You will be given a definition of a task first, then an example. Follow the example to solve a new instance of the task.
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Earth rotates on its axis once times in one day.
Solution: How many times does Earth rotate on its axis in one day?
Why? The given output correctly poses a question based on the scientific fact given.

New input: Cardiac muscle kind of muscle cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods, allowing the heart to fill with blood for the next cycle.
Solution:
What kind of muscle cells undergo twitch-type contractions with long refractory periods followed by brief relaxation periods, allowing the heart to fill with blood for the next cycle?