Orlick/Weider: Abdominal Exercises for Strength and Power (1944)
The following exercises are of a strenuous nature and should be performed only by those who are already quite advanced in their training. Where number of repetitions are not given, repeat exercise until you have to strain and at this point stop and rest. Do this three or four times in succession with each exercise. In this way your body will act as its own barometer telling you pretty well what you can and what you cannot do. As the abdominal muscles grow stronger you will naturally be able to do more repetitions and at no time will you be forcing yourself beyond sensible physiological limits.