The way muscles are put to work is more electrical than chemical. Yes, the electrical 'twitches' give off chemicals, it's not like you could drink a magic potion to get them where they need to be. Even worse, you wouldn't be able to dosage them, which would probably result in a tremor/rigor mortis kind of reaction.
How it works?
Your nerve stimulates motor units, which starts to release ACh, which binds with the muscle fiber. If the stimulation is steady enough and enough ACh is released, an electrical charge will be transmitted along the muscle fiber and open ion gates, this is called depolarization and sends off an action potential.
This causes the SR to release calcium, which bind with troponin on the actin filament (within the muscle), so that the myosin crossbridges can connect. According to the sliding filament theory, this then shortens the muscle fiber, because mysion is drawn into action.
Then ADP (from ATP) connects to myosin and it releases again, but if there's enough stimuli, others will reconnect. Shortening the muscle further and further.
Given that the ion gates have to open to start this whole process, you need nerve stimulation to get even start a contraction. That off course leaves out that you need your heart and lungs to be working to get rid of all the waste and supply your hard working muscles with oxygen. And if you wanted to maintain it for longer than a minute, your liver has to start supplying you with new energy sources.
So I suggest you look into muscle physiology some more! I can highly recommend Physiology of Sport and Exercise, because it covers almost anything regarding exercise you'd want to know!