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Articles in PresS, published online ahead of print May 3, 2002
J Appl Physiol, 10.1152/jap.00764.2001
Submitted on July 24, 2001
Accepted on April 23, 2002
1 Department of Life Science, University of Tokyo, School of Arts and Sciences, Meguro, Tokyo, Japan
* To whom correspondence should be addressed. E-mail: kouzaki{at}idaten.c.u-tokyo.ac.jp.
To determine quantitatively the features of alternate muscle activity between knee extensor synergists during low-level prolonged contraction, a surface electromyogram (EMG) was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) in 11 subjects during isometric knee extension exercise at 2.5% of maximal voluntary contraction (MVC) for 60 min (experiment 1). Furthermore, in order to examine the relation between alternate muscle activity and contraction levels, six of the subjects also performed sustained knee extension at 5.0, 7.5, and 10.0% of MVC (experiment 2). Alternate muscle activity among the three muscles was assessed by quantitative analysis based on the rate of integrated EMG sequences. In experiment 1, the number of alternations was significantly higher between RF and either VL or VM than between VL and VM. Moreover, the frequency of alternate muscle activity increased with time. In experiment 2, alternating muscle activity was found during contractions at 2.5% and 5.0% of MVC, though not at 7.5% and 10.0% of MVC, and the number of alternations was higher at 2.5% than at 5.0% of MVC. Thus, the findings of the present study demonstrated that alternate muscle activity in the quadriceps muscle 1) appears only between bi-articular RF muscle and mono-articular vasti-muscles (VL and VM), and its frequency of alternations progressively increases with time, and 2) emerges under sustained contraction with force production levels equal to or less than 5.0% of MVC.
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