Shares of Tivity Health (TVTY) are sharply lower in midday trading, which is being attributed to UnitedHealth (UNH) announcing a new fitness benefit for certain members of its Medicare Advantage plans. Tivity Health operates the SilverSneakers, Prime Fitness and WholeHealth Living networks. The SilverSneakers senior fitness program is offered to members of Medicare Advantage, Medicare Supplement, and Group Retiree plans. Tivity is down 31.5% to $32.90 near noon following UnitedHealth’s announcement.