Remember that for all of the above guidelines, no matter whether your goal is strength, size, or endurance, you should be overloading your muscles for each set. If your muscles are not being subjected to overload then you are just going through the motions and wasting your time. So the obvious question then, is how can you make sure that you are overloading your muscles for each set? The best way to achieve overload is to lift until failure. In other words, this means that you should lift until you are unable to complete another repetition with proper form. If this happens you have exhausted your muscles to the point where they can't continue, and there can be no question that you have subjected them to overload