Biological Systems MCAT Practice Exam

During which phase of muscle contraction is ATP used to reset the myosin head?

1. Power stroke

2. Release phase

3. Resting phase

4. Calcium influx phase

The correct answer is: Resting phase

ATP is utilized to reset the myosin head during the resting phase of muscle contraction. In muscle contraction, the myosin heads attach to actin filaments to form cross-bridges, which pull the actin filaments toward the center of the sarcomere during the power stroke. After this power stroke, the myosin heads need to detach from the actin to allow the cycle to continue. The detachment of myosin from actin occurs during the release phase, where ATP binding to myosin is crucial. However, the resetting of the myosin head—returning it to its initial high-energy conformation to prepare for another cycle of contraction—occurs in the resting phase. This is because after ATP binds to myosin, it is hydrolyzed for the myosin head to return to its original position, priming it for the next cycle of attachment and pulling. During this phase, calcium levels are low, and the muscle is not actively contracting. Understanding this process is important, as it highlights the role of ATP not just in enabling muscle contraction through energy release but in preparing the muscle fibers for subsequent activations.