While it is true that the horse only has one foot on the ground at any one
time, the rack is a smooth gait with no suspension. In a racking horse the
head of the rider traces out a stright line. Hence there is no increased
force on the legs cause by the absorbtion of the kenic energy generated by
the suspension of the trot. So as far as stress it is probably a wash.
The true rack is a fast gait. My walking horse can rack between 10 and 15
mph - this is fairly common for a rack. If a troting horse has a
predisposition to do this gait there are two places that will most likely
happen. Pushing the horse on at a trot while maintaining contact can cause
a transition into the rack. Riding a fast trot down mild downhills can
also cause a transition into a rack.
Truman
Truman Prevatt
Sarasota, FL