Vestiges of -ˀ- in Syr. rather point to *-ˀ- in the protoform; otherwise a biconsonantal root, with doubling of -n- (and probably with insertion of -ˀ as a second radical).
PS
*mV(ˀ)n(-at)- - tendon, sinew; muscle
(SED I No. 183)
Cf. *matn- (1) ‘sinew, tendon, nerve’ (No. 191), related with metathesis?