Threshold and Graded Response

Why doesn't the muscle respond at lower voltages?

We dissected the gastrocnemius from a frog and ran some experiments on it. This experiment was incrementally increasing the stimulus voltage to see the response in tension. I know this is because the threshold is not reached, but I don't know how to explain this physiologically?

Why doesn't the muscle response continue to increase beyond maximum with increasing stimulus voltages?

I'm not really sure about this one. I know that the muscle can only contract to the length of its sarcomere within the muscle fibers, but that's not really answering the question.

Please Help!