The practice of any physical yoga posture will necessarily be difficult in its early stages because it throws the body into unfamiliar and unaccustomed positions. The muscles need to be re-educated little by little. It is dangerous to try to force oneself into such a posture all at once. Therefore, the exercise should be done for a few seconds only at the beginning, and the period extended by a few more seconds after several days, and further extended after a few weeks. In any case, it must be followed by a rest period before being repeated.
-- Notebooks Category 5: The Body > Chapter 5: Exercise > # 75