The moving parts of our skeleton operate as levers, powered by skeletal muscles. Levers are nifty mechanisms which make force application more efficient. Using a crowbar is easier than grunting a boulder out of the dirt.
It's high-school physics applied to the body: Levers are classified according to the relative placement of the fulcrum, the resistance and the force.
Any form of exercise can be made more interesting by thinking about the sort of lever action applied in single-joint exercises (like bicep curls or calf raises) or the multiple lever actions applied in complex, multi-joint movements like squats or running.
If you are fortunate enough to use the services of a professional trainer, that trainer will not be focusing on your muscles. He or she will focus on the mechanics of your movement(s) - proper lever use - to make your motion or activity as safe and effective as possible. That's why a trainer will make frequent corrections to your technique: Palms out (or in), wider stance (or narrower), head up (or down), chest up (or down) etc.
Even if you are not a fitness person, everybody knows that there is a right and wrong way to pick up something heavy, or to get out of a chair. It's all leverage.
