From The School of Biomedical Sciences Wikimyofibril of skeletal muscle cells. The sarcomere is split into the H-zone, A-band, I-band, M line and Z line. The H-zone consists of myosin only, the I-band consists of actin only and the A-band contains both actin and myosin. The M-line holds together the thick myosin filaments. The Z-line differentiates between each sarcomere. During the contraction of skeletal muscle cells the A-band stays the same length whereas the H-zone and I-band decrease in length. The distance between the Z-lines also decreases.