Canterbury-based Amy's Challenge, who has run the fastest Beyer Speed Figures of the year for a 2-year-old filly, has been ruled out of Keeneland's Grade 1 juvenile stakes this weekend after an episode of tying up, a condition causing severe muscle cramping, trainer Mac Robertson said Tuesday morning. Amy's Challenge romped in her debut at Canterbury by 16 1/2 lengths, earning a Beyer of 91, and posted a three-quarter-length score in the Shakopee Juvenile Stakes on Sept. 16, earning a 92. The filly arrived at Keeneland last Wednesday for a tilt in either the Grade 1 Alcibiades against her own sex on Friday, or the Grade 1 Breeders’ Futurity against the boys the following day. But her first trip to the Keeneland track brought on an episode of tying up, the common term for equine rhabdomyolysis syndrome. The affliction causes severe muscle cramping, particularly in the large muscle groups along the back and hindquarters. Episodes, which can be sporadic or chronic, can occur due to stress, fatigue, or sometimes electrolyte and mineral imbalances. "She came here and she tied up - she was sore behind, and her numbers [on a blood test] were really high," Robertson said. "I think she just got stressed and she tied up a little bit. She'll walk a couple days, and then just take it easy for a couple days, and then we'll draw blood on her again. "Canterbury is a beautiful place - it's quiet. Here, there's a little more noise, and a different surface. And maybe she's had 11 1/2 months of training and she needs a break. That could be it, too. Most things with Thoroughbreds, it's stress." Robertson didn’t rule out running Amy's Challenge in the Grade 3 Jessamine Stakes on Oct. 11 on the Keeneland turf, but any future plans for the filly are pending the results of another blood test and her general attitude in the coming days. Robertson also didn’t rule out putting the filly away for the year. "She won't run this week," he said. "Maybe next week in that grass race, if her numbers come down. We'll see. She ran really hard last time - she was a little tired after the race. Maybe she's had enough for a while." Robertson stressed that his main goal in the upcoming weeks would be to do the right thing for his charge’s long-term health and career prospects. "Anytime you can't run in a big race, it's disheartening, but I never feel bad for doing the right thing,” he said. “You can always cheat horses, but eventually it cheats you."