Assessment of Elbow Flexors Activity Levels during Incremental Isometric Contractions Using Supersonic Shear Imaging

Our previous study has shown that supersonic shear imaging (SSI) can be accurately used to estimate muscle activity level. The present study was designed to assess the muscle activity level of the main elbow flexors using this technique during linear torque ramps of 30 s from 0 to 40% of the previously determined maximal voluntary contraction (MVC). The echographic probe was randomly placed on the short head of the biceps brachii (BBSH), the long head of the biceps brachii (BBLH), the brachialis (BA), the brachioradialis (BR) and the long head of the triceps brachii (TB). A good repeatability of the shape of the shear elastic modulus changes was observed. The shear elastic modulus patterns highlighted changes in load sharing between the elbow flexors. The torque was mainly produced by BR and BA at low torque levels, and the increase in torque after ~10% of MVC was mainly due to the BB. This phenomenon has not been identified earlier due to lack of information regarding the BA muscle (deep muscle). This study provides a step towards confirming the viability of SSI as a tool to estimate muscle stress or force, but the accuracy of such estimation will have to be assessed.