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Standing while counting:
 Admur 489/4; Michaber 489/1; Rambam 7/23; Yireim 261; Rosh Pesachim 10/41
 Michaber ibid writes “One needs to count standing” however Admur ibid writes “One needs to bless the Sefira standing”. This is based on different wordings used by the Rishonim ibid. Vetzaruch Iyun!
 The reason: The verse [Devarim 16/9] states “Bikamah Tachel Lispor”, and the Sages expounded that this word should not be read Bekama but rather Bekomah which means upright, and the verse is hence saying that one is to say the blessing while standing. [Admur ibid; Taz 489/2] See Minchas Elazar 1/48 and Piskeiy Teshuvos 8/4 footnote 30, 34 for sources in Geonim and Rishonim for this Derasha, although it is not mentioned in the Talmud.
 Admur ibid; M”A 489/3; Rambam ibid; Chok Yaakov 489/5; Elya Raba 489/2; Chayeh Adam 131/4
The reason: As the teaching from the above verse is only a support used by the Sages for their decree [i.e. “Asmachta] and it is not of actual Biblical weight. [Admur ibid; M”A 489/3; Kesef Mishneh on Rambam ibid]
 Based on Admur 585/1 regarding Shofar and the same would apply here by Sefira, as the entire law of standing by Shofar is derived from Sefira!; Rivivos Efraim 1/129; Piskeiy Teshuvos 489/4; See however Daas Torah 585/1 in name of Rivash, Chacham Tzevi and Birckeiy Yosef 585/4 that it is permitted to lean in all cases that sitting does not invalidate the Mitzvah
 The reason: As in such a case this leaning is not defined as standing but as leaning. [Admur ibid; See also 607/7]