A while ago we commented on a study out of New Zealand from the team headed by Dr CW Rogers (no relation to the author here!) that looked at the risk factors associated with getting a horse to its first public appearance, in this case a barrier trial (published gate work ~4f).
The same team has just released another paper in the American Journal of Veterinary Research on the relationship between yearling exercise (total hand walking time and mechanical walker time accumulated during sales preparation) and interruptions during race training in Thoroughbred racehorses. The results of the paper, which does state that it does need further research, are probably not what sales consignors around the world would wish to hear.
The results identified an association between early exercise during sales preparation and decreased risk of voluntary interruption, defined as a stop on training where no known condition or disease was identified. The bad news for sales consignors was that early exercise during sales preparation showed an increased risk of involuntary interruption during training of 2-year-old Thoroughbred racehorses with involuntary training interruption defined as due to presence of a condition or disease. The study went on to add that it was "longer cumulative distances at a canter" when the horse was broken in and on the track that were significantly associated with decreased risk of involuntary interruptions.
It seems counter intuitive that early exercise at the yearling stage wouldn't help a horse mature and run at two, but this report would indicate that it does more harm than good. As I said, it is a report that needs further verification and understanding, but it is an interesting one nonetheless.
You can read the abstract to the paper here.