An antagonistic muscle is a muscle that

Question

BlackTom AI · Accepted Answer

Antagonistic muscles work in pairs: when one muscle (the agonist) contracts to produce a movement, its antagonist must relax to allow that movement to occur smoothly.

1. Relevant knowledge points and concepts
- Agonist vs antagonist: The agonist generates the primary force for a movement; the antagonist opposes that movement. Reciprocal inhibition is the neural mechanism that reduces antagonist muscle activity when the agonist contracts. Examples: elbow flexion—biceps brachii (agonist) contracts while triceps brachii (antagonist) relaxes; knee extension—quadriceps contract, hamstrings relax.

2. Step-by-step reasoning process
- To bend the elbow: motor neurons fire to the biceps, causing sarcomere shortening and contraction.
- Sensory feedback and spinal interneurons trigger reciprocal inhibition of motor neurons supplying the triceps, causing the triceps to relax.
- With the antagonist relaxed, the agonist’s contraction produces smooth movement and prevents opposing forces that would limit motion or cause injury.

3. Intermediate steps (physiology sequence)
- Agonist motor unit activation → cross-bridge cycling in muscle fibers → muscle shortening → movement.
- Simultaneous interneuronal inhibition → decreased antagonist motor neuron firing → antagonist muscle relaxation → reduced opposing tension.

4. Why the correct answer is correct
The defining characteristic of an antagonistic muscle is that it relaxes when the opposing (agonist) muscle contracts, enabling coordinated, controlled movements. This reciprocal relationship is central to musculoskeletal function and motor control.

5. Why the other options are incorrect
- “Is considered part of a large-muscle group.” and “is considered part of a small-muscle group.”: Antagonistic refers to functional relationship, not muscle size or grouping.
- “Contracts as the surrounding muscles contract.”: That describes a synergist (a muscle that assists), not an antagonist; antagonists oppose or relax relative to the agonist.

Conclusion: Therefore the correct answer is “relaxes as an opposing muscle contracts,” which matches the provided answer.

An antagonistic muscle is a muscle thatSingle choice

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