wish I knew where my paper was... for my prevet class in college I wrote a term paper about the increasing instances of breakdowns in racehorses in the US v. UK (if I remember correctly) due to over protecting while overcondition on over-controlled situations.
In short - more horses break down bc in the US we overprotect - wrap legs, don't t/o with others if they get turned out at all, papmer them, etc.
We condition them in overly controlled situations - may growing racehorses never see a wet or muddy track, and uneven field, or get outside to do anything other than go around on a nicely groom well footed track in perfect conditions.
We then overcondition them with repetitive workouts under the same conditions causing stress injuries. I.e. Stress fractures and weakening of the stay apparatus for the constant work into the point of fatigue without the proper variation of conditions (think how a a good athlete regardless of sport cross trains rather than training in just one area). Without varying conditions and movements, the horse's bones can not remodel properly for density during growth and the stay apparatus can not be stressed to strengthen because the same parts are being constantly used without time to recoup or be used in varying ways.
In ireland at a race barn my friend worked at, colts were galloped through fields, over hills, rocky conditions, various footing, and turned out with others. This allows different movements - lateral, vertical, and differ load bearing on joints, bones, and stay apparatus to allow the horse's structure to remodel throughout growth. Raising our US horses in a bubble leads to that bubble bursting in the all too common form of a tragic - and often very dramatic - death.
Eight belles did not break her legs from a misstep on the track. If you watch her video very closely you will find that her stay apparatus goes first - starting with suspensory, then superficial and deep digital flexors, and only THEN after her stay apparatus is gone do the bones break. Of course - because they had nothing left to hold them in place. This was likely caused by too much stress on bones in the same repetitive situations (ever get shin splints? Yeah that), coupled with no dynamic workouts to strengthen the supporting soft tissue of the lower legs (below the knee horses have no muscles and rely solely on ligaments and tendons which require a significant amount of work in the form of direction changes, playing, different footing, etc. to truly strengthen.
Was eight belle's death her owner or trainer's fault? Not entirely imo. The issue is with the standards we set for racing and how we teach our trainers to condition horses. Are there always going to be freak accidents? Yes. Barbaro was a true misstep due to being clipped by another horse. The best conditioning in the world couldn't prevent that. But we certainly can reduce the instance of situations such as eight belles if we did a better job of educating our trainers and horse racing professionals on the physiology and performance of the horse specifically as it relates to the muscular and skeletal systems rather than turning them loose to try and win money....