To me, mind-muscle is a synergy, a two-way communique with both muscle and mind talking to one another to achieve a goal, or for that matter, to accomplish anything. It concerns the mind listening to the muscle for what it requires for growth and sustenance, while the mind takes this information, processing it like the supercomputer it is, for optimal efficiency with the task at hand. It is the muscle “thinking” of new, innovative ways for stimulating growth, as the mind logically and systematically determines the best methods for safety and contraction. It is not accepting failure when a certain lift or weight doesn't go according to plan. It accepts the challenge, breaking down methodically the requirements to get stronger and break that plateau. It's sublimity at its core!
The spark driving all of this concerns the impulses not only of the muscles and the chemistry behind their function, but the sheer desire, the sheer will to break through any barrier, to dutifully accomplish any goal with the utmost intensity. It’s re-assessing a failure and using that as energy, as fuel to keep the ignition constantly driving one to get better each and every day.