Question

What is the greater applied voltage doing to stimulate greater contraction?

Answer 1

There are wide range of motor units in a muscle. Smaller motor units has low threshold motor neuron and therefore they can respond to a low stimulus. So low voltage actually stimulate these motor units. As voltage (stimulus) progressively rises, larger motor units with higher threshold motor neuron gets stimulated. This increasing activation of motor units due to increasing voltage is actually termed as Recruitment, which leads to activation of larger muscle fibers and consequently, greater contraction.