In order to be efficient exercise must be regular and at relatively short intervals.

All exercise should tend toward using all of the muscles of the body.

In fatigue a person has lost control over his muscles.

The process of getting into condition, therefore, is directed more toward strengthening the nervous system in its control work over the muscles rather than in increasing sheer muscular strength.
Pure creative mental work, although requiring no out-put of physical energy, is perhaps the most productive of fatigue.

The brain gets more blood during physical activity and waste products are much better removed.