The phrase "No Pain, No Gain" has been around since I was a kid. Probably longer.
We all know it - to gain from exercise, you've got to hurt. If you don't work hard enough to feel that "burn" or feel sore or just ache from lifting a heavy weight, you aren't getting anything.
It's not true.
Let's look at it one part at a time.
First, "no pain." No pain implies that pain is required. Pain, in fact, is good. You have to feel the burn, feel the ache, suffer from some pangs of pain before you get anywhere. "Feel the burn." The "burn" is just muscular fatigue from lots of repetitions. It's not much more than just irritation. It is an indication of hard work, but not necessarily of good work. In fact, pain tells you one thing - you've overdone it. You've gone too hard. If you work out until you feel acute pain, you've injured yourself. And in fact, you can go too hard and suffer overtraining - a syndrome that results from going too hard over and over again, causing you to lose fitness and show a decline in results. Post-workout soreness is more of a sign of novelty (you did something your body isn't adjusted to yet) than of a good workout. Don't get me wrong - soreness does mean you worked hard. But lack of soreness doesn't mean you didn't. Some exercises, like dragging a sled or pushing a car are notoriously hard work but for various reasons don't induce a lot of soreness (no eccentric portion, which means no deceleration of the weight). They're still effective exercise.
So pain is bad, and pain doesn't indicate a good workout. But what about the "no gain" part?
Second, "No gain." The straight-out statement here is that unless you get the pain we discussed earlier, you won't gain. If it doesn't hurt, if it's not crushingly hard work, it's not going to help. That's bunk. If you apply progressive overload to your system - you do more work today than you did last time - your body must supercompensate and make you more able to handle that workload. It doesn't matter if it hurt or not, you felt a burn or not, or if you went as hard as possible or not. It just has to be more work - more reps, less rest, or more weight, or some combination of all three. It doesn't need pain, soreness, or misery. It just needs to be more than last time.
It's a pretty simple statement. It sounds so clear and true - no pain, no gain. But it's more like "No hard work in excess of last time's outing, no gain." Quite a mouthful, huh? And even that stops being true when you're an intermediate lifter or advanced lifter.
Beware of the simple weightlifting truism. They often are simple, but not true.