Tigrinya

mnat - arm muscle (Bass. 103)

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?