If you were to start lifting the same amount every workout, eventually you would find the amount easier to lift, because your muscles have adapted to the required specifications – no less, and no more. But then, your gains would stop.
If you are to continue building muscle, you need to partake in what is known as progressive resistance training. Training with weights just so happens to be the most efficient method of adding progressive resistance that man has devised… and always will be.
It’s simple: you add a weight which challenges you, quickly (within several sets and reps) causing your muscles to fail, because they simply can’t lift it any more.
Progressive resistance training is all about pushing to your current capabilities. It’s about maximum effort during a lift/exercise until you simply can’t lift any more, or your technique starts to falter because of muscle fatigue. At this point, and only at this point, have you given your body the message you need it to hear: I need to grow to be able to lift this weight more easily, so grow body, and grow!
Weight training is as simple as that.
The next important thing is sufficient nutrition and rest, as discussed in other articles on the site. You need to nail nutrition and rest, because without them, you won’t grow at all – as progressive resistance in lifting requires progressive dieting and resting.