Most of the time, the sarcomere is in a state of relaxation. Because it is longer than a thick filament, there is a region at either end of the sarcomere that contains only thin filaments. The thin filaments rotate polarized light very little; therefore, the region of the sarcomere on either side of the Z band where thin filaments are not overlapped by thick filaments is called the isotropic band, or I band. In the relaxed state, the thin filaments of a single sarcomere that are attached to adjacent Z bands point toward each other but do not touch. Thus, there is a region in the middle of the sarcomere where thick filaments are not overlapped by thin filaments, which is called Hensen’s disk, or the H zone.

The three-dimensional structure of the sarcomere is very regular. On cross section, each thick filament is surrounded by six thin filaments and each thin filament is equidistant to three thick filaments.