In order to rack, a horse has to be in a hollow frame
This is a HUGE misconception, and one of the things that bothers me most about the horse world. I don't have a clue where it came from... maybe so many people allow
their gaited horses to travel in a hollow frame that people started to assume it was necessary? Gaited horses, just like non-gaited horses (except those with natural collection, such as Andalusians) prefer to be uncollected. It's easier for them and they don't understand the long-term effects.
Gaited horses don't need to be in a hollow frame to do any gait except the pace. It's probably possible to be collected at the pace, but it would be extremely
difficult. But the foxtrot, running walk, rack, etc... no. Collection should be taught to all gaited horses. I'm a firm believe in teaching gaited horses to do everything you would teach a non-gaited horse to do. So many people take them on the trail and just gait, gait, gait, that the non-gaited horse world has started to view gaited horses as horses that can't do anything but gait in a straight line!
I rein with my Fox Trotter. She can spin, stop, rollback, do flying lead changes, sidepass, and canter (obviously). She can collect and extend in every one of her gaits. Same with my Paso Fino (except I don't rein with him). I also occasionally trot both of my horses. A good horse (along with a knowledgeable rider) will ALWAYS be able to go back to a gait. As long as you don't do something stupid like do nothing but trot for months and then expect your horse to gait perfectly, it should be extremely easy to get them to gait again. There are certain muscles that are only used in the trot, and that's why it's good to work gaited horses in a trot every once in a while.
My Paso Fino's sire won all kinds of versatility awards and also won Horse of the Year 3 years in a row. He barrel raced, jumped, reined, AND competed in and won Paso Fino performance classes (so his gait was obviously not ruined by all the other things he did).
Here's a Walker with a weak hind end who cannot canter properly: http://www.horsemanmagazine.com/wp-c...ng-horse-1.jpg
He's doing a "tranter," which gaited horses are famous for thanks to people not teaching them balance and collection.
I hate to see things like this: http://images01.olx.com/ui/3/50/32/57094832_1.jpg
This poor horse doesn't have an ounce of muscle in his hind end: http://i25.photobucket.com/albums/c6...HappySmall.jpg
Could this horse be any stiffer? http://www.hitchinpoststables.net/images/100_0572.jpg
And unfortunately, this has become the norm. People see gaited horses being ridden like that, and assume it's the right way. So they ride like that, and so on...
This horse is wonderfully collected, and yet is still performing a perfect running walk (made obvious by the blue ribbon): http://i205.photobucket.com/albums/b...m6919/jpeg.jpg
Anyway, back on topic... My point is that travelling in a hollow frame is bad for any horse - gaited or non-gaited. And any horse can travel in a hollow frame or a collected frame - gaited or non-gaited. It doesn't have anything to do with the rack.
I think you'll find this article interesting: You Can Collect That Gaited Horse | Gaited Horse Training Articles