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2000 Publications

DIFFERENCES IN MEAN POWER FREQUENCY BETWEEN ECCENTRIC AND CONCENTRIC QUADRICEPS McHugh MP, Tyler TF, Greenberg SC, Gleim GWNicholas Institute of Sports Medicine and Athletic Trauma, New York, NY - last modified 2013-02-10 00:00
Medicine and Science in Sports and Exercise. 32: S55, 2000.


Previous studies using indwelling EMG electrodes have indicated that fast-twitch motor units are selectively recruited for low intensity eccentric contractions.

However, several studies have failed to confirm these results using surface EMG measurements. These disparities may be due to the insensitivity of surface EMG measurements to changes in recruitment. Alternatively, results may differ with respect to muscle group and contraction intensity.

Therefore, the purpose of the present study was to compare the surface EMG signals from quadriceps muscles during eccentric, concentric and isometric contractions at varying intensities. Surface EMG signals were recorded from the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) muscles during isometric, eccentric and concentric (isokinetic) knee extension contractions at 25%, 50%, 75% and 100% of MVC for each contraction mode (n=10 men).

Mean power frequency (MPF) computed from 30 to 60 degrees during eccentric contractions and from 60 to 30 degrees during concentric contractions. MPF was higher (p<0.0001) for eccentric versus concentric contractions in all three muscles, with similar differences at each contraction intensity. MPF was insensitive to increasing eccentric or concentric contraction intensity but increased with increasing isometric contraction intensity (p<0.001). EMG amplitude (RMS) increased with increasing contraction intensity in each contraction mode (p<0.0001).

However, RMS increased by only 2.6 mV/Nm for eccentric contractions compared to 3.7 mV/Nm and 4.0 mV/Nm for concentric and isometric contractions respectively (p<0.001). Higher MPF for eccentric versus concentric contractions at various contraction intensities is consistent with selective recruitment of fast-twitch motor units.

Additionally, eccentric contractions became more mechanically efficient (lower EMG/torque) as contraction intensity increased.