16The aim of this study was to investigate whether the resting Vastus Lateralis (VL) muscle shear 17 elastic modulus (µ), evaluated by shear wave elastography, represents peripheral fatigue during 18 repetition of isometric maximal voluntary contractions (MVCs) of the knee extensor (KE) muscles. 19 Eight healthy well-trained males repeated 60 isometric MVCs of the KE muscles (6 × 10 MVCs; 5 s 20 on/5 s off). Single and double electrical stimulations were delivered to the femoral nerve every ten 21 MVCs during contraction and at rest. The amplitude and properties of the potentiated torque 22 following single (Tw pot ) double electrostimulation and the amplitude of the concomitant VL 23 compound action potential were considered to be indicators of peripheral fatigue. The resting VLµ 24 was measured during a 5-s rest period after each MVC and electrical stimulation series. 25 The resting VLµ significantly decreased (-21.8 ± 3.9%; P < 0.001) by the end of the fatigue protocol, 26 decreasing from the 10 th MVC to the end of the exercise (60 th MVC) for all participants, with the loss 27 ranging from 18 to 29%. The potentiated doublet and single twitch torque (Tw pot ) decreased by 42.5 28 ± 10.8% and 55.7 ± 8.8%, respectively, by the end of exercise (P < 0.001 for both). The relative 29 mechanical properties of Tw pot , i.e. electromechanical delay (P <0 .001), contraction time (P = 30 0.004), and maximal rate of torque development/relaxation (P < 0.001) also changed significantly 31 during exercise. 32 This study shows that the kinetics of the resting VLµ is associated with changes in both voluntary 33 and electrostimulated torque amplitudes and electromechanical properties of the single twitch during 34 the repetition of maximal voluntary fatiguing exercise. Changes in the resting VLµ may reflect a 35 decline in muscle function, e.g. impairment of excitation-contraction coupling, contractile processes, 3 36 and/or elastic properties, throughout the increase in muscle compliance, directly affecting force 37 transmission. Neuromuscular fatigue (NMF) is classically defined as "an exercise-induced reduction in the ability 40 of skeletal muscle to produce power or force, irrespective of task completion" [1]. The study of NMF 41 and underlying mechanisms of recovery, both central and peripheral, is important to prevent overuse 42 injuries [2] and develop new strategies to enhance muscle recovery [3].
43NMF and, more specifically, the peripheral modifications that occur within the muscle, at or distal to 44 the neuromuscular junction [4], have been evaluated by artificial stimulation of the skeletal muscle 45 and/or motor nerve structures [5][6][7]. Modifications in mechanical and/or electromyographic (EMG) 46 response amplitudes delivered by single, double, or tetanic stimulation to a relaxed muscle are 47 considered to be good indicators of peripheral fatigue [1]. More specifically, the study of these 48 electrically-induced responses allows non-invasive investigation of changes of the (i) neuromuscular 49 propagation/tr...