Step 1: The Design of Cross-Bridges

• When excitation-contraction coupling causes the sites of myosin binding on actin to be released, the cycle begins. Myosin heads, forming cross bridges, connect to these sites.
• Myosin begins in its high-energy state, in which ADP and inorganic phosphate occupy the ATP.
• The head of the myosin is ready for the power stroke to pivot.

Step 2: The Power Stroke

• Inorganic phosphate release from the myosin head causes the head to pivot, pushing the thin filament toward the core of the sarcomere.
• ADP is then also released. The head of the myosin is now in a low-energy state but is still bound to actin.
• Eventually, the energy for the power stroke is provided by the ATP that binds to the head of the myosin.

Step 3: The Cross-Bridge detachment

• Until an ATP molecule attaches to the empty ATP-binding site on the myosin head, myosin remains attached to actin.
• This weakens the link between the head of the myosin and the actin and detaches the head of the myosin.
• In its low-energy state, the disconnected myosin head remains.

Step 4: It is Myosin head Reactivation

• This happens when the myosin head-bound ATP undergoes hydrolysis to ADP and inorganic phosphate. During hydrolysis, the energy released reactivates the head of the myosin, pivoting it back to its high energy state.
• The cross-bridge loop begins again when the energized myosin head creates another cross-bridge.
• Cross bridge cycles occur between the thick and thin filaments along the entire length of each sarcomere during muscle contraction.
• But not all myosin heads are simultaneously attached or detached. They may be in various stages of the cycle of cross-bridges.
• Note that a myofibrilcomprises thousands of sarcomeres to read the definition and thousands of myofibrils to read the definition in a muscle fibre.

• Cross bridge cycling causes the entire muscle to contract on both of these sarcomeres