If you mean after you lift 10,20,40 pounds after 10 sets of 30 (or whatever) you finally get so worn out that you can't even lift a 5 pound weight once; That's normal for a beginner. It's the lactic acid building up and the muscle being broken down. That shouldn't be your sole technique. Conditioning your lactic acid to hit a higher threshold is a part of working out most big tough bro-dudes miss. The opposing technique is moving through heavy weights. Instead of doing a total of 100 reps, you might do 5x5. 3x3 and keep adding the weight until you get to 1 of 1. You get through the day being able to lift 5 pounds but you're sore the next day and you grow mass. It has to be a mix of the two to get "big." Your left arm gives out quicker because you don't use it as much. It isn't conditioned from everyday life the way the right one is.