When a muscle fiber contracts, the I bands diminish in size, the H zones disappear, and the A bands move closer together but do not diminish in length.

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Quizplus · Accepted Answer

This is true. During muscle contraction, the actin filaments slide past the myosin filaments, causing the I bands (which contain only actin filaments) to shrink in size, the H zones (which contain only myosin filaments) to disappear as the actin filaments overlap them, and the A bands (which contain both actin and myosin filaments) to move closer together but not change in length.

When a Muscle Fiber Contracts, the I Bands Diminish in Size