St Theophan the Recluse, in his invaluable handbook to the spiritual life, The Path to Salvation, writes with great clarity about the nature and practice of bodily exercises in the ascetic life:
The body is pure. Therefore we must not only estrange from it unnatural cravings and strengthen it in those things which are natural to it; in other words, we must return it to its natural state. Beside this, the body should assist the soul as its constant companion. Therefore, besides returning it to its natural state, we must turn the very satisfaction of its basic needs to the benefit of the soul and spirit. In satisfying these needs, some sort of exercise should be assigned to each bodily function as another means of healing our fleshiness, thus benefiting us spiritually as well.These are harsh and, for many of us, unwelcome words; we aren't likely to find them in the average self-help book. But bear in mind that, as St Theophan clearly states, and as I've written earlier in reference to the life of St Anthony the Great, the ascetic life is not bodily mortification so much as it is bodily revitalization or revivification. But this involves real work and real discomfort. We trust, however, that the reward will be worth the effort, to say the very least; and if we do it truly out of love, it will involve hardly any real effot at all, as God will aid and assist us by His Divine Grace.
Here are the prescribed rules:
1) For the senses: guard the senses altogether, especially the hearing and vision (nervous system). 2) Guarding the tongue. 3) Abstinence and fasting (the stomach). 4) Moderate sleep and vigilance (the stomach). 5) Physical purity (the stomach).
For the body in general. Wear out (muscular), constrain (nervous system) and emaciate yourself (the stomach). It is obvious how through these ascetic practices the body little-by-little returns to its natural state, becomes alive and strong (muscular), bright and pure (nervous system), light and free. It becomes a most capable instrument of our spirit and a worthy temple of the Holy Spirit. (p. 243)