I’d imagine horse training is difficult because the horse can’t communicate how it’s feeling. Too much money invested to risk pushing the envelope or innovated training and injuring the horse. A human can communicate sore spots and adjust training and therapy to stay healthy. An injured horse and don’t really know unless it’s completely ruined.
I think you are correct that thoroughbreds as a breed are fragile compared to other horses. I don't have an opinion on whether they are more fragile now than 50 years ago. But I think we agree that thoroughbreds need not be fragile. Consider the career of Huaso:
Started as racehorse. No success by age 6; too nervous.
Next was trained for dressage. Sustained a serious injury (not apparently due to training as such) and could no longer hope for success.
Finally trained for high jump. At age 16 set the equine HJ record of 2.45 meters (1949).
Could it be that thoroughbreds started to become fragile when horses were no longer valued for military purposes?
I think you are correct that thoroughbreds as a breed are fragile compared to other horses. I don't have an opinion on whether they are more fragile now than 50 years ago. But I think we agree that thoroughbreds need not be fragile. Consider the career of Huaso:
Started as racehorse. No success by age 6; too nervous.
Next was trained for dressage. Sustained a serious injury (not apparently due to training as such) and could no longer hope for success.
Finally trained for high jump. At age 16 set the equine HJ record of 2.45 meters (1949).
Could it be that thoroughbreds started to become fragile when horses were no longer valued for military purposes?
I wouldn't say that they were fragile compared to other breeds. Most sport horse breeds are quite fragile now. Most jumping horses are contemplating retirement at age 16 and generally need a lot of veterinary intervention to keep them going. But most sport horses also have a lot of thoroughbred in them and are also very inbred.
I'll finish by saying 'too much inbreeding' is something of a myth. To begin with there are multiple measures of inbreeding (at least four, some dealing with close inbreeding close up in the pedigree another, the Ancestral History Coefficient with deep inbreeding) and paper inbreeding (what the pedigree shows) and genetic inbreeding are not identical. There have been great horses with a wide range of inbreeding, from the European horse Coronation V, who was by a son of a stallion out of a daughter of the same stallion, to Secretariat, who was a complete outcross at five generations.
I'm not sure what you mean by saying that "too much inbreeding is something of a myth". Plenty of studies have found that heavily inbred horses are statistically less likely to win races than those which aren't. Those which succeed, like the Arc winner Enable, are very much in a minority.
We all know what is meant by inbreeding. We all know it means lack of genetic diversity. Obviously now in the racehorse, the male line lacks genetic diversity (Secretariat's male line has long since died out) but this has not been due to natural or even true performance selection, its also been heavily influenced by operations such as Coolmore who are such large players on the market that they can actually influence market and breeding trends. I think we can state that we are talking about inbreeding to the speed gene and to the very limited number of stallion in the past 50 years that provide that with reliability.
Are you confusing the criticism of inbreeding with the study by EW Hill, which linked it to higher levels of musculo-skeletal failure? We know how to measure inbreeding coefficient, whether its by blood test or on paper. We know that heavily inbred horses can still do well, but they are the exception to the rule and lead to a lot of "wastage".
What Professor Hill has also done is to isolate the gene which encodes for the protein regulating muscle development and muscle fibre type, basically the C/C gene variant which leads to early maturing racehorses which develop muscle more quickly than their counterparts. Those racehorses are also sprinters running a mile or less. The so-called "speed gene". However, inbreeding in the racehorse is not done particularly well, because genes which influence soundness and stamina are not being preserved and "bad" genes, such as bleeding, are not being bred out.
The horses which you mention - Coronation V and Secretariat - are still far less inbred than the typical racehorse today. Coronation is a particularly interesting example because of course his close up inbreeding to Tourbillion means he is descended heavily from the Byerley Turk, the male line of which has almost been lost now, and is therefore far more outcrossed than the typical modern racehorse of today.
Obviously, the modern racehorse with its closed stud book was already developed from a heavily inbred population - there were only so many of those Arab/Turkomen/Barb imported stallions around. Nevertheless, some commentators maintain that a type if still discernible today from the 3 surviving founder lines, e.g. the Godolphin Arabian type is typically later maturing. We have long since known that the Northern Dancer male line descending from the Darley Arabian is now quicker maturing and more likely to produce sprinters/speed horses.
What is being lost is hybrid vigour. Its not possible to introduce outcrosses and the little remaining foundation line diversity is being lost as we speak. The female line is more diverse but still remarkably small.
Many racing devotees are genuinely worried about the future of the TB racehorse because those breeding lines which produced the champions of the past are long gone, in favour of many stallions who either didn't race due to unsoundness or who had a limited race programme over short distances only. Are we going to end up with another type of Quarter Horse in effect? Maybe one that runs 5 or 6 furlongs instead of quarter of a mile?
The other factor which hasn't been mentioned so far is riding style. Around the 1920s, jockeys began to stand up in their stirrups while keeping their heads low in a crouching position, thus reducing drag and the weight carried on horses' backs during the race. This reduced load and enabled the horse to run more efficiently. This cannot really be improved upon now - we cannot have lighter jockeys and equipment has just about peaked and is restricted by rules to ensure uniformity and safety.
I was talking about the fairly loose use of "too inbred." If we are looking at Inbreeding Coefficients (Wright's Coefficient) that measure relative recent inbreeding, then G1 winners (highest level) have less close inbreeding that G2 winners, G3 winners and average horses.
There are some differences when it comes to the Ancestral History Coefficient (deep inbreeding) and inbreeding to inbred ancestors.
Generally, though, you are absolutely correct - in terms of five or six generation inbreeding horses like Enable are the exception rather than the rule.
As far as the lack of diversity in the male-line, I think that has very little to do with overall levels of inbreeding as the y-chromosome carries very little genetic material.
I'm very aware of Professor Hill's work - I actually know her. With regard to the MSTN work (MSTN = myostatin which controls proportions of muscle type), there is very little conscious breeding based on actually knowledge of MSTN type (CC, CT, TT) especially in the US. Generally breeders in the US, at least, don't know the MSTN make of a given stallion (and some can be quite a surprise, running contrary to what one might have guessed from the horse's racing performance), Certainly CT horses can be found in the ranks of top class horses anywhere from 1200m to 3000m, with other factors (including biomechanical efficiency, cardio and other genes) paying a major part.
Some time on from the original "Speed Gene" work, it was also discovered that the bulky, pure sprinter type is brought about by a SINE insertion in the MSTN gene which is very commonly found in the Quarter Horse. There are CC horses without the MSTN gene that have won the Kentucky Derby.
Professor Hill has done more than one study on inbreeding. One was for the US Jockey Club, which did show that genetic inbreeding (as measured by identical runs of DNA) has increased over recent years. That led to a short-lived bid by the Jockey Club to limit the number of mares that any one stallion could cover (the Standardbred - which has far more recent inbreeding - did successfully enforce such regulation).
The other study that you mention specifically related to dna and cartilage, and that horses with a higher degree of inbreeding (close, first-time identical by descent) were more likely to have the negative variant, and less likely to run.
In terms of "modern inbreeding" that is close up paper inbreeding, Coronation V was far more inbred than Secretariat. At 8 generations her Coefficient of Inbreeding is 14.3 against 1.17 for Secretariat, so she's off the charts high, where Secretariat is low by any standard.
What is interesting is that Coronation V is inbred to Tourbillon himself an inbred horse, which would mean that she scores well on Ballou's Coefficient - the possibility that a random allele has been previously exposed to inbreeding.
In terms of Ancestral History Coefficient, Secretariat does score highly, so he is an outcross close up, but with a lot of the same strains further back.
I will note that elite horse's back from the further in time, even through to the 60s (for example) did have higher degrees of recent inbreeding, but that is because the breed was emerging from a remarkably wide background (for example Sadler's Wells, possibly the greatest European stallion of all time, is from the same mtDNA - female line - as the Shetland Pony and Icelandic horse; and there is at least one example of a cross with a Cart Horse).
Anyway, this has probably all become somewhat esoteric for most readers of this board.
The bottom line is that the thoroughbred reached what is probably the limits of performance in genetic terms about 60 years ago.
Faster times through tracks and equipment hasn't taken place (unlike humans).
Training methods haven't improved a whole lot in that time, and there is a limit to how much you could increase volume and intensity compared to humans.
The thoroughbred is more inbred that it was 60 years ago, but that isn't the cause of the stagnation.