Final answer:
Cross-bridges between actin and myosin generate active force during muscle contraction. This process is part of the power stroke within the sliding filament model, resulting in muscle contraction and shortening of the sarcomere. The correct option is B.
Step-by-step explanation:
When cross-bridges form between actin and myosin during muscle contraction, they generate active force. This is part of the sliding filament model of muscle contraction, where the myosin head attaches to actin and then proceeds through the power stroke.
During the power stroke, the actin is pulled toward the M line. ADP and inorganic phosphate (Pi) are released, which causes the myosin head to bend, and as a result, pulls actin along, shortening the sarcomere and producing muscle contraction.
At the end of the power stroke, a new ATP molecule binds to the myosin head, causing the actin-myosin cross-bridge to detach, allowing the cycle to repeat. Hence, active force is produced when cross-bridges are formed, but not ATP; ATP is required to break the cross-bridge so the cycle can continue. The correct option is B.