The Off-Season Crash: Understanding the Physiological Changes Racehorses Experience When Training Stops

by Dr. Lindsay Korac for Structure Supplements

From skeletal muscle adaptation and energy metabolism to gastric health and recovery, what does scientific research tell us about the transition from the racetrack to pasture?

For Thoroughbred and Standardbred racehorses, the conclusion of a racing season represents a significant transition. Horses that have spent months undergoing intensive conditioning, structured exercise, and carefully managed feeding programs may suddenly experience substantial reductions in physical activity as they move into an off-season period of rest or pasture turnout.

Although this period is intended to promote recovery, owners and trainers sometimes report an unexpected decline in their horses' physical appearance or general condition. Horses that previously displayed well-developed musculature, consistent energy levels, and strong athletic condition may appear to lose topline definition, become quieter, experience changes in body weight, or exhibit differences in appetite and behaviour.
Within the racing industry, these observations are sometimes described as an “off-season crash.”

While the term is not a recognized veterinary diagnosis, it raises an important physiological question: what happens to a highly conditioned athlete when the demands placed on its body change substantially? Research examining equine detraining, skeletal muscle physiology, energy metabolism, and gastrointestinal health suggests that the transition from intensive exercise to relative inactivity involves a complex series of adaptations. Understanding these processes is important for distinguishing expected physiological changes from conditions that may require additional nutritional or veterinary support.

The Physiology of Detraining: When the Athletic Stimulus Is Removed

Racehorses undergo considerable physiological adaptations in response to repeated exercise. Cardiovascular function, oxygen delivery, skeletal muscle metabolism, and energy utilization are all influenced by the intensity and duration of training. These adaptations enable the horse to sustain high rates of energy production and meet the substantial oxygen demands associated with racing.

However, it should be noted that many training-induced adaptations are reversible.

When exercise intensity or frequency decreases, the body begins to adjust to its new level of activity through a process known as detraining. Importantly, detraining is not inherently pathological. It represents the physiological response to a reduced training stimulus. Mukai et al. (2017) investigated the effects of different detraining strategies in 27 Thoroughbred horses. Following 18 weeks of treadmill conditioning, horses underwent 12 weeks of reduced exercise, consisting of continued cantering, walking, or stall rest. The researchers demonstrated that aerobic capacity and cardiovascular performance declined during detraining. However, the magnitude of these changes depended on the level of exercise maintained throughout the rest period. These findings are important because they demonstrate that the removal of intensive training can produce measurable changes in physiological capacity, even in otherwise healthy horses.
Similarly, Tyler et al. (1996) investigated the effects of prolonged training and detraining in Standardbred horses. Maximal oxygen consumption declined following six weeks of detraining, demonstrating that the cardiovascular benefits developed during conditioning can diminish when the exercise stimulus is reduced. Maximal oxygen consumption, commonly referred to as VO₂max, represents the maximum rate at which the body can utilize oxygen during intense exercise. It is an important indicator of aerobic capacity and is closely related to the ability of an athletic horse to sustain high-intensity performance. A reduction in VO₂max does not necessarily mean that the horse is unhealthy. Rather, it indicates that the cardiovascular and metabolic systems are adapting to reduced exercise demands. For racehorses entering the off-season, these findings demonstrate that the transition away from training is an active physiological process, rather than simply a period in which the body remains unchanged.

Skeletal Muscle: Why Racehorses May Lose Their Topline

One of the most noticeable changes during the off-season is a reduction in muscular definition, particularly across the topline, back, and hindquarters. To understand why this occurs, it is important to consider how skeletal muscle responds to training.

Muscle tissue is metabolically active and highly adaptable. During repeated exercise, skeletal muscle undergoes changes that improve its ability to generate energy, utilize oxygen, and sustain muscular contractions. These adaptations include changes in mitochondrial enzyme activity, muscle fibre characteristics, capillary supply, and intramuscular energy stores. Mitochondria are responsible for much of the aerobic energy production within muscle cells. Enzymes such as citrate synthase and 3-hydroxyacyl-CoA dehydrogenase are commonly used as indicators of oxidative metabolism and the capacity to utilize substrates for energy production.

Research by Snow and Guy (1979) demonstrated that training increased the activity of enzymes associated with oxidative metabolism in equine skeletal muscle. Following five weeks of detraining, the activity of 3-hydroxyacyl-CoA dehydrogenase was moving back toward pretraining values. Similarly, McGowan et al. (2002) investigated skeletal muscle adaptations in Standardbred horses subjected to 34 weeks of intensive training followed by 12 weeks of detraining. Their findings demonstrated that prolonged conditioning altered muscle metabolic characteristics, with detraining influencing the maintenance of these adaptations. Further evidence comes from research examining the effects of endurance training and detraining on equine skeletal muscle. Following eight months of training, horses exhibited changes in muscle fibre composition, oxidative enzyme activity, and capillary density. After three months of detraining, many of these adaptations had reversed.

When training is substantially reduced, the metabolic requirements of the muscle change, and some of the adaptations developed during conditioning are no longer maintained to the same degree. However, a reduction in muscle oxidative capacity is not necessarily equivalent to a measurable loss of muscle mass. Changes in enzyme activity, fibre characteristics, and visible musculature represent related but distinct physiological outcomes. This distinction matters when interpreting the loss of topline condition reported by owners.

Visible changes may reflect reduced muscle size, altered body composition, changes in posture, or a combination of factors. The magnitude of these changes will depend on the horse's previous conditioning, nutritional status, activity during the off-season, age, and individual physiology.

Energy Metabolism: A Change in the Horse's Nutritional Demands

The nutritional requirements of a racehorse are closely related to its level of physical activity. During active training, horses require energy to support muscular contraction, cardiovascular activity, thermoregulation, tissue maintenance, and recovery from exercise. Energy is supplied through the metabolism of carbohydrates, fats, and other dietary substrates. Within skeletal muscle, glucose and fatty acids contribute to adenosine triphosphate (ATP) production, providing the energy required for muscular contraction.
When training stops, the energy expended during structured exercise decreases, and the horse's overall nutritional requirements must be reassessed. However, the relationship between exercise and energy expenditure is more complex than simply removing the calories associated with a daily workout.

A study examining the energy requirements of racehorses in training, published in
Translational Animal Science (2020), evaluated energy expenditure under commercial racing conditions. The researchers reported that horses actively racing had approximately 12% higher maintenance energy requirements than horses in training that were not yet racing. The study also demonstrated that energy requirements in racehorses may differ from conventional estimates, emphasizing the importance of evaluating energy balance under actual management conditions. These findings are particularly relevant to off-season management because they demonstrate that nutritional requirements are influenced by more than the duration of exercise alone. When racehorses transition to pasture, their energy requirements may change because of reduced training, differences in spontaneous
activity, environmental temperature, growth, and changes in body composition. Consequently, feeding programs designed for intensive racing may no longer be appropriate.

Why reducing feed is not always straightforward

An important distinction must be made between reducing energy intake and maintaining adequate nutrient supply. Although a horse may require fewer calories during the off-season, it still needs sufficient dietary protein, essential amino acids, vitamins, minerals, and essential fatty acids to support normal physiological function. Protein is particularly important because skeletal muscle is continually undergoing protein synthesis and degradation.

Muscle maintenance depends partly on the balance between these processes, with exercise and amino acid availability contributing to the regulation of muscle protein turnover. When exercise intensity decreases, the stimulus supporting training-related muscular adaptation is reduced. If this occurs alongside an inadequate supply of dietary protein or essential amino acids, the maintenance of lean tissue may become more challenging. Conversely, maintaining an energy-dense racing ration when exercise demands have fallen may contribute to excessive weight gain. The objective should therefore be to adjust dietary energy while maintaining an appropriate balance of nutrients. The National Research Council's Nutrient Requirements of Horses (2007) emphasizes that dietary requirements vary according to physiological state, exercise intensity, age, and other individual factors. For horses entering the off-season, this supports the importance of reassessing the entire feeding program rather than simply reducing feed quantities without considering nutrient density.

Gastrointestinal Health: An Overlooked Factor in Off-Season Condition

Changes in gastrointestinal health may also contribute to the physical changes observed when racehorses transition away from active training. Equine Gastric Ulcer Syndrome (EGUS) is a common condition among performance horses, particularly Thoroughbred and Standardbred racehorses. Gastric disease is generally classified into two distinct conditions: equine squamous gastric disease (ESGD), affecting the non-glandular region of the stomach, and equine glandular gastric disease (EGGD), affecting the glandular mucosa. These conditions differ in their underlying mechanisms, risk factors, and responses to treatment. Research has consistently demonstrated a high prevalence of gastric ulceration in racehorses undergoing intensive training. Dionne et al. (2003) examined 275 Standardbred racehorses from training centres and racetracks in Quebec. The researchers found that actively racing horses were more likely to have gastric ulcers than horses at rest, with poor body condition also associated with increased lesion severity. The European College of Equine Internal Medicine consensus statement on EGUS further identifies racing and intensive exercise as important risk factors for gastric disease.

During high-intensity exercise, increases in intra-abdominal pressure and changes in gastric contents can increase acid exposure to the vulnerable squamous mucosa. Feeding practices, including prolonged periods without forage and the consumption of large concentrate meals, may further influence gastric conditions. Although transitioning to pasture can provide opportunities for increased forage consumption and reduced exercise-associated gastric stress, it should not be assumed that existing gastric disease resolves automatically. Horses entering the off-season may still have clinically relevant gastric lesions, particularly if ulceration developed during the racing season. This becomes important when evaluating horses that appear to lose condition despite reduced exercise demands. Changes in appetite, body weight, or behaviour may be associated with gastric disease, although none of these signs is specific enough to diagnose ulceration. A horse that appears to be experiencing an off-season crash may therefore require a broader assessment of gastrointestinal health rather than an assumption that the changes are solely related to detraining.

From the Racetrack to Pasture: Changes in Behaviour and Stress Physiology

The transition from active racing to pasture also introduces changes in the horse's physical and social environment. Racehorses are often managed according to highly structured schedules involving feeding, exercise, grooming, and housing. During the off-season, these routines may change substantially. Increased turnout may allow greater freedom of movement, social interaction, and opportunities to express natural behaviours. These changes can support welfare, particularly for horses previously maintained in relatively restrictive housing environments. However, the response to environmental change depends on the individual horse and the circumstances of the transition.

Research examining turnout management has demonstrated that opportunities for movement and social interaction can influence behavioural and physiological indicators of stress. For example, studies investigating turnout conditions have identified differences in cortisol concentrations and social behaviour depending on the space available to horses. These findings highlight the importance of appropriate turnout management, including adequate space, compatible social groupings, and safe environmental conditions. Nevertheless, it is important not to confuse behavioural adaptation with a generalized hormonal crash. There is currently insufficient evidence to conclude that horses routinely experience a sudden collapse in cortisol production, adrenal function, or immune regulation following the cessation of racing. A horse that becomes quieter after leaving the racetrack may simply be responding to reduced exercise demands or changes in its environment. Conversely, persistent lethargy, reduced appetite, or unexplained deterioration in condition should not be dismissed as normal behavioural adjustment.

Is the Off-Season Crash Actually a Combination of Physiological Changes?

The term off-season crash implies a sudden and potentially harmful decline in the horse's physiological condition. However, the available scientific literature suggests that the changes observed during this period are more accurately understood as several overlapping processes. Detraining can reduce cardiovascular capacity and reverse some skeletal muscle adaptations developed during intensive conditioning. Changes in exercise intensity alter energy expenditure and may require substantial adjustments to dietary management. Reduced muscular stimulation, particularly when combined with inadequate nutritional intake, may contribute to changes in body composition. Underlying gastrointestinal disease may influence appetite and body condition, while changes in housing and social management can affect behaviour.

Importantly, these processes do not necessarily occur simultaneously or affect every horse in the same way. Some racehorses may gain weight and improve their general condition during the off-season. Others may lose muscle definition while maintaining an appropriate body condition. A smaller proportion may experience concerning changes requiring veterinary investigation. Therefore, although the observations described as an off-season crash may be genuine, there is insufficient evidence to identify them as a single, predictable physiological syndrome.

The more useful question is not whether racehorses crash during the off-season, but which physiological changes occur during the transition and how management can support healthy adaptation.

Supporting the Racehorse During the Off-Season

Understanding the science of detraining has practical implications for racehorse management. The purpose of the off-season should be to provide appropriate recovery from intensive exercise while supporting the horse's changing physiological requirements. This requires an individualized approach rather than assuming that all horses will respond similarly to pasture rest. Body weight, body condition, and topline development should be monitored throughout the transition. These measurements provide information about changes in body composition and may help identify horses experiencing unexpected deterioration. Nutritional programs should also be reassessed to reflect changes in energy expenditure while maintaining adequate nutrient intake. Forage quality, protein supply, amino acid balance, and overall dietary composition are important considerations.

Where appropriate, opportunities for voluntary movement and carefully managed low-intensity exercise may help maintain aspects of physical conditioning. However, exercise recommendations must account for the reason the horse has been removed from training. Horses recovering from musculoskeletal injuries or other medical conditions may require restricted activity according to veterinary recommendations. Gastrointestinal health should also remain a consideration, particularly in horses with a history of gastric ulceration or unexplained changes in appetite and condition.

Finally, changes in behaviour, feeding patterns, and general appearance should be interpreted in the context of the individual horse. An off-season management program should support recovery without overlooking potentially important changes in health.

References

1. Mukai, K., Hiraga, A., Takahashi, T., Matsui, A., Ohmura, H., Aida, H., & Jones, J. H. (2017). Effects of maintaining different exercise intensities during detraining on aerobic capacity in Thoroughbreds. American Journal of Veterinary Research, 78(2), 215–222. https://doi.org/10.2460/ajvr.78.2.215
2. Tyler, C. M., Golland, L. C., Evans, D. L., Hodgson, D. R., & Rose, R. J. (1996). Changes in maximum oxygen uptake during prolonged training, overtraining, and detraining in horses. Journal of Applied Physiology, 81(5), 2244–2249. https://doi.org/10.1152/jappl.1996.81.5.2244
3. Snow, D. H., & Guy, P. S. (1979). The effect of training and detraining on several enzymes in horse skeletal muscle. Archives Internationales de Physiologie et de Biochimie, 87(1), 87–93. https://doi.org/10.3109/13813457909070488
4. McGowan, C. M., Golland, L. C., Evans, D. L., Hodgson, D. R., & Rose, R. J. (2002). Effects of prolonged training, overtraining and detraining on skeletal muscle metabolites and enzymes. Equine Veterinary Journal, 34(S34), 257–263. https://doi.org/10.1111/j.2042-3306.2002.tb05429.x
5. Ebert, M., & Moore-Colyer, M. J. S. (2020). The energy requirements of racehorses in training. Translational Animal Science, 4(4), txaa196. https://doi.org/10.1093/tas/txaa196
6. National Research Council. (2007). Nutrient requirements of horses (6th revised ed.). National Academies Press. https://doi.org/10.17226/11653
7. Dionne, R. M., Vrins, A., Doucet, M. Y., & Paré, J. (2003). Gastric ulcers in Standardbred racehorses: Prevalence, lesion description, and risk factors. Journal of Veterinary Internal Medicine, 17(2), 218–222. https://doi.org/10.1111/j.1939-1676.2003.tb02437.x
8. Sykes, B. W., Hewetson, M., Hepburn, R. J., Luthersson, N., & Tamzali, Y. (2015). European College of Equine Internal Medicine consensus statement—Equine gastric ulcer syndrome in adult horses. Journal of Veterinary Internal Medicine, 29(5), 1288–1299. https://doi.org/10.1111/jvim.13578
9. Hartmann, E., Søndergaard, E., & Keeling, L. J. (2012). Keeping horses in groups: A review. Applied Animal Behaviour Science, 136(2–4), 77–87. https://doi.org/10.1016/j.applanim.2011.10.004

Additional sources from the expanded research list- These were included in the additional references but were not explicitly named in the article.

1. Harris, P. A. (1997). Energy sources and requirements of the exercising horse. Annual Review of Nutrition, 17, 185–210. https://doi.org/10.1146/annurev.nutr.17.1.185
2. Rodgers et al. (2026). Study examining the effects of turnout duration on physiological and behavioural indicators of stress in stalled horses. Journal of Equine Veterinary Science. https://doi.org/10.1016/j.jevs.2026.106075.