This video introduces the concept of progressive overload in fitness training.
Progressive overload is highly related to the theory of stimulate-recover-adapt. The overload event is the stimulus that causes the adaptation. Adaptation is cumulative and therefore in order to continue to adapt, and make progress, the overload event must be increased. It can't be increased too much or else we'll annihilate instead of stimulate - this is the idea of progressive overload.
The overload event must be sufficient to cause a disruption in homeostasis, but it can't be large enough that it can't be recovered from. Later, we'll talk about maximum recoverable volume. For this introduction, it's important to know that we'll train inside a range of progressive overload.
For strength, we'll train with sets of 1-5 reps at 70-95% intensity at 10-20 sets per week. For hypertrophy we'll train at sets of 6-15 reps per set, 15-30 sets per week, and 60-75% intensity. Endurance training will be done 3-5 times per week at 60-95% of intensity. Intensity is measured in weight for strength and hypertrophy and heart rate for endurance.
There's different ways of causing overload events. We increase the intensity, sets, reps, decrease rest time, change the range of motion, alter the exercise, increase the time under tension, and increase the frequency.
You've got to use periodization to ensure that you can train across these ranges while managing fatigue (another topic for later) and ensure you're training hard enough. Smart training is hard training.
There are specific overload considerations for fat loss and general work capacity. Maintaining a caloric deficit is important for fat loss, but there is a healthy range in which this can be done. Constant variance will also aid in increasing metabolic after burn and causing stress that will lead to adaptations in your work capacity.