Final answer:
It is false that myosin heads are always cocked when muscles are relaxed; they enter this high-energy state during the cross-bridge cycle. ATP binding to myosin triggers the cocking of the myosin head, a crucial step for muscle contraction and for the cycle to repeat.
Step-by-step explanation:
The statement that myosin heads exist in the "cocked" or ready position even when muscles are relaxed, so long as ATP is present, is false. When a muscle is relaxed, the myosin heads are not necessarily in the cocked position. The cocked position of the myosin head is a high-energy conformation that is important during the cross-bridge cycle of muscle contraction. This cycle begins when ATP binds to myosin, causing the myosin head to enter the cocked position.
After the power stroke, the myosin head is in a low-energy position. ATP then binds to the myosin, which allows the cross-bridge to detach and the myosin to return to the cocked configuration. If another ATP attaches, the cycle can recur, which is essential for continued muscle contraction.