Arabische Pferde IN THE FOCUS Nr. 2/2019 - Preview

Horses in the focus of science
bone fatigue in Race
and Endurance horses
Science
Horses breaking their leg during endurance competitions and races
demonstrate the potential dangers of these sports.
But these events should not be considered accidental or acceptable.
Recent research including that from the
Equine Limb Injury Prevention Program
at The University of Melbourne has progressed
our understanding of when, how and
what kind of injuries occur in racing but a lot
more work is needed to discover what trainers
can do to reduce breakdown and deaths.
Bone, like most materials, weakens when
subjected to repeated high loads. Most bone
and joint injuries in racehorses are the result
of fatigue damage, which means they occur
due to weakening of bone over the course
of training and racing, rather than because
of a single incidental injury such as a fall or
trip. The bones of racehorses endure extreme
loads repeatedly every time they gallop,
with the highest loads occurring in the joints
that are most commonly injured: the fetlock
and knee (carpal joint). Thankfully complete
fractures of bone occur infrequently, but less
severe fatigue injuries to the joint surface are
common in racehorses. Such injuries can result
in poor performance and lameness but
are hard to identify unless sophisticated imaging
equipment like scintigraphy is used.
The most important thing to understand
about bone fatigue is that it builds up over
time. Due to fatigue, tiny cracks will accumulate
in the bone during training. This means
that it is not what the horse was doing on the
day an injury occurred that is most important,
rather what the horse was doing in the days,
weeks or months leading up to the injury. With
every fast gallop or race, a proportion of the
lifespan of the bone is used up. Training and
racing intensely in the short term might seem
to be a successful strategy if there is no immediate
consequence, but by depleting the bone’s
reserves it may have set the horse up for future
injury. Bone injuries are most likely to occur
in horses that do a lot of high speed distance
work over the course of their career, or in horses
that complete a very large amount of work
in a short period of time.
Managing Bone to prevent Injury
Bone is an amazing tissue. It is not inert but
highly dynamic and in the right training environment
can adapt to increase its resistance
to injury. Bone also has a repair mechanism
that removes older fatigued bone and
replaces it with fresh new bone. During training,
the horse’s bone adapts to even short
bursts of speed by laying down new bone to
increase the total amount of bone (a process
called modelling). Understanding and exploiting
these processes is the secret to preventing
injury.
Bone adapts under Training Loads
Most trainers know that a horse’s skeleton
needs to get used to high speed exercise in
order to reduce the risk of limb injuries, but
accurate and sufficient information about
the time it takes for bone to adapt to changes
in exercise intensity throughout a racehorse’s
training program is not yet available.
New research from the Equine Limb Injury
Prevention Program indicates that any horse
rested from training for longer than about a
week is at risk of injury from high speed work
because of rapid bone loss or loss of adaptation.
This means that any return to high speed
training after such a rest should be gradual.
Because it is time consuming, costly, and
potentially unethical to assess the effect of
different training loads on subchondral bone
fatigue in racehorses, we are developing mathematical
models to better understand this
complex process.
In one of our models we looked at the effects
of gradually increasing or decreasing
training load for three classes of racehorses:
untrained two-year-olds, trained horses that
had rested (spelled) for at least four weeks,
and trained horses that are currently in training
for more than four weeks.
The model showed that for a young horse, at
least 16 weeks of training is needed before
bone has completely adapted to racing speeds
– this is the case even for the most rapidly progressive
training regimen. In practice, more
time will be needed because training speeds
must be increased slowly to avoid injury in a
poorly adapted skeleton. There are no shortcuts.
Complete adaptation might be several
weeks less for racehorses that have undergone
at least one full preparation.
Typical fractures in racehorses include
condylar fractures of the cannon bone above
the fetlock. - Typische Knochenbrüche bei
Rennpferden sind Frakturen des Röhrbeinknochens
über dem Fesselgelenk.
all images: Equine Limb Injury Prevention
Program, The University of Melbourne
On the other hand, bone loss (‘de-adaptation’)
can be quite rapid when race training is
interrupted. The model shows that even with
short rest periods of ten days up to 10% of
bone is lost, making it considerably weaker
and therefore prone to injury if high speed
exercise is reintroduced too quickly. Based
on the model, such a loss would take 20 days
to be recovered following the careful reintroduction
of training and intense speed work
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