Fitness can be thought of as the ability of a horse to complete the required amount of physical activity without experiencing exhaustion, distress, or injury.
Horses that are not conditioned sufficiently to cope with the amount of exertion that is being asked of them will fatigue faster and are more likely to injure themselves.
Using an objective measure to gauge your racehorse's fitness will not only help you tailor your conditioning programs to the individual needs of each horse but also help you to pick up any abnormalities in your horse's body before they become a bigger problem.
Heart rate and the cardiac recovery interval (CRI)
Heart rate changes in a predictable manner with exercise. Heart rate monitoring during incremental exercise is the most practical means for assessment of fitness.
Incorporating this type of testing into your horse’s conditioning program will provide a useful guide regarding fitness and the effects of different conditioning strategies. Click here for a good article: Is your horse fit for the task?
Resting heart rate for most horses is between 30 and 40 beats per minute (bpm). During race conditions the standardbred can achieve heart rates as high as 240 bpm (an astounding four beats per second).
Unlike humans, resting heart rate and the maximal heart rate reached by horses are predetermined genetically that do not change with increasing fitness. They are not used to monitor fitness in the horse.
Other changes do occur in the body over a relatively short time that increase the volume of blood that is pumped out by the heart per beat. It is these changes that create increasing cardiovascular capacity with training and will be covered in a later article.
Rather, it is the time taken to for the heart rate to come back to resting levels (Cardiac Recovery Index or CRI) that is universally accepted as the best measurement of equine fitness during and after competition across a most horse sports.
Fit healthy horses do not take long to drop their heart rates post exercise. A fit standardbred will have a heart rate under 90 bpm within 10 minutes of finishing a race or completing fast work in non-humid conditions.
They should be back close to pre-exercise resting rates within 30 minutes provided they are allowed to cool down adequately. Heart rates remain elevated if:
The level of work being demanded is higher than the horse's current fitness level
There is an underlying disease or injury producing pain or interfering with normal physiological functioning (e.g., dehydration, exercise induced pulmonary haemorrhage, atrial fibrillation etc)
The combined ambient temperature, windspeed and humidity is greater than 28 as measured by the Wet Bulb Globe Temperature Index (which can be found at the Bureau of Meteorology Thermal Comfort webpage).
The horse is not provided with an opportunity to warm down
The horse's CRI should drop as it gets fitter. It is good practice to measure and record your racehorse's heart rate prior to commencing a work or race session and to monitor the recovery time post exercise. Ideally the measurements would be performed at the same time post exercise so that changes across training sessions can be detected.
Shortening recovery times across time indicate that the training level is appropriate and continuing to produce exercise induced adaptation within the horse's body. Sudden increases in recovery time may indicate injury or disease. Plateauing recovery times indicate either an increase in training demand is warranted, or the horse is reaching its genetically determined fitness limit. Incrementally increasing recovery times across sessions can indicate overtraining, which will result in poorer race performance.
While horses' respiratory rate can be easily detected by observing the expansion and contraction of the abdomen with each breath or feeling the expiratory breath on your hand held close to one nostril, it is not useful for monitoring fitness. It can be used to detect heat stress.
A heat stressed horse will often have a respiratory rate will be higher than heart rate. This phenomenon is called inversion. Horses displaying inversion should be urgently managed for heat stress. Please note that heat stress can occur in colder weather if the horse is unfit, experiences overexertion or has an underlying medical condition.
Clinical pathology screening (blood tests) is used by some to help evaluate fitness levels.
Most routine clinical pathology tests are designed to identify disease processes rather than wellness or "readiness to race".
The role of clinical pathology in detection or support of subtle or subclinical disease and overtraining that can have a large impact on performance should not be underestimated.
Racehorses do have some recognised changes in their blood biochemistry and blood cells that are different to unfit horses.
The emphasis given to subtle changes in pathology results from a fit racehorse and its paddock ornament cousin is like that of comparing the tuning specifications of a racing car compared to that of the commuter version of the same model. The same things are measured but the range of acceptable variance from the ideal is much less in the performance unit.
As a consequence, ratios between white blood cells, morphology of red blood cells and biomarkers such as enzymes and waste products provide information about the health of the horse. Monitoring haematology and biochemical results over time can provide valuable indicators of each individual's health and fitness.
Using lactic acid to measure fitness levels
One blood test that can give immediate feedback on horse fitness is the concentration of lactic acid present both during and immediately after exercise.
Lactic acid is a by-product of anaerobic energy production in the horse's cells, especially in the muscles. Greater anaerobic energy production occurs at intense exercise overwhelming the muscle cell's ability to process the lactic acid that is formed. The excess is transported into the blood stream for processing in the liver.
Horses need to train at a level where some lactic acid is produced in order to induce the physiological and biochemical changes associated with increased fitness and better performance.
Lactic acid concentrations above 4 mmol/L are considered the minimum level necessary to invoke these training adaptations. Lactic acid concentrations that are very high may indicate a training level that is too demanding for the horse's current level of fitness. Training at this level can lead to injury and other health conditions such as tying up (recurrent rhabdomyolysis).
It only takes a single drop of venous blood in a lactic acid monitor to provide meaningful stall side data during and immediately after training and can be used as a leading indicator of fitness.
Researchers are constantly looking for biomarkers to evaluate fitness and inform training decisions. For example: serum amyloid A in serum is already being used to detect inflammation while a stall side test to monitor for levels of ammonia (a product of anaerobic energy production) is being trialled in Canada.
Speak to your veterinarian about options for fine tuning how you monitor fitness in your racehorses.
McGowan, Catherine Clinical Pathology in the Racing Horse: The Role of Clinical Pathology
in Assessing Fitness and Performance in the Racehorse Vet Clin Equine 24 (2008) 405–421