The Collaborative International Dictionary of English v.0.48:

musclebuilder \musclebuilder\ n.
   someone who does special exercises to develop the
   musculature; a bodybuilder.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

muscle-builder
    n 1: someone who does special exercises to develop a brawny
         musculature [syn: bodybuilder, muscle builder, muscle-
         builder, musclebuilder, muscleman]