During the muscle can no longer physiologically respond, i. e. contract.

The answer to the question, would be: during fatigue.

Explanation:

Normally, a muscle fiber will respond to neurological stimulation to perform movement. In this case, the whole process of movement of the fiber results from the biochemical processes that lead the muscle to respond to calcium and use ATP, so that the chain reactions can be carried out, and the fibers can move. However, there is a point at which, ATP is depleted, and there is an overproduction of lactic acid, which prevents muscle fibers from further responding to stimulation from the motor neuron, regardless of the amount of ACh released to bind with the sarcomeres´ receptors. At this point, then, the muscle fibers are incapable of further responding to stimultation, and thus movement is stopped.