ANATOMY

Detailed explanation-1: -When muscle contracts, The Z discs come close together. The width of the I bands decreases. The width of the H zones decreases.

Detailed explanation-2: -During muscle contraction, the I band reduces in size as the overlap between the thick filaments and the thin filaments increases. The H zone also decreases in length because of the same reason.

Detailed explanation-3: -The H zone-the central region of the A zone-contains only thick filaments and is shortened during contraction. The I band contains only thin filaments and also shortens.

Detailed explanation-4: -When muscle contracts, the H zone (central region of Azone) which consists of thick filaments is shortened and the I band which contains only thin filaments is also shortened during the time of contraction.

Detailed explanation-5: -So, the correct answer is ‘Decrease in length of actin myofilaments’.