What is an agonist muscle?

An agonist muscle is the primary muscle responsible for executing a specific movement by contracting while other muscles relax. This muscle is often referred to as the "prime mover" because it is the one that actively generates the force needed for the intended action. For example, during a bicep curl, the biceps brachii acts as the agonist, contracting to lift the weight while the triceps brachii relax. Understanding the role of agonist muscles is crucial in fields like sports medicine, as it helps in identifying how muscles work together during physical activity, aiding in rehabilitation and training programs.

The other options do not correctly describe the role of agonist muscles. A muscle that limits joint movement refers to an antagonist, which works in opposition to an agonist to create balance during movement. The mention of a muscle that works only under strain does not accurately capture the function of an agonist, as agonists can operate under various levels of resistance, not just strain. Lastly, a muscle that contracts and lengthens simultaneously describes eccentric contraction, which is not the defining characteristic of an agonist.