The Collaborative International Dictionary of English v.0.48:

Brawniness \Brawn"i*ness\, n.
   The quality or state of being brawny.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brawniness
    n 1: possessing muscular strength [syn: brawn, brawniness,
         muscle, muscularity, sinew, heftiness]