Training programmes differ enormously in their details and share one principle underneath. Understanding it explains why almost every credible programme looks structurally similar despite the disagreement about specifics.
Adaptation is a response to insufficiency
The body maintains only the capacity it currently needs, since tissue and metabolic machinery are expensive to build and to keep.
A demand that exceeds current capacity registers as a shortfall, and the repair and rebuilding that follows overshoots slightly, leaving a little more capacity than before.
A demand comfortably within existing capacity produces no shortfall and therefore no signal, which is the mechanism behind an unchanging routine producing unchanging results.
The demand has to keep moving
Once capacity has risen, the previous demand is no longer sufficient, so the same session that drove adaptation last month is maintenance this month.
Progression can come through load, repetitions, distance, pace, range of movement or reduced rest, and these are alternatives rather than a hierarchy.
Which is why programmes for strength, endurance and mobility all specify some increasing variable, however different the activities look from the outside.
Adaptation happens between sessions
The session creates the signal, and the rebuilding occurs during the hours and days that follow, given sufficient rest and adequate protein and total energy.
Training again before that process has run leaves the previous session incompletely consolidated, and repeated across weeks this accumulates rather than resolving.
Which is why programmes prescribe rest days and lighter weeks as part of the design rather than as concessions, and why more frequent training is not straightforwardly better.
The margin is narrower than it sounds
The demand must exceed current capacity enough to signal and not so far that recovery cannot keep pace with it.
Too small an increase produces slow progress, while too large an increase produces excessive soreness, missed sessions and elevated injury risk, particularly in connective tissue.
Tendon and ligament adapt more slowly than muscle, which is why people whose strength rises quickly often meet an injury rather than a plateau.
Why progression stalls in practice
Plateaus are frequently read as a need for a new programme, when the more common causes are inadequate sleep, insufficient food, or progression that quietly stopped.
Programme variety has genuine value for interest and for training different qualities, but novelty is not itself a stimulus for adaptation.
Anyone training around an existing injury or a diagnosed condition should have the progression set by a qualified professional, since the tolerable margin in that situation is narrower and specific to the individual.