H Zone

The H zone, also called the H band, is the central region of the A band in a sarcomere where thick, myosin-containing filaments are present without overlapping thin, actin-containing filaments. It lies around the center of the sarcomere and appears relatively lighter than the surrounding parts of the A band because those surrounding regions contain overlapping thick and thin filaments .^1,2

The M-line lies at the center of the H zone and helps organize the thick filaments. The H zone should therefore not be confused with the entire A band. The A band spans the full length of the thick filaments, including both the central H zone and the regions where thick and thin filaments overlap. The H zone specifically describes the portion of the thick-filament region that lacks thin-filament overlap .^1,2

During sarcomere shortening, thin filaments slide farther toward the center of the sarcomere, increasing their overlap with the thick filaments. As a result, the H zone becomes narrower and may become very small as the sarcomere shortens. The A band, by contrast, remains essentially the same width because the thick filaments themselves do not appreciably shorten during this process .^1,2