Fitness-fatigue model

The fitness-fatigue model, also called the two-factor theory, explains performance as the combined effect of fitness gained and fatigue produced by training.^{1Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 29.} Fitness changes slowly; fatigue can change quickly.^{2Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 27.} A workout produces a moderate, lasting fitness gain alongside a larger, shorter-lived fatigue effect.^{1Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 29.}

For an average workout, the model roughly estimates that fatigue lasts a third as long as the fitness gain: 24 hours of fatigue would accompany about 72 hours of fitness benefit.^{3Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 30.} Periodization uses this difference: during the heaviest training, both fitness and fatigue rise, and performance dips.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 601.} When training load drops, fatigue clears faster than fitness and performance rises.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 601.}

In practice

• Space sessions so fatigue from the previous workout has cleared while the fitness gain remains.^{3Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 30.}
• Plan deloads and tapers as deliberate cuts in volume that allow residual fatigue to subside, during training and before competition respectively.^{5Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 56.}
• For peaking, Zatsiorsky and colleagues describe keeping the number of sessions while lowering the load in each, with sessions resembling a warm-up in the final days.^{1Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 29.},^{3Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 30.}