Muscles do not contract instantly. There must be a build up of action potential to a threshold for the muscle to contract.
Can you control the level of action potential not to achieve the threshold of muscle contraction?
"Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. These channels are shut when the membrane potential is near the resting potential of the cell, but rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open, they allow an inward flow of electrical current, which produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electrical current, etc. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential, often to the extent of briefly reversing its polarity......."
At which time the muscles contract.