Progressive overload put simply is the slow increase of the work done during a workout. This increase can be done via the weight lifted, the repetitions per set or the sets themselves.
The body yearns to be in a state of homeostasis, if you push it past its normal point it will get your muscular system and your nervous system to a point where it returns to being comfortable. So if you were to do 3 sets of 8 of 60kg on the bench press your body would get used to this quickly (assuming its not too heavy for you) so you could either : - increase the weight by a small number, or the repetitions. For example you could either do 3 x 8 at 62.5kg on the next workout, or you could do 3 x 9 at 60kg on the next one.
If you were to follow the repetition increase I would go with adding a rep each week until you hit sets of 12, then you would increase the weight by 10% and go back to sets of 8. This simple progression scheme is what I did when I first started and it worked remarkably well for a good while. The reason this works is that by adding one rep to your set you are, theoretically, increasing your one rep max by 2.5% each time - hence the 10% increase.
Don't be fooled by it being called an overload, you do not want to make big jumps too suddenly. This can lead to injury or overtraining and either would be detrimental to your training.
Steven J. Fleck and William J. Kraemer, Designing Resistance Training Programs 2nd edition
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