Final answer:
When skeletal muscles lengthen, sarcomeres also lengthen, reversing the shortening process that occurs during muscle contraction.
Step-by-step explanation:
When skeletal muscles lengthen, sarcomeres lengthen. During muscle contraction, sarcomeres shorten as the myosin heads pull the actin filaments inward, decreasing the distance between Z discs and increasing the overlap between the thin and thick filaments. Conversely, when muscles lengthen, the sarcomeres are stretched, increasing the distance between Z discs and reducing the overlap between the filaments.
A sarcomere is defined by the region between two Z discs, and it contains both thick (myosin) and thin (actin) filaments. When the muscle fiber contracts, the I band and the H zone within the sarcomere get smaller as the Z discs come closer together, while the A band remains the same width. This process is critical for muscle function and is powered by ATP, which provides the energy necessary for the myosin heads to bind to actin and perform their power stroke.