No Progress without "progressive overload"
When it comes to the fundamentals of weight training, the "principle of progressive overload" is the most important, most known and yet often wrongly or inadequately applied training principle. Whereas most gym enthusiasts know that weight has to be added to the bar on a regular basis many just simply fail to implement this knowledge into their training effectively - and therefore miss a vast amount of potential gains.
The principle of progressive overload
But let's recall respectively introduce the concept of progressive overload. Therefore it is necessary to understand why it is important to progressively overload in the context of training. If you are involved into real training, you will always train for a certain goal (like getting stronger, having more endurance or performing better athletically overall). And as you train, you want to progress closer and closer towards your goal. Everything else would just be exercising for the sake of exercising.
According to Hans Selye's (Austrian-Canadian endocrinologist; 1907-1982) model of the General Adaptation Syndrome (GAS), body systems can react to the exposure of an outside stressor with an adaption, as long as this exposure lies within the body's potential of recovery. Adaptation in this context simply means, the body repairs damages induced by the outside stressor and also slightly improves the affected system in order to prevent future damages from a similar stressor. This is one of the human's evolutionary advantages and the reason why we evolved.
In order to come closer to your training goal, your body has to go through plenty of so called stress-recovery-adaptation cycles (SRA). And each time, the stressor has to be increased, otherwise there won't be any new adaptations (=gainz). Increasing the grade of stressor-exposure on a regular basis is, what lies behind the principle of progressive overload.
If for example you are exposed to direct sunlight for 10 minutes (=systemic disruption), your body will react with repairing your skin and creating a tan, which will protect you from 11 minutes of direct sunlight (=adaptation). If the next sunbath lasts 10 minutes too, your tan will prevent any damage to your skin, since the time of exposure lies within your achieved protection provided by your tan. In order to become more tanned, you have to increase the time of sunbathing to an amount that exceeds your current sunlight protection potential (in our example this would be 12-13 minutes).
This principle does also work for weight training. In order to squat, bench or pull more weight, you simply have to squat, bench or pull more weight, than you did before. It's as simple as that.
How to implement the principle of progressive overload into training
Depending on your current training level (beginner, intermediate or advanced) you have to progressively increase the load on the bar. Whereas a novice lifter can increase the load from one training session to the next session (in general 5 kg for squat and deadlift, 2,5 kg for bench and pulls every time), the intermediate lifter will most likely be able to do those load-increases from week to week and the advanced lifter from month to month - as a rule of thumb.
If you are aiming for hypertrophy, but strength is not your priority, than another way to progressively overload would be increasing the reps or the sets of an exercise and thus creating more training volume. Of course, by increasing the load, volume will automatically be increased if all the other training parameters stay consistent.
So obviously it's not enough to bench those 100 kg for 3x5 every other week over months, as often observed in many gyms. In order to implement progressive overload for more gains, I recommend to stick to the rule of thumb mentioned above. Over time you should develop a feeling of how often and how much you can increase the load (or the reps/sets). By consulting an experienced coach who provides you a proper progressively overloaded program and performance tracking, you can get even more out of your training in the first place - faster and more effective than by doing trial and error on your own. Either way, you will get better results by really sticking to this important training principle.
I wish you much success and more gains with progressive overload!