(pl.t.). To be analyzed as man-ān-, with -ān- as a plural suffix. In most contexts, however, can be equally treated as manān-ū/ī, with a common masc. pl. suff. ū/ī.
PS
*mV(ˀ)n(-at)- - tendon, sinew; muscle
(SED I No. 183)
Cf. *matn- (1) ‘sinew, tendon, nerve’ (No. 191), related with metathesis?