Ok after reading practical programming, below in my own words is some of what I think I understand from reading the book. But I’m not sure if I’m correct. Could you comment?
Training programs work because they push the individual to achieve more without overtaxing recovery. Through Volume, Intensity or some combination of both a stimulus is applied to disrupt homeostasis and force adaptation. After the Stimulus is applied there must then be enough recovery time given to allow some adaptation to occur so that an even greater stimulus than the previous one can be applied. Recovery does not have to be complete but needs to be enough to apply a greater stimulus than what has been experienced before.
The closer one is to their genetic potential (i.e. the stronger someone is) the greater the application of stimulus (training stress) will need to be; which will make more time to recover necessary. At some point the time for the body (Comprehensive recovery processes) to recover from the stimulus will be greater than the time for the muscles to recover (Metabolic and structural fatigue). This is where it is necessary to vary the level of training stress so that there are times when enough stress is applied to completely disrupt homeostasis, while at other times apply only enough stress to keep the muscles (metabolic and structural fatigue) from regressing in adaptation while not interfering with the body (comprehensive recovery processes), and still at other times allowing for full recovery.
This is where the concept of “light”, “medium” and “heavy” periods comes in. In training everything should be preparing you to apply that one stimulus that is greater than previous stimuli experienced in order to drive up adaptation. After this “heavy” stimulus is applied adequate recovery must occur, however the time it may take to allow enough recovery to occur to again apply a stimulus greater than before may cause detraining at the muscular level (Metabolic and structural fatigue) so “medium” and “light” days are needed to stimulate the muscles without hampering the body’s (comprehensive recovery process) ability to recover in preparation for the “heavy” day stimulus.
The challenge to programming/periodization is knowing when to apply “light”, “medium” or “heavy” to balance homeostatic disruption and recovery for a particular individual in order for them to make continued progress as quickly as possible without overtaxing their recovery ability and cause them to plateau (get stuck)
Every program that works, works because it balances training stress and recovery in the way described above.