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2002 Publications

Differences in activation patterns between eccentric and concentric quadriceps contractionsby McHugh MP, Tyler TF, Greenberg SC, Gleim GW. - last modified 2012-11-27 08:41
Journal of Sports Sciences. 20:83-91, 2002.


Purpose: Previous studies analyzing electromyographic (EMG) activity from indwelling electrodes have indicated that fast-twitch motor units are selectively recruited for eccentric contractions. However, these studies were limited to low intensity contractions and have not been confirmed by analysis of the frequency content of the surface EMG signal. The purpose of the present study was to compare the frequency content of surface EMG signals from quadriceps muscles during eccentric, concentric and isometric contractions at various contraction intensities.

Methods: EMG signals were recorded from the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) muscles during isokinetic (1.05 rad.s-1) eccentric and concentric knee extension contractions at 25%, 50%, 75% and 100% of MVC for each contraction mode (n=10 men). Additionally, isometric contractions (70°) were performed at each intensity. Mean frequency (MF) and root mean square (RMS) of the surface EMG signal were computed.

Results: MF was higher for eccentric contractions compared with concentric contractions (p<0.0001). Greater differences between eccentric and concentric contraction modes were seen in the RF (7+/-3 Hz) and VL (6+/-5 Hz) compared with the VM (3+/-4 Hz) (mode x muscle p<0.05). The difference in MF between eccentric and concentric contractions decreased with increasing contraction intensity (8+/-5 Hz at 25%, 6+/-4 Hz at 50%, 5+/-4 Hz at 75%, 2+/-4 Hz at 100%; mode x intensity p<0.05). MF increased with increasing contraction intensity for isometric contractions (p<0.001) and concentric contractions (p<0.01), but not for eccentric (p=0.27). EMG amplitude (RMS) increased with increasing contraction intensity similarly in each contraction mode (p<0.0001).

Discussion: Higher MF for eccentric versus concentric contractions at sub-maximal contraction intensities is consistent with a greater proportion of fast-twitch motor units being active during eccentric contractions.