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

Abstract

The aim of this study was to investigate whether the resting Vastus Lateralis (VL) muscle shear elastic modulus ({micro}), evaluated by shear wave elastography, represents peripheral fatigue during repetition of isometric maximal voluntary contractions (MVCs) of the knee extensor (KE) muscles.\n\nEight healthy well-trained males repeated 60 isometric MVCs of the KE muscles (6 x 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 (Twpot) double electrostimulation and the amplitude of the concomitant VL compound action potential were considered to be indicators of peripheral fatigue. The resting VL{micro} was measured during a 5-s rest period after each MVC and electrical stimulation series.\n\nThe resting VL{micro} significantly decreased (-21.8 {+/-} 3.9%; P < 0.001) by the end of the fatigue protocol, decreasing from the 10th MVC to the end of the exercise (60th MVC) for all participants, with the loss ranging from 18 to 29%. The potentiated doublet and single twitch torque (Twpot) 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 Twpot, 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.\n\nThis study shows that the kinetics of the resting VL{micro} 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 VL{micro} 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.