I know that my children are told in school that halakhah requires that they stand for the Shemoneh Esrei with their legs perfectly straight and their feet together with heels and toes touching, but I am wondering where this actually comes from. Thus question arose when one of my children asked if they could stand with their feet slightly apart because it is easier for them.
Is it a custom? Is it required by the halakhah or a dictate of kabbalistic practices? What is the origin of this practice?