 So you need to keep in mind, when you're doing these exercises, that you have to always be moving under control. Again, with the example of picking up or attempting to pick up a car. If you take the weight and you try to yank or jerk at it as fast as you can, you're going to pull or strain something. If on the other hand you attempt to lift it gradually, and as you approach the end of the lifting movement, you gradually, so to stop, you reverse direction, you bring it back down, you anticipate reaching the start, you slow down for that so that you're able to reverse direction smoothly. You'll be able to lift as heavy a weight as your muscles are capable of, but you're not going to pull, strain, wreck or damage anything in the process. In a healthy person, your tendons, your ligaments, your other connective tissues, everything will be able to more than adequately handle the heaviest weight that your muscles are capable of contracting against, as long as you're moving in a strict and controlled fashion. The other thing that tends to stop people from pushing as hard or pulling as hard as they possibly can during exercise is physical pain or fear of physical pain. For most people, the exercise stops when it starts to become uncomfortable. If you look at a lot of people in a gym, if a person is really going all out, by the time they get to the point where they are at their actual physical limits, they will have slowed down considerably. A person who can comfortably curl at a moderate pace if they're using a heavy enough weight, their last repetition is going to look like this, they're just going to be barely moving it. Watch a lot of people, their speed doesn't change too much, they get to a point where it starts to get uncomfortable, they set it down. The problem with that is the point where you start to become uncomfortable is those last couple of most productive reps where you reach that point Doug was talking about where you're sending the message to your body that it is about to die or it thinks it's about to die. If you stop when it starts to burn, if you stop when it starts to become uncomfortable, you are stopping right at the point where the real exercise begins. Now again, keep in mind that you're not going to injure yourself if you're moving in a controlled manner. The burn is not an indication that any kind of actual large-scale physical damage is being done. Now there is damage, very, very small damage. You have little tears, it's called microtrauma, structural level. The lactic acid Doug discussed is going to have some effect on that discomfort too, but there is no kind of real injury occurring. Now something that helps is if you keep in mind the difference between pain, which is informative, pain is basically telling you that, hey, something is wrong and doing so so that you stop doing whatever is causing the damage and what is called exertional discomfort, which is actually what's happening during exercise, which is non-informative. It doesn't tell you anything you don't already know. Your muscles are working hard, okay? We already know that, but again, there's nothing happening there that's going to cause any damage. You can safely ignore that.