A persistent back ailment has raised doubts about the future of reigning Horse of the Year Tiznow.The son of Cee's Tizzy had been diagnosed with a bad back in mid-April, with the injury, pinpointed under nuclear scintigraphy, located in the lumbar section of his vertebral column. Though adequate rest and a positive follow-up scintigram allowed 4-year-old Tiznow to resume jogging a month ago, trainer Jay Robbins still noticed signs of discomfort as the colt's training increased."When we started galloping him," Robbins said, "he didn't really want to leave the pony. He doesn't gallop like himself. He had trouble extending himself."Consequently, Tiznow went in for yet another nuclear scan the morning of June 19. The results were clearly encouraging, leading Dr. Rick Arthur, Tiznow's veterinarian, to gather that the trouble now exists in the colt's lower back muscles."In my mind, the way he'd been moving, I really thought that the scan yesterday would show that that vertebrae was in trouble again," Robbins said Wednesday morning from his Santa Anita barn. "It looks great. That's what makes Dr. Arthur think it's a muscular problem."Arthur has prescribed a muscle relaxant for Tiznow, and Robbins mentioned that the colt will continue to train lightly. Tiznow jogged 1 1/2 miles on June 20 and is scheduled to gallop tomorrow morning. Owned by Michael Cooper and the family of the late Cecilia Straub-Rubens, Tiznow won the March 3 Santa Anita Handicap (gr. I) in his last start."It's a bit of a relief, not a huge relief, because I still see him this morning jogging virtually the same way," Robbins admitted. "We're just playing it by ear. We should have a better idea in five or six days if it's getting any better."