Undulating periodization is also known as nonlinear periodization
The undulating design is based on the concept of Hans Seyle’s General Adaptation Syndrome. G.A.S. explains the way your body restores itself to balance, or homeostasis, when faced with stressors.
With undulating design, there is enough variation in stressors to continually make progress without allowing your body to fully adapt to all the stressors taking place. All this while still accounting for the recovery or restoration needed.
In undulating design, the stimulus is varied either within a weekly model (WUP) or in daily undulating periodization (DUP) where daily changes are made to either volume or intensity.
Some studies have shown this modeling can be more favorable for increases in strength gains than in typical linear modeling in well-trained athletes. Studies also purported that DUP may be more beneficial for elite athletes as it helps them avoid the dreaded plateau effect that can happen in well-trained individuals.
DUP modeling has also showed a favorable increase in strength gains and CNS adaptation without the added muscle mass, which could benefit athletes in groups where weight classes are of importance.