The Collaborative International Dictionary of English v.0.48:

musclebuilding \musclebuilding\ n.
   exercise that builds muscles through tension; bodybuilding.

   Syn: bodybuilding, anaerobic exercise.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

musclebuilding
    n 1: exercise that builds muscles through tension [syn:
         bodybuilding, anaerobic exercise, muscle building,
         musclebuilding]