The effects of a doublet stimulus and force level on the electromechanical delay.

The purpose of this study was to examine the effects of a doublet stimulus evoked during isometric submaximal contractions on the electromechanical delay (EMD). Twenty-three healthy individuals performed isometric step contractions ranging from 10 to 70% maximal voluntary contraction (MVC) with a doublet stimulus applied during the contractions. A 2-way repeated-measures analysis of variance (gender [male vs. female] × contraction intensity [10% vs. 20% vs. 30% vs. 40% vs. 50% vs. 60% vs. 70% MVC]) was used to analyze the EMD. The EMD at 10% was significantly less than at 30% (p = 0.022), 40% (p = 0.044), 50% (p = 0.012), 60% (p = 0.041), and 70% MVC (p < 0.001). In addition, the EMD at 70% MVC was significantly higher than at 20% (p = 0.002), 30% (p = 0.006), 40% (p = 0.009), and 50% MVC (p = 0.021). The EMD is the time difference between muscle activation and the onset of muscle force production, with the stretching of the series elastic component (SEC) as the major contributor. A doublet stimulus applied during an isometric contraction minimizes the effects of the SEC during the interpolated twitch technique. Therefore, when the SEC is minimized, the EMD may be lengthened because of mechanisms associated with the excitation-contraction coupling at higher contraction intensities. The EMD measurement, produced with a doublet stimulus, could potentially be a useful tool to examine the effects of fatigue, exercise, stretching, and so on, on the excitation-contraction coupling during various force levels. However, future research is needed to further elucidate the mechanisms that result in a lengthening of the EMD under these conditions.