Final answer:
During muscle contraction, the A bands remain unchanged in width, while the I bands narrow and the Z lines move closer together due to the sliding of actin and myosin filaments.
Step-by-step explanation:
The Sliding Filament Model provides the best explanation for the process of skeletal muscle contraction. During this process, the configuration of bands within the muscle sarcomere changes significantly. The dark A bands remain the same width, as they correspond to the length of the thick filaments (myosin). The lighter I bands, which are located on either side of the A bands, become smaller as the thin filaments (actin) slide towards the center of the sarcomere. This movement causes the Z lines, which define the edges of each sarcomere, to move closer together. In fully contracted muscles, the actin and myosin filaments overlap completely. The shrinking of the H zone, where actin and myosin do not overlap, is another indication that contraction has occurred, as it nearly disappears when the muscle is fully contracted.