Congruency of fatigue-mediated changes in shear wave velocity, upper limb force, muscle activity, and kinematics of the scapular stabilizer muscles

Ultrasonographic shear wave elastography is a new technique for assessing muscle stiffness in vivo. Such insights may be valuable to uncover fatigue-mediated muscle stiffness and control changes at the shoulder, for which much variability remains unexplained through EMG-based methods. This study assessed fatigue-mediated changes in shear wave velocity (SWV), EMG root-mean-squared (RMS) amplitude, EMG mean-power-frequency (MdPF), and force output from upper trapezius, middle trapezius, lower trapezius, and serratus anterior, to determine whether SWV may provide a significant predictive variance in the quantification of muscle fatigue and fatigue-mediated scapulothoracic kinematics changes. 15 males and 15 females had their scapular stabilizer muscles and scapulothoracic kinematics assessed before and after a targeted muscle fatigue protocol. Results indicate that EMG RMS amplitude and EMG MdPF were most sensitive to fatigue-mediated changes at 100% contraction intensity, while SWV appeared most sensitive at 30% and 50% contraction intensity. Scapulothoracic kinematics changes appeared most sensitive to fatigue-mediated EMG MdPF changes. These results suggest that EMG MdPF changes may be elusive in a highly redundant muscular system like the shoulder due to its association with kinematics changes altering load-sharing strategies. SWV increases at contraction intensities ≤ 50% suggest post-exercise muscle edema is a likely physiological mechanism.