Which statement about sarcomere contraction is incorrect? a. The A bands shorten. b. The H zones shorten. c. The I bands shorten. d. The sarcomeres shorten.

A sarcomere is the basic functional unit of a muscle fiber, consisting of overlapping thick (myosin) and thin (actin) filaments. It is divided into different regions: the A band, the H zone, the I band, and the Z lines. - The A band contains the entire length of the thick filaments and also includes overlapping portions of the thin filaments. - The H zone is the lighter region in the middle of the A band, consisting of thick filaments only. - The I band is a lighter region containing thin filaments only. It is located on either side of the A band. - The Z lines are darker lines that anchor the thin filaments and separate one sarcomere from the next.

Sarcomere contraction occurs through the sliding filament theory: the thick and thin filaments slide past each other, shortening the sarcomere and pulling the Z lines closer together. Importantly, the filaments themselves do not shorten, only the overlapping regions change.

Let's analyze each statement: a. The A bands shorten: Since the A bands represent the entire length of the thick filaments and also includes overlapping portions of the thin filaments, they do not actually shorten during contraction. Only the regions of overlap between the thick and thin filaments change. b. The H zones shorten: This statement is correct. The H zones are the regions of the thick filaments without any overlapping thin filaments. As the sarcomere contracts, the thin filaments slide past the thick filaments, reducing the length of the H zones. c. The I bands shorten: This statement is also correct. The I bands represent the regions containing only the thin filaments. As the sarcomere contracts and the filaments overlap more, the I bands shorten. d. The sarcomeres shorten: This statement is correct. The overall length of a sarcomere decreases as a result of the increased overlap between the thick and thin filaments during muscle contraction.

Based on our analysis, the incorrect statement is: a. The A bands shorten.