Proximo-distal medial gastrocnemius neuromechanical behavior: combining HDsEMG with SSI elastography

The medial gastrocnemius (MG) plays a crucial role in lower limb biomechanics, yet its pennate architecture and regional variations in excitation complicate its neuromechanical characterization. This study investigates whether the relationship between muscle excitation and stiffness is region-specific within MG, by combining high-density electromyography (EMG) and SuperSonic Imaging (SSI) elastography. Eighteen participants performed low-level, force-increasing contractions while EMG amplitude and shear modulus (μ) were concurrently measured in proximal and distal regions of the MG. The distribution of channels detecting EMGs with greatest amplitude values revealed regional differences in MG excitation, with more distal fibers being recruited as force requirement increased. A strong correlation was found between EMG envelope amplitude and μ across all subjects for both portions (mean ± std: r = 0.85 ± 0.06 proximally; r = 0.91 ± 0.05 distally). Moreover, Fisher-transformed correlation coefficients were significantly lower (p = 0.004) in the proximal portion (mean ± std: rf = 1.32 ± 0.26) compared to the distal one (rf = 1.63 ± 0.34). This difference indicates that the preferential recruitment of distal fibers during increasing contraction drives a more consistent relationship between excitation and stiffness in the distal MG relative to the proximal region. These findings underscore the importance of considering muscle region-specificity when assessing MG excitation and stiffness, and highlight the value of combining high-density EMG and SSI for a comprehensive neuromechanical assessment of muscle function.