How much training is too much for a racehorse? New bone research has some answers

The old "more is better" mindset in racehorse training is changing. And with no link identified between higher racehorse training workloads and a trainer's success, there's a big opportunity to change things.

Trainers still largely rely on experience and intuition, tracking the horse's fitness and performance to judge when a horse is ready to race.

Despite this, in Victoria alone, an injury occurs once every 47 race starts—or about two to three injuries every day in Thoroughbred racing.

The importance of reducing such injuries is evident.

Most racehorse fatalities are due to injuries, specifically limb fractures. This presents a risk not only to the horse but also to the jockey riding them. Fewer racehorse injuries therefore mean fewer jockey injuries.

To better guide trainers, our research team examined how much training might be too much for racehorses' bones.

Why training and rest matter

Just like any athlete, horses need to train before competing in a race. Training builds horses' fitness, and being fit is essential to performing well in competition.

However, it also plays a critical role in ensuring their bones are strong enough to withstand repeated impact at high speeds during racing.

Although more training may seem beneficial for improving fitness, it can be harmful to the skeleton.

High workload volumes have been shown to increase the risk of musculoskeletal (muscle and bone) injuries, including fractures.

Bone is a dynamic tissue that continually responds to how it is used.

During a race, horses gallop. This is the fastest way they can run, and during one phase of their stride, all four feet are off the ground at once.

So when each limb hits the ground, it puts around 4 metric tons of load through the bones of their lower limbs with every stride.

Over time, the bone adapts to tolerate this force by changing its density and strength. But during this process, tiny cracks or "microdamage" start to build up.

Bones can repair these tiny cracks on their own. Specialized bone cells constantly break down and remove old or damaged bone while other cells build new bone in its place.

But when microdamage builds up faster than it can be repaired, bone becomes vulnerable to fracture.

This is why resting the horse from intense training is important: It allows the bone to catch up and repair itself.

How can mathematics help with horse training?

It is extremely challenging to determine the degree of injury risk each individual racehorse faces before an injury occurs.

This is where mathematical models can help.

Our first model showed how the bone adapts to different training speeds. We then showed how high-speed training contributes to injury development by accumulating bone damage.

Finally, we showed how the bone repairs itself when a horse rests.

The new model uses all these aspects of bone adaptation, damage and repair to simulate how a Thoroughbred racehorse's bone responds to different training and rest patterns over time.

We tested and calibrated the model using real bone data from postmortem studies of thoroughbred racehorses that had undergone periods of training and rest.

We used the fetlock joint on the lower leg because it is the most common site of severe and fatal injuries and is subjected to the highest forces at high speeds.

Our model helps us understand what is happening inside the bone. It helps because we can change multiple things at a time without risking harm to the horse.

The model estimates:

How strong the bone becomes (measured as "bone volume fraction") under different training loads

How quickly the tiny cracks (measured as microdamage) build up

How much of the damage is repaired during rest or periods of less intensive training

The key components of safer training programs

To understand what safer training workloads might look like, we started with the average training routine of local trainers in Victoria, Australia.

We then tested alternative training programs around this average by adjusting the total gallop distance and how often and how long horses rested to see how bone responded.

Results showed that speed and the total distance a horse trains at top speed are linked to injury risk. That is, longer distances at faster speeds mean higher injury risk.

We also found that lower training volume, with some high-speed work, can still achieve sufficient bone adaptation without increasing injury risk.

Overall, managing horses with two or more rest periods of at least six weeks each year noticeably lowers injury risk by providing more time for bone repair.

Longer and more frequent rest periods clearly reduced bone damage.

Trainers can also choose low-intensity training in place of rest, as long as the horse has at least one significant rest period per year.

Getting the balance right

Naturally, horses that never train have the least bone damage, but they will not be fit or strong enough to race safely.

Our study shows that by modifying racehorses' training programs, we can reduce injury risk without affecting their likelihood of winning.

Mathematical modeling can help trainers design and tailor individual programs to find that sweet spot: building strong bones for successful racing without training so much, or for so long, that horses face increased risk of injury and fracture.

It can also help veterinarians design evidence-based return-from-injury programs that protect bone health while keeping the horse competitive.

Ultimately, we want horses to maintain bone health throughout their racing careers.

This would also allow them to have second careers in other equestrian disciplines like eventing or show jumping, and retire without injuries that would stop them from enjoying life after racing.