Resting shear elastic modulus as a marker of peripheral fatigue during maximal isometric contractions in humans

The aim of this study was to investigate whether the resting Vastus Lateralis (VL) muscle shear elastic modulus (µ), evaluated by shear wave elastography, represents peripheral fatigue during repetition of isometric maximal voluntary contractions (MVCs) of the knee extensor (KE) muscles. Eight healthy well-trained males repeated 60 isometric MVCs of the KE muscles (6 × 10 MVCs; 5 s on/5 s off). Single and double electrical stimulations were delivered to the femoral nerve every ten MVCs during contraction and at rest. The amplitude and properties of the potentiated torque following single (Tw pot ) double electrostimulation and the amplitude of the concomitant VL compound action potential were considered to be indicators of peripheral fatigue. The resting VLµ was measured during a 5-s rest period after each MVC and electrical stimulation series. The resting VLµ significantly decreased (-21.8 ± 3.9%; P < 0.001) by the end of the fatigue protocol, decreasing from the 10 th MVC to the end of the exercise (60 th MVC) for all participants, with the loss ranging from 18 to 29%. The potentiated doublet and single twitch torque (Tw pot ) decreased by 42.5 ± 10.8% and 55.7 ± 8.8%, respectively, by the end of exercise (P < 0.001 for both). The relative mechanical properties of Tw pot , i.e. electromechanical delay (P <0 .001), contraction time (P = 0.004), and maximal rate of torque development/relaxation (P < 0.001) also changed significantly during exercise. This study shows that the kinetics of the resting VLµ is associated with changes in both voluntary and electrostimulated torque amplitudes and electromechanical properties of the single twitch during the repetition of maximal voluntary fatiguing exercise. Changes in the resting VLµ may reflect a decline in muscle function, e.g . impairment of excitation-contraction coupling, contractile processes, and/or elastic properties, throughout the increase in muscle compliance, directly affecting force transmission.