extensor

/ɪkˈstɛnsər/
noun
  1. A muscle that straightens a part of the body, such as an arm or leg.
    • Strengthening your back extensors can improve your posture.
    • After the injury, the doctor said the extensor muscle in his leg was strained.
    • The extensor in your forearm helps you straighten your fingers.
Antonyms
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