Which of the following describes an agonist muscle? A muscle that is lengthening during an exercise

A muscle that is resisting the motion

The primary muscle that is shortening during the exercise

Both B and C

D. Both B and C

Explanation:

The agonist in a movement is the muscle(s) that provides the major force to complete the movement.  Because of this agonists are known as the ‘prime movers’.  In the bicep curl which produces flexion at the elbow, the biceps muscle is the agonist, as seen in the image below.

The agonist is not always the muscle that is shortening (contracting concentrically).  In a bicep curl the bicep is the agonist on the way up when it contracts concentrically, and on the way down when it contracts eccentrically.  This is because it is the prime mover in both cases.

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