, Fisher-transformed correlation coefficients were significantly higher in the distal (rf = 1.76 ± 0.31) compared to the proximal portion (rf = 1.34 ± 0.33), suggesting a more consistent relationship in distal MG. This study underscores the utility of combining HDsEMG and SSI to investigate muscle mechanics, especially in pennate muscles like MG, with implications for clinical, rehabilitation and sports science.