The two may be set to face off on Dec. 8 at UFC on Fox 5, but that doesn’t stop the verbal war from happening now. The ‘Twitter beef‘ kicks off as MacDonald tweets an Instagram (pic above) of his new found muscle. Penn, 10 years his senior, responds with a message that reminds us that this is experience versus youth.

The verbal volleyball is left in Penn’s court as of now. For Penn, the match comes after almost a year layoff due to a short lived retirement after a loss to Nick Diaz in Oct. 2011. MacDonald will be returning from a TKO victory over Che Mills at UFC 145. The two were originally expected to face off Sept. 22 at UFC 152, but a severe forehead cut kept MacDonald from the bout, pushing it back to Dec.