Periodization involves planned variation in the volume and intensity of training; for instance, dividing a training program into cycles or phases. Ensuring variation in volume and intensity of training is important for optimal strength gain. Hans Selye’s General Adaptations Syndrome describes three phases of adaptation, shock adaptation and staleness (also known as alarm, resistance and exhaustion).
Shock occurs after initiation of novel stimulus in which case your body suffers from soreness and decreased performance. Adaptation occurs during repeated training exposure leading to increased performance. When your body has fully adapted to the new program, you reach the staleness phase, a time when the same stimulus no longer produces further adaptation; as a result, performance may plateau. To ensure further increase in performance, rest or a change in stimulus may be required.