-u- in Akk. (alongside with -i-) and Arm. (Syr. and Mnd.) is likely a secondary development under the influence of b-.
For the meaning ‘lap’, see *barak- ‘chest, thorax’. See discussion in [Holma 1911 95] where the author tends to treat birku ‘knee’ and birku ‘genitals’ as the same word, quoting, at the same time, Arb. rakab- ‘pubis’ as a possible cognate to the latter.
bäräk (< *bark-; suff. birk-). Note Hbr. pB. bōräk ‘knee-shaped shaft of a plough’ [ibid.].
Biblical Aramaic
birkōhī - (du. suff. 3ms) knee
(HALOT 1839)
Jewish Palestinian Aramaic
du. bärkayin
Jewish Babylonian Aramaic
Syriac
burkā - knee; bowing down, genuflection; middle part of plow
(LSyr. 96, SL 131)
Mlaḥso
berko - Knie (knee)
(Jastrow 1994: 170)
Turoyo
barko - Knie (knee)
(RW 56)
Barwar
bǝrka - knee
(Khan 2008: 1248)
Hertevin
berka - Knie (knee)
(Jastrow 1988: 183)
Formed after the active participle pattern; strangely enough, usually not quoted in comparative studies (see [DRS], [LGz]). Cf. also brk ‘s’agenouiller’ [BK 1 116]. As for bark-, birk- ‘genou’ quoted in [GD 153], it is not clear whether it is Daṯ. or Mhr.
Harari
bərḫi - the unit between two joints (in a finger, sugar cane)
(EDH 45)