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Laboratory of Sports Sciences, Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, 153-8902, Japan
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, 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
on the basis of 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, although 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 biarticular RF muscle and monoarticular vasti muscles (VL and
VM), and its frequency of alternations progressively increases with
time, and 2) emerges under sustained contraction with force
production levels 
5.0% of MVC.
electromyogram; synergistic muscles; quadriceps muscle
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