What defines the smallest contractile segment of a muscle fiber, known as the sarcomere?

Explanation

The sarcomere is the fundamental unit of muscle contraction, stretching from one Z-line to the next. The Z-line marks the boundary of each sarcomere and anchors the actin filaments, facilitating contraction. Other structures like the H-zone, M-line, and A-band are components within the sarcomere but do not define its boundaries.