During muscle contraction, which area remains the same size?Group of answer choicesH zonesI bandsnone of the listed regions remain the same sizeA bands

During muscle contraction, the area that remains the same size is the A band.

Here's a step-by-step explanation:

1. Muscle contraction is regulated by the sliding filament theory. This theory explains how muscle fibers shorten or lengthen during contraction and relaxation.

2. The muscle fiber is composed of many myofibrils, which are made up of thick and thin filaments. The thick filaments are made of a protein called myosin, and the thin filaments are made of a protein called actin.

3. The myofibril is divided into repeating units called sarcomeres, which are the functional units of muscle contraction. Each sarcomere is composed of an A band, an I band, and an H zone.

4. The A band is the region of the sarcomere that contains the entire length of the thick filaments (myosin). The I band contains only thin filaments (actin), and the H zone contains only thick filaments.

5. During muscle contraction, the thin filaments slide over the thick filaments, which causes the sarcomere to shorten. This shortening of the sarcomere leads to the shortening of the entire muscle fiber and thus muscle contraction.

6. While the I band and the H zone shorten during this process, the A band does not change in length because it includes the entire length of the thick filaments, which do not change in length during contraction.

So, the A band remains the same size during muscle contraction.