A motor neuron cannot make a single muscle fibre pull harder by shouting louder — every action potential is the same size, and each one triggers the same brief twitch: calcium floods the fibre, tension rises in tens of milliseconds, then relaxes over a couple hundred.
The one lever it has is rate. Fire a second stimulus before the first twitch has relaxed and the two tensions simply add — temporal summation. Fire faster still and the ripples shrink until the trace goes smooth: a fully fused tetanus, three to four times a single twitch.
Past fusion, firing even faster buys almost nothing — the calcium machinery is already saturated, so force plateaus. That is how your motor system grades force: not with bigger spikes, but by choosing how often to fire and how many fibres to recruit.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.