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J Appl Physiol (June 18, 2004). doi:10.1152/japplphysiol.00418.2004
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Submitted on April 19, 2004
Accepted on June 12, 2004

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction

Motoki Kouzaki1*, Minoru Shinohara2, Kei Masani1, and Tetsuo Fukunaga3

1 Department of Life Sciences, University of Tokyo, Meguro, Tokyo, Japan
2 Department of Integrative Physiology, University of Colorado, Boulder, Colorado, USA
3 School of Sport Sciences, Waseda University, Tokorozawa, Saitama, Japan

* To whom correspondence should be addressed. E-mail: kouzaki{at}idaten.c.u-tokyo.ac.jp.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The standard deviation (SD) of force and average EMG (AEMG) of each muscle were calculated for 30 s periods during alternate muscle activity. Power spectrum density of force was calculated for the low- (≤ 3 Hz), middle- (4-6 Hz) and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant, but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF (0.736 ± 0.095, P < 0.05) compared with VL and VM. Large changes were found only in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component (0.832 ± 0.087) was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.




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