Which structure, bordered on either side by Z-discs, is described as the functional unit of a skeletal muscle fiber?

1. A myofibril running the length of a muscle fiber is built from many repeating contractile units. 2. Each of these units is bordered by Z-discs and is called the sarcomere. 3. Because it is the smallest unit that actually shortens, the sarcomere is described as the functional unit of the fiber. 4. A myofibril contains thousands of sarcomeres, a fascicle is a bundle of whole fibers, and a triad is a T-tubule/SR complex so none of these is itself the Z-disc-bordered contractile unit. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 10 "Muscle Tissue", section 10.2, p. 410_