Those who have any part or organ of the body in a defective or weakened condition, which has led their physician to forbid their imposing any strain upon it, should consult him or her before practising any of these exercises. This is because the latter do achieve their results by imposing strains.
Those whose advancing age suggests a similar carefulness may, with their physician's prior consent, take up the easiest only of these exercises. But they ought to proceed toward mastery very patiently and by very slow degrees.
-- Notebooks Category 5: The Body > Chapter 5: Exercise > # 140