CVS Practice Test 2026 - Free CVS Practice Questions and Study Guide

The A band is the section of the sarcomere that contains thick filaments (myosin) and is responsible for the dark appearance of the striation in skeletal muscle. During muscle contraction, the components of the sarcomere undergo specific changes. The important aspect of the A band is that it retains its length regardless of whether the muscle is contracted or relaxed. This stability is because the A band is defined by the length of the thick filaments, which do not change during contraction.

In contrast, other components of the sarcomere do change during contraction. I bands, which contain only thin filaments (actin), narrow as the filaments slide past each other. The H zone, which is the area within the A band that contains only thick filaments, also decreases in size as contraction occurs. The Z lines, which mark the boundaries of each sarcomere, come closer together as the sarcomere shortens during contraction.

Thus, the A band remaining unchanged during contraction highlights an essential aspect of sarcomere function in muscle contraction.