agonist
ag.o.nist \'ag-*-n*st\ n [LL agonista competitor, fr. Gk ago-niste-s, fr. ago-ni] [back-formation fr. antagonist]zesthai to contend, fr. ago-n 1: one that is engaged in a struggle 2: a muscle that is checked and controlled by the opposing simultaneous con traction of another muscle