Final answer:
During isometric muscle contractions, cross-bridges rebind in the zone of overlap within the A band of the sarcomere, where thick (myosin) and thin (actin) filaments interact.
Step-by-step explanation:
In an isometric contraction, where the muscle does not change length, the cross-bridges rebind in the zone of overlap within the sarcomere. This zone of overlap is where myosin (thick filaments) and actin (thin filaments) interact, specifically in the area of the A band that remains a constant length during contraction. The cross-bridges do not rebind at the M Line, H Zone, or Z Disc, because these are not the sites where myosin and actin overlap. During muscle contraction, the H Zone shortens and the I band gets smaller as the Z lines move closer together. The A band remains the same width, even when the sarcomere contracts and the thin filaments overlap at full contraction. Consequently, the appropriate answer for where cross-bridges rebind during isometric contractions is the A band.