The action or result of extending or expanding in an opposite direction, or a counter-expansion that opposes or balances another extension.
From counter- + extension (from Latin extensio, meaning 'spreading out'). Used in anatomy, physics, and conceptual discussions.
Your bicep muscle creates flexion (bending), while your tricep creates counterextension (straightening)—your body is literally full of opposing muscles working together, which is why balance is more important than raw strength.
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