Did you know there is an observed law known as the law of diminishing marginal returns?
In terms of exercise, fitness, and strength building, it is what is known as plateauing. It is when the same exercises that initially experienced significant gains in function, slow down with each time they are repeated to the point improvement stops.
Periodization is the process of designing a training program that works around this law to provide sustainable improvement. The idea is to consistently create adaptations through different cycles that include progressive overload. To stimulate adaptation and progress the stimulus or type of training must change. Changes can be very big such as the type of training, say switching running to swimming, or small, changing the angle you do a bench press exercise to stimulate different fibers.
If it is a multi-year plan you could have macro-cycles of 1 year and within them meso-cycles of 4-12 weeks, and micro-cycles of a week.
Principles of periodization include adequate rest which is directly proportional to exercise intensity, age, and larger muscles being exercised. As well as sound nutrition to promote recovery and hormone optimization if needed.
An example of a microcycle might be in one week to train the same muscle group in resistance training with different repetitions to work on strength vs. hypertrophy or, sprinting one day vs running a mile a different day as an extreme example.
A mesocycle can be developed with a specific sport in mind. Athletes will divide it up on season, a pre-season to promote sport specific activity. An in-season for maintenance, preventing injury, and remaining in peak physical condition that is possible. An active rest season immediately following the season, and an off-season to identify baselines and prescribe a program for progressive overload. An example may be to spend 4 weeks on hypertrophy, strength, then power during the off-season workout program. Ideally with the exception of the active rest cycle which can last 1-4 weeks, the above cycles would each be about 12 weeks or so.
For athletes not necessarily concerned about peak competition performance, or for non-athletes – cycles can be divided up in any number of ways as long it promotes adaptation and progressive overload.
The competition for everyone is to improve health, quality of life and disease prevention. This is how we age well. The above principles can only help achieve wellness goals.