The core principle that all training is based on Im, of course, talking about adaptation.
Training is or rather, it shoulder be a systematic process where the body and mind are exposed to stressors of varied volume and intensity.
It forms the basis of program development and is a fundamental principle of a biological organism. This includes us probably not so humble homo sapiens. No adaptation, no survival!
To make progress in strength and physical fitness, we must be exposed to continually increasing stressors. It's essential that these stressors are varied over time to avoid excessive accumulation of fatigue and a regressing in capability.
A strong, resilient and capable body is the result of many years of well planned, systematic training.
"Master the mundane" - Travis Mash
In essence, we training we provide the body with a stressor, this stressor causes fatigue and breakdown of tissues of the body on a microscale. Sort of like vaccinations. We give our selves a small dose of the poison our body then goes about recovering from. This process common to biological organisms is increases the threshold of the body a little, be that in immunity to a disease as a result of vaccination or with increase muscle mass, thicker stiffer tendons and the ability for our nervous system to orchestrate the movements of our limbs with greater precision and control.
It's all in the dose!
Too much of the poison and you can expect fatigue, overtraining and injury. Too little and the stress will be so low that its already within our capability and not cause the adaptation process to take place.
Increasing Stimulus -> Adaptation -> Increase in performance.
If the same load and or volume is used, there will be an increase in performance in the beginning, but without and increasing stressor you will not be able to improve performance further beyond your current state.
Lack of stimulus -> Plateau -> Lack of improvement beyond the current state.
Another essential thing to note is that if the stimulus is overly varied, you will struggle to make meaningful improvements this is because adaptation works both ways. We have all heard of the use it or lose it principle. Physical fitness is something that must be kept up if one wishes to remain in peak condition. Once we stop providing the body with the nessecery stimulus our body with begin to regress to the stressors at hand which are often less than current levels.
Overly varied stressors fail to take advantage of the adaptation cycle as the random frequency means it's hard to make sure that then next stimulus falls during a period of adaptation or maladaptation. Any progress made here is mostly down to luck and not skill.
So when forming your program, make sure to have adaptation as the cornerstone of your principles. Without it, you might be giving a lot of effort with little, not no results.
Unless of course, your goal is to be tired.