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.
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.
Antagonistic muscle pairs exist because muscles contract but cannot extend. Therefore, to achieve both directions of limb movement, muscles must exist on both sides…