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 VO2max. It has also been suggested that
VO2max correlates to athletic performance in horses. However, there
was no a relationship between left ventricular size and VO2max 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 VO2max was reported in a
group of 18 Thoroughbred racehorses exercising on a high-speed treadmill (Young
et al, 2002). VO2max 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
VO2max 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
VO2max 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.
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