Force Depression and Electromyography of Knee Extensors

Abstract

Force depression (FD) of a muscle is a well-documented phenomenon showing a reduction of isometric force output after an active shortening. This has been investigated mostly with external stimulation, and few studies have employed voluntary exertion. In those studies the electromyography (EMG) has been the method mostly employed to access FD but in small muscles, and they have not focused the analysis of the EMG in frequency domain. PURPOSE: This study investigated FD after isokinetic (ISK) and submaximal isotonic (IST) exertions through surface EMG signal of knee extensors during isometric actions. METHODS: 23 health subjects (12 males and 11 females) with age 24 (±2.4) years old participated in this study after a written consent. The experimental protocol was approved by ethical committee for human research. The EMG of vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM) were registered by a DataLINK (Biometrics) during isometric tests as following. All subjects first exerted a maximal isometric test (REF), with the knee at 120o of flexion in a BIODEX machine. Further, the ISK (ω=30o/s) and IST exertions were randomly performed in the BIODEX, once by each subject, and followed by a 5s isometric action. The IST had the load set at 60% of the value obtained in REF. The isometric test that followed the ISK was maximal and the IST was 60% of the maximal. An isometric reference at 60% with the knee at 120o of flexion was also monitored. During concentric actions the knee moved from 80o to 120o. The RMS-EMG and the median frequency (MF) of the isometric tests were compared trough repeated measures ANOVA. RESULTS: Significantly lower (p=0.007) isometric torque was found after ISK (105±12 Nm) exertion than during REF (120±16Nm). A lower MF after ISK than REF was found in VL (p=0.0005) and VM (p=0.028). The RMS-EMG was higher after IST than in its isometric reference in VL (p=0.015), RF (p=0.006), and VM (p=0.002). CONCLUSION: FD after maximal concentric exertion was observed in the knee extensors due to the torque decrease. The increase in EMG activity after IST exertion may be also accounted to the FD. As FD is not a fatigue related phenomenon, the decreasing in MF is not well explainable, but if a change in pennation angle does occur due to FD, the orientation of the electrodes in relation to the fibers might have an influence on such reduction.