Muscle failure

The growth of muscles is in reality the breaking down of our muscle fibers. The fibers are torn when stressed, and then the tear is filled in with organelles that form additional muscle protein strands. When working out to increase muscle weight, you want to tear your muscles (yes, it sounds pretty gruesome). The best way to achieve that is by reached muscle failure. That is when your arms, lets say you are working on your biceps, cannot lift the weights no matter how hard you try to complete that rep. Essentially, what you have to do is challenge your body. When you think you cannot make one last rep, just try pushing a little harder or get a spotter. This helps to keep your body growing instead of keeping yourself in a "maintenance" workout.