Newton's Laws 1. An object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless acted upon by some force.2. Force=mass × acceleration 3. Every action has an equal but opposite reactionNewton's First Law especially applies here. To perform that squat you have to apply a force to your knees, enough to overcome inertia. (resistance to change) Newton's Second Law is more for calculations, and the third doesn't affect much. You are exerting a force on the ground, though it is minute… more mass=more inertia.