PERFORMANCE-BASED SELECTION OF ATHLETIC HORSES

PERFORMANCE-BASED SELECTION OF ATHLETIC HORSES

One of the earliest themes in exercise physiology of horses was the selection of physiologically superior athletes. The concept of using a measurement that accurately predicted success in racing or other athletic events is still popular. In this session a review will be presented of the methods that have been used more recently. What is being measured? What is the evidence for those measurements?

Selection of horses based on a physiological measurement has usually focussed on cardiac measurements. Electrocardiographic assessment of heart size was popular for decades, and now cardiac ultrasound is used by some investigators. Muscle biospsies for assessment of fibre type distribution were briefly popular, but results are unreliable for racehorses.

There is good rationale for focus on the heart. Athletic performance depends on aerobic and anaerobic energy supply. The dependence on aerobic ATP resynthesis is higher in events with longer duration. It has been estimated that all thoroughbred races with durations exceeding 1000 m have greater dependence on aerobic than anaerobic energy supply. In a race with 2-3 minutes duration, aerobic energy might supply 70-80% of the total requirement. 

Assessment of anaerobic energy output in human athletes usually relies on 30 second bicycle or treadmill test of maximal energy output. No such equivalent test exists for horses, and more research is needed in this area.

Aerobic energy output is predicted by the Fick equation

Oxygen uptake = (HR x stoke volume) x peripheral oxygen use

Assessment of heart structure with cardiac ultrasound  in a resting horse as part of an examination to predict future performance attempts to predict stroke volume during exercise. This is a large extrapolation from resting measures of cardiac wall mass, end-diastolic volumes and indices of contractility. As well, indices of cardiac myocardial wall function during ultrasound depend on HR.  Other factors that could limit the accuracy of cardiac ultrasound to predict cardiac output at maximal exercise intensities include;

Absence of the effects of increase in blood volume due to splenic contraction

Cardiac contractility during high HRs

Effects of training is very different in individual horses

Measurements are not expressed relative to horse body weight or age – a serious limitation

Absence of maximum HR (varies by up to 15%), so excludes estimate of maximum cardiac output

Maximal heart rate can vary from 195 to 235 beats per minute, so in horses with the same sized heart, the blood blow from the heart during a race can vary by 20%. Assuming a stroke volume of 1.3 litres, the range of HRmax can explain a range of cardiac output of 50 litres per minute! This variation shows the difficulty of trying to predict future performance with cardiac ultrasound in yearlings to assess heart size. Of course the changes in the heart’s ability to pump blood is also likely to vary greatly between horses as they grow, train and exercise, due to differences in genetics, and changes in myocardial structure and function (contractility).

In human athletes, there is evidence to support a link between left ventricular mass and VO_2max. It has also been suggested that VO_2max correlates to athletic performance in horses. However, there was no a relationship between left ventricular size and VO_2max in 6 Thoroughbreds exercising on a treadmill. However, this and most other studies focussed on flat race Thoroughbreds or Standardbreds that generally run over distances of less than 3,200 m (2 miles).

More recently a strong relationship between left ventricular mass and other measurements of cardiac size with VO_2max was reported in a group of 18 Thoroughbred racehorses exercising on a high-speed treadmill (Young et al, 2002).  VO_2max was significantly correlated with left ventricular (LV) internal diameter in diastole (r=0.71; P=0.001), estimated LV mass (r=0.78; P=0.0002) and LV short-axis area in diastole (r=0.69; P=0.003). When indices of heart size were indexed to body weight the correlation between VO_2max and indices of heart size were lower or similar, (LV diastolic diameter (r=0.57; P=0.01), LV mass (r=0.78; P=0.0002) and LV short-axis area (r=0.69; P=0.003). 

Whilst a relationship between left ventricular dimensions and VO_2max had been established, whether a similar relationship existed for heart size and athletic performance was still in doubt:  However, recent data from a large cross-sectional study of racehorses competing on the flat or over jumps in the United Kingdom did demonstrate a relationship between derived left ventricular mass and published rating (quality) in horses racing over longer distances in jump races (P£0.001), although the strength of the association with left ventricular mass was less for horses in flat races (Young et al..). Rather, left ventricular ejection fraction and left ventricular mass combined were positively associated with race rating in older flat race horses running over sprint (<1408m) and longer distances (>1408 m), explaining 25-35% of overall variation in performance, as well as being closely associated with performance in longer races over jumps (23%). Predicted differences between otherwise equivalent horses with small and large hearts was thus able to explain a significant proportion of the difference between elite and non-elite racehorse performance (Evans and Young, 2010). 

However, these results conflict with findings in 370 Thoroughbred yearlings, where there was no relationship between echocardiographic measurements of heart size and prospective race performance (Leadon et al., 1991). 

Use of echocardiography to predict future performance of racehorses should nevertheless still be used with caution, because the relative proportion of energy supply from aerobic metabolism probably varies widely. For example, in Thoroughbred races, with a range of 800-3200 metres distance, the relative contributions of aerobic energy output could be 40-80%.  With the possible exception of horses used for high level endurance riding, the technique is also likely to have limited value for horses other than those that race, as other skills are likely to be equally or more important influences on their athletic success than aerobic capacity. Additionally the level of skill required to obtain repeatable images of the equine left ventricle with an echocardiograph for this purpose is high and the confounding effects of gender, fitness, age and body size must always be taken into account. Prediction of maximal cardiac output and maximal oxygen uptake from estimates of stroke volume in a resting horse will be also confounded by variation in maximal heart rates during exercise.

Success of endurance horses will depend on superior aerobic fitness and economy of locomotion, reflected in lower HRs during exercise. 

Muscle biopsies have also been investigated for prediction of endurance horse performance (Rivero et al.)  . The disadvantage of muscle biopsies is their invasive nature, but a combination of muscle biopsy with assessment of heart rates during exercise should offer the best opportunity for accurate assessment of athletic potential in endurance horses.

What is the future for performance prediction if cardiac ultrasound measurements at rest are inaccurate. More studies are needed to investigate the use of heart rate and blood lactate measurements during field and treadmill exercise. Such studies will necessitate use of large numbers of commercially trained horses, with assessments scheduled during exercise undertaken before their first races. Treadmill or other mechanical testing equipment offers a methodology for such studies.

DNA analysis?

New commercial services offering genetic testing are now marketed. Each service should be scrutinised carefully for the evidence that supports the product, and thought given to the limitations of the service.

For example, a service that only estimates sprint capacity is probably ignoring a very important factor in the performance of most athletic horses; their aerobic capacity.

References

Evans D.L. and Young, L (2010) Cardiac responses to exercise and training. In: Cardiology of the Horse. Editor Marr, C. M., published by Elsevier Saunders, London

Young LE. Diseases of the heart and vessels. In: Hinchcliff KW, Kaneps AJ, Goer RJ, editors. Equine Sports Medicine and Surgery: Basic and Clinical Sciences of the Equine Athlete. Edinburgh: WB Saunders; 2003. p. 728–769.

Sampson SN, Tucker RL, Bayly WM. Relationship between VO2max, heart score and echocardiographic measurements obtained at rest and immediately following maximal exercise in thoroughbred horses. Equine VetJ Suppl 1999;30:190–194.

 Young LE, Marlin DJ, Deaton C, Brown-Feltner H, Roberts CA, Wood JLN. Heart size estimated by echocardiography correlates with maximal oxygen uptake. Equine Vet J Suppl

2002;34:467–472.

 Leadon D, McAllister H, Mullins E, Osborne M. Electrocardiographic and echocardiographic measurements and their relationships in Thoroughbred yearlings to subsequent performance. In: Persson SGB, Lindholm A, Jeffcott LB, editors. Equine Exercise Physiology 3. Davis, CA: ICCEP Publications; 1991. p. 18–22.