There is a unique law regarding the Amida: it must be recited while standing. When one stands, she summons her complete being, from head to toe, to prayer. In addition, standing expresses the reverence and awe felt toward the King of the world. Therefore, one must not lean against anything while reciting the Amida, for anyone who is even partially supported is not standing in awe. In extenuating circumstances, for instance, if one is weak and must lean against something, she should try to lean only slightly, such that if the support should be taken from her, she would remain standing on her own. In that way, although she is not standing in fear, she is at least considered to be praying in a standing position (SA 94:8; MB 22).
One must put her legs together so that they look like one leg. The reason for this is that the separation of one’s legs exposes one’s material side and represents the pursuit of worldly matters. Thus, we keep our feet together in prayer just like the kohanim who, in their ascent of the altar, would walk heel-to-toe to avoid spreading their legs. Furthermore, putting one’s legs together symbolizes the annulling of the powers in one’s legs, demonstrating that we have but one desire, to stand before Him in prayer. The Sages learn this from the angels, of whom it is said: “Their legs are a straight leg” (Yeĥezekel 1:7), meaning that their legs were placed so close together that they appeared as one leg (Berakhot 10b; y. Berakhot 1:1; see Maharal, Netiv Ha-avoda §6).
One must put the entire length of her foot next to the other so that they will seem like one leg to the extent possible, and not merely put her heels together (SA 95:1; Talmidei Rabbeinu Yona). However, be-di’avad, if one prayed with her feet apart, she still fulfilled her obligation (MB ad loc. 1; Kaf Ha-ĥayim 2). Someone who has trouble putting her feet together fully should place them together to the degree possible.
One who is ill and cannot stand may recite the Amida while sitting. If she is unable to sit, she may pray while lying down (MB 94:27; Kaf Ha-ĥayim 34).
Even one who must recite the Amida while sitting or lying down should try to put her feet together and bow her head at the appropriate places. When one sitting in a wheelchair finishes her prayer, she should wheel herself slightly backwards, approximately the distance of the three steps with which a healthy person would take leave of her prayer (based on Rema 94:5).