Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Muscle groups are controlled by the motor cortex .
Muscle groups are controlled by what mechanism?

40 m/s north of the following represents the velocity of a moving object.
Which of the following represents the velocity of a moving object?

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.
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?