This article explains and breaks down training theory into an extremely basic form. Much of the information comes from Vladimir Zatsiorsky, a world renowned strength coach.
1. Stimulus Magnitude (Overload)
Adaptation (changes) can only take place when the training load (weight being lifted) is above the normal level. This means that in order to elicit change, the training loads need to be increased, or the exercises have to be changed. For example, to get from 405 lbs on deadlift up to 500 lbs, at multiple points the weight or reps are going to need to push you beyond what you can do at 405. If repping 350 lbs is simple, then the weight must increase in order to force adaptation to get beyond the 405 max. In other words, it requires a larger stress than the 350 lbs. The other way would be to change stances. From conventional to sumo stance. It forces the body to adapt to a different stimulus.
As Zatsiorsky put it, “a response to a constant stimulus decreases over time. If you move from the 405 lbs up to 500 lbs on deadlift, then the 405 lbs will feel much, much lighter. Although it was once extremely heavy, your body adjusts to this and responds differently. Where before a single rep would kill your body for days after, now a single rep is basically a warmup.
Training adaptations are highly specific. If you want to run a marathon, preparing by deadlifting heavy makes absolutely no sense. It doesn’t mean you can only do one thing, but if the goal is to be elite in either, then you can’t do both. Maybe at separate times in life, but not at the same time.
Everyone has different bodies and some are going to be better at certain lifts than others. Some people lose weight fast, some people gain it fast. The important thing is to know your body, and figure out what to do about it. If you are training one way and not improving, then it may be time to switch up the program and try something new.
This is a very generalized approach to each of these subjects, but they all come into play when talking about lifting. In order to take your body to the next level, your brain is going to need to get there first.