What period is a muscle in when it is temporarily unable to respond to stimuli?

The period a muscle is in when it's temporarily unable to respond to stimuli is known as the refractory period. It's caused by the closing of sodium ion channels in the muscle cells, which makes them unable to receive nerve signals until they open again.

The refractory period is divided into the absolute refractory period and the relative refractory period. The absolute refractory period is the time in which all the sodium ion channels are closed and the muscle cannot be reactivated at all. The relative refractory period is the time directly after, when the channels begin to recover but still require a stronger signal than before to activate the muscle.