How much weight should you be lifting?
The oldest known lifting regimen is that of Milo of Croton, a champion Olympic wrestler who lived in the 6th century BC. He trained—or so the story goes—by lifting the same calf over his head every day until it became a full-grown cow. There’s a lot of myth in that, but the underlying principles are true: A) the more you lift, the more you’re able to lift; B) you gain strength by gradually and consistently increasing the weight you are lifting; C) results take time.
With free weights particularly—dumbbells, barbells, kettlebells—the instinct is that heavier is the better, more heroic choice. The reality is that the muscle doesn’t know the weight you’re holding, and it’s not impressed with whatever number is on the side of it. The aim is to use the weight that will get the right response out of the muscle. You want to stimulate it, not annihilate it. The deeper you can stretch a muscle, the more potential you’re going to have for muscle fiber recruitment, muscle breakdown, muscle growth, and strength development.
Weight—that number on the side of the dumbbell—is secondary to range of motion and tempo. Lifting more weight will develop more muscle, true, but only when the muscle is properly loaded and working hard throughout the full range of motion. The basic rule of thumb is that you should be lifting a weight heavy enough such that you can complete the reps, but just barely: you get them done, but you couldn’t do another one.