Physiological adaptations of

equine athletes to training

General concepts

ADAPTIVE training responses are the body’s attempts to continue to produce energy for muscular contraction in the face of increasing work load with minimal disruption to homeostasis. For responses to occur there must be some over-reaching, with responses task-specific and dependent on repetition, summation and duration of the stimulus.

Two main factors that influence performance are genetics and environment such as training, with the appropriate type of training required for an athlete very much dependent on the genetic make-up of the individual as well as the intended type of athletic performance.

It is important to recognise genetic differences when establishing a training programme so that the genetic potential to perform a specific type of task is maximised while minimising injury.

Injury during training is of great concern for equine athletes, especially during high-intensity exercise, with training programmes often modified to reduce risk of injury at the expense of an optimal training stimulus. For example, thoroughbred racehorse training is not as intense or as frequent as human athletic sprinters in order to reduce musculoskeletal injuries.

Overall, this means that there is no ideal blueprint for training!

It is important to remember that although maximal performance involves the coordinated and optimal functioning of body systems, a lack of fitness is a main limiting factor.

The primary systems that need to be healthy and work well together include the respiratory, cardiovascular and musculoskeletal systems, with training programmes aimed to enhance these areas.

Fitness is subjective and can be defined as the horse’s physical capability to perform to the best of its ability in a chosen competition. Thus, the definition of fitness depends on what the horse is being asked to do.

Physiological adaptations

and measurement of fitness

The respiratory and cardiovascular systems work together to bring in and deliver oxygen to (and remove waste from) the musculoskeletal system during energy production for muscular contraction. The two main systems that produce energy are the aerobic (oxidative, requiring oxygen) and anaerobic (glycolytic, no oxygen required) metabolic systems. It is easiest to objectively measure aerobic metabolic fitness by measuring pre- and post-training physiological responses such as heart rate to exercise stimuli.

Anaerobic metabolic fitness is difficult to directly measure, with blood lactate often used as an indirect measurement. The gold standard for assessing aerobic capacity and fitness is measurement of maximal oxygen consumption per unit of time (VO2max), with the factors contributing to VO2max defined as ‘central’ and ‘peripheral’. The central factor is the heart (cardiac output, the volume and rate of oxygen-rich blood delivery to muscles, dependant on heart rate and stroke volume), while the peripheral factor is the muscle (the amount of oxygen taken up by the muscle from the blood).

Regular exercise training results in an improvement in the aerobic metabolic system due to an increased capacity to deliver oxygen to skeletal muscle (termed conductive oxygen movement) as well as an increased capacity for skeletal muscle to take up and use oxygen for energy production (termed diffusive oxygen movement).

VO2max increases in response to most types of exercise training due to increases in the central and peripheral components, and can either be directly measured using a mask (for the most part restricted to Universities with appropriate equipment) or indirectly measured using heart rate and a published equation. Horses respond extremely rapidly to training in comparison to other domestic athletic species, with the VO2max improving within two-six weeks of training.

Most of the initial improvement in VO2max is due to central improvements with peripheral adaptations occurring a bit more slowly. An increase in the central component is primarily due to increases in stroke volume (increased heart size and contractile efficiency) with no change in resting and maximal heart rate.

An increase in the peripheral component is primarily due to increases in aerobic enzyme activity and the size and number of capillaries and mitochondria, with the latter being the actual place in the cell in which oxygen is used to produce energy.

Increases in capillary length and number increases the delivery of oxygen to skeletal muscles as well as the rate and volume of oxygen movement into the mitochondria since the capillaries and mitochondria are now closer together, with the distance oxygen has to travel decreased.

Although there is typically no change in resting or maximal heart in horses, heart rates at submaximal exercise intensities decrease as the horse becomes fitter. This means that a horse can exercise at greater distances and speeds before reaching maximal heart. Thus, associations between heart rate and speed (i.e. velocity, assessed with a stopwatch, tachometer, GPS) are useful indicators of aerobic fitness. The velocity at which a horse reaches a heart rate of 200 beats per minute (VHR200) has been shown to be a reliable indicator of fitness, with VHR200 increasing as the horse becomes fitter. It is important to consider that VHR200 can be artificially affected by excitement, rough gait changes, rapid acceleration and track type.

RECOVERY TIME

Another useful indicator of aerobic fitness is the velocity at which a horse reaches maximal heart rate (VHRmax), with VHRmax increasing as the horse becomes fitter.

Heart rate recovery can also be used to assess fitness. As fitness improves, heart rate should either recover to a pre-determined point more rapidly, or to a lower point within a pre-determined timeframe.

It is important to consider that the type of exercise performed and external stimuli during the recovery period (person approaching the horse, horses being fed, horse walking by the stall etc.) will affect heart rate recovery. Heart rate can also be used to assess individual work load, allowing training sessions to be tailored for a horse’s inherent capability.

Skeletal muscle is made up of different fibre types characterised by speed of contraction (slow vs. fast) and the metabolic pathway used to produce energy (oxidative vs. glycolytic).

The three main skeletal muscle fibre types in horses are Type I (slow-twitch oxidative), Type IIA (fast-twitch oxidative) and Type IIX (fast-twitch glycolytic).

Skeletal muscle groups have all fibre types represented, with the ratio of fibre type representation differing between muscle groups and breeds of horses. Muscular responses to training are dependent on the type of stimulus and basal muscle profile (i.e., genetics).

Equine skeletal muscle has great capability to respond differently depending on the type of training stimuli, resulting in changes in the ratio of fibre type distribution.

Training responses result in hypertrophy (muscle gets bigger), remodelling (function and structure change, no change in size) or mixed responses with remodelling the primary response observed in horses. The main outcome of hypertrophy is greater peak force capacity which enhances performances such as sprinting (greater acceleration rate) and showjumping (greater force).

Remodelling enhances things such as buffering capacity (can better deal with acid by-products) resulting in a delay in fatigue. Long-term high-intensity training can also increase anaerobic capacity in horses due to increases in MCT4 expression. MCT4 is a transporter abundantly expressed in fast-twitch muscle fibres that facilitates lactate removal from muscle during high-intensity exercise.

The association of the onset of blood lactate accumulation with velocity (VLa4) is another potential way to assess fitness, with VLa4 increasing as the horse becomes fitter. This assesses aerobic rather than anaerobic fitness and typically requires at least two exercise tests so is not as straightforward to do in a field setting.

Conclusion

Unlike humans, resting measurements are not valid indicators of fitness in horses.

Fitness evaluation in a field setting is practical, allows testing to occur in an environment similar to the one in which the horse will be competing but can be difficult to standardise. Heart rate measurement is an affordable and practical way to directly assess fitness.

Lisa M Katz, DVM, MS, PhD, DipACVIM, DipECEIM, CertUTL, MRCVS, is an Associate Professor in the School of Veterinary Medicine, University College Dublin, Ireland, having joined the school in 2003. Lisa obtained her DVM from the University of Georgia in 1994, following which she completed an internship in equine medicine and surgery at Peterson, Smith, Matthews, Hahn & Slone in 1995.

By 1998, Lisa had completed a combined large animal internal medicine residency program and a masters program in equine exercise physiology at Washington State University. She became an American College of Veterinary Internal Medicine diplomate in 2001, completed a PhD at the Royal Veterinary College in 2003, investigating the pathophysiology of equine acute laminitis, and became a European College of Equine Internal Medicine diplomate in 2005. Her current clinical and research interests include equine exercise physiology and equine genomics.