If you engage the bicep muscles and the triceps relax, this is known as:

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If the biceps engage and the triceps relax, this is called reciprocal inhibition. The contraction of the agonist (muscle on one side of the joint) can help release the tension in the antagonist (muscle on the opposite side of the joint) allowing for a much more consistent stretch. This requires focus and intention, as the natural tendency of the muscles is to both activate at the same time.

If You Engage the Bicep Muscles and the Triceps Relax