Antagonistic muscle action
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Antagonistic muscle action refers to pairs of opposite muscles working to create movement in opposite directions. Examples include the bicep-tricep pair in arms, and the quadricep-hamstring pair in legs. Confusingly (?!), the term forelimb is used to refer to any limb (leg, arm) in terrestrial vertebrates.
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Only one muscle in the antagonistic pair can contract at a given time. The contraction causes either flexion i.e. bending of the limb, or extension i.e. straightening of the limb.
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Antagonistic muscle pairs exist because muscles contract but cannot extend. Therefore, to achieve both directions of limb movement, muscles must exist on both sides…