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When Ca²⁺ ions combine with troponin contraction of muscles initiates. During contraction the Z lines come closer together and the sarcomere becomes shorter. The length of A band remains constant. I bands shortens and H-band narrows.

1. When Ca²⁺ ions combine with troponin contraction of muscles initiates.
2. During contraction the Z lines come closer together and the sarcomere becomes shorter.
3. The length of A band remains constant.
4. I bands shortens and H-band narrows.

Correct answer: When Ca²⁺ ions combine with troponin contraction of muscles initiates.

Solution

Calcium binding to troponin exposes actin’s binding sites, which initiates muscle contraction. The other statements describe what happens after contraction begins, such as sarcomere shortening and band changes.

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