1. The body does not just adapt – it overcompensates.
If the stimulus of the training load is of sufficient strength, the cell/organ/system will respond by fatiguing below its normal state. Almost immediately after the stimulus, the compensation to the fatigue state will occur. This is a period where the cell/organ/system will replenish its energy stores, oxygen, and any other metabolic substrate (Carbohydrate, Fat, Minerals, and H2o). During the compensation period, the reaction is such that it will exceed the normal biological state and greater stores of energy will be available during the next stimulus. This allows the athlete to handle heavier workloads, if necessary.
2. The body needs time to adapt and recover.
The body needs time to recover between training sessions and to plan when one runs hard or easy during the various microcycles or mesocycles. There is also a need to incorporate planned periods of easy running into training. Many athletes think that training hard day after day will make them better runners, but the truth is that a day off may do them more good than another hard day of training. Runners should not confuse the necessity for taking it easy and having a recovery day (or period) with not training because it is raining, too cold, or they just don’t feel like it.