Triaxial Mechanomyography of the Biceps Brachii Muscle during Sustained Submaximal Isometric Contractions

The biceps brachii (BB) is a bi-articular muscle that acts in the elbow and shoulder joints. Mechanomyography (MMG) is a technique that uses accelerometers to monitor the muscle waves. The aim of this study is to model the behavior of BB MMG signals and elbow torque during sustained submaximal isometric contractions. Three MMG axes (Z, Y and X) and torque were recorded from BB during sustained elbow flexion at 70% of maximum voluntary contraction (MVC) and kept the longest time the volunteers could get. MMG and torque signals were analyzed in 3 different (Initial, Middle and Final) time windows (TW) with 1s duration specific to the protocol. Temporal parameters [Absolute mean, root mean square (RMS), number of zero-crossings, peak count, and absolute integral] were calculated for all signals and every TW. The ANOVA analysis showed that absolute mean, RMS and absolute integral parameters presented statistical difference only for X axis (accelerations in the proximal-distal direction) increasing amplitude with time. The Z axis (accelerations in the anterior-posterior direction) showed decrease of the zero-crossings in the first half of the test and had a second decrease between the Middle TW and the Final TW, but no difference was observed at any moment on X axis during this analysis. The unique parameter that presented statistical difference for all axis (Z, X, Y) and modulus among the three TW was number of peak counts and only X axis is not different between the Initial and Middle TWs. One possible hypothesis would be a tension increase in muscles that actuate on the scapula since the direction of the action force is parallel to the X axis. A purely spectral parameters analysis could help comprehending what does occur with MMG signal in sustained submaximal isometric contractions.