Thick and thin filaments are organized into contractile units called sarcomeres. A sarcomere extends from one Z line to the next and represents the fundamental unit of muscle contraction.From Copstead, 1995.sar·co·mere/ (sahr´ko-mēr) the contractile unit of a myofibril; sarcomeres are repeating units, delimited by the Z bands, along the length of the myofibril.