Probably, *-ˀ- in the protoform; otherwise a biconsonantal root, with triconsonantizing strategy, namely inclusion of the -at- suffix into the root as a third radical.
PS
*mV(ˀ)n(-at)- - tendon, sinew; muscle
(SED I No. 183)
Cf. *matn- (1) ‘sinew, tendon, nerve’ (No. 191), related with metathesis?