Dynamically Characterizing Skeletal Muscles via Acoustic Non-linearity Parameter: In Vivo Assessment for Upper Arms

It is crucial to model skeletal muscles for muscle-centered health care, such as prosthetics. Here we hypothesize that the acoustic non-linearity parameter (B/A) can be utilized to partially represent the contraction state of skeletal muscles. Although previous work commonly measured the B/A value of tissues in vitro, the present study targets the biceps brachii muscle to investigate the relationship between the B/A value and the dynamics of the elbow. Furthermore, it is proposed that a correction method based on the angular spectrum theory be applied in vivo, and the dynamic metrics of the B/A value and its feasibility be verified through an underwater experiment. Seven participants were invited for the in vivo experiment, in which elbow torque and B/A values were measured simultaneously. The non-plane reflection was approximately treated through an integral method, leading to a modified B/A value. Then, linear regression was applied to characterize the B/A–torque relationship, with the calculated coefficient of determination (R²) ranging from 0.85–0.93. Experimental results indicate that the modified B/A value of the biceps brachii correlates well with elbow torque. This study not only paves the way to dynamic measurement of the B/A value of skeletal muscles in vivo, but also confirms that B/A can be used as a more comprehensive assessment criterion for muscle functions.