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This article explains the knowledge available about physical exercise during the rearing phase of horses in a comprehensive and understandable manner. It shows that this phase is very important for our horses, and that we need to consider what we do in it. We also highlight that we use this knowledge in combination with our sport exercise knowledge to optimise this phase as EquInnoLab.

Injuries in the equine sport

Across all equestrian disciplines, lameness and musculoskeletal injuries are highly prevalent and form the primary reasons for wastage and lost training days. In Swedish warmblood horses, musculoskeletal injury was even found to be the most common cause of death. An important question therefore is: how can we try to prevent these injuries in order to prolong the equine sporting career and improve equine welfare?

What do we know?

Several studies have found that horses that started competition at a younger age tended to have longer careers. Starting physical exercise early in life has a beneficial effect, because at this time the tissues of the musculoskeletal system are most sensitive to exercise stimuli due to the rapid growth and development that is taking place. Physical exercise can additionally stimulate the developmental processes, thereby priming the tissues of the musculoskeletal system and making them better prepared for the training load later in life. In this way, the susceptibility of the horse to musculoskeletal injuries can be reduced while also improving performance, resulting in a successful and healthy athletic career.

How is this knowledge currently used?

In The Netherlands, physical training usually starts when the horse is three years old. At this age, a warmblood horse is already in a well-advanced stage of development, meaning that the optimal time to prime the musculoskeletal is over. It is therefore worthwhile to consider initiating physical exercise during the rearing phase in the form of ground work, for example canter exercise. Currently, most foals and young horses are kept at pasture for the majority of the time, which offers a fair amount of physical activity during the day. However, in European countries the weather conditions often require housing of the foals during the winter. This can result in a significant decrease in the access to physical activity, which can proof detrimental since this time is lost to prime the musculoskeletal tissues. Apart from physical activity at pasture, physical exercise is not yet a common element in the rearing phase of the equine athlete.

What does research tell us about physical exercise during the rearing phase?

Several studies have investigated the response of the musculoskeletal tissues to physical activity and exercise in young horses. The results of these studies are described below for each separate tissue.

Since the horse needs to be up and running within hours of birth, the equine bones are capable of load-bearing very soon. Physical activity is essential for the further bone growth and development since a lack of physical activity in the first months after birth will delay this process. Horses that are kept at pasture during the rearing phase show sufficient bone development and maturation in order to resist the physical loads later in life. In the first 1.5 years of life, the imposition of mild track-based exercise on top of normal pasture activity can stimulate the development of bone even more. However, in contrast, a high-intensity training program imposed on box rest resulted in long lasting negative effects on the equine bone. These data indicate that sufficient physical activity is critical for bone growth and development and that mild exercise at a young age can stimulate this pre-programmed process. However, despite the important initial bone development early in life, bone remains capable of adapting to mechanical loading during training throughout life.

Equine muscle is highly capable of adaptation to physical exercise throughout life. In the first months after birth, muscles undergo developmental changes in fibre type composition including a transition from more anaerobic to more aerobic fibres. However, to date only a trend towards a stimulating effect of physical exercise imposed on normal pasture activity has been found on muscle fibre type composition in the young warmblood horse. Further research with more horses is needed to determine if physical exercise is truly able to elicit a response in muscle fiber type composition. Another study found that physical exercise on top of box rest had an effect on equine muscle; it increased the capacity of the Na+/K+-pump, an important factor in the excitation of muscle fibers. The muscles of foals kept at pasture did not show such effects. Hence, the current evidence indicates that physical exercise is able to prime the muscular system for mechanical loading, but further research with more horses is needed to determine if physical exercise is also able to elicit a higher response in muscle fiber type composition than normal pasture activity alone.

The adaptive capacity of tendon tissue to physical exercise only spans the first year of life. At 5 months after birth, warmblood foals maintained at pasture had thicker and stronger tendons than foals that experienced either box rest or box rest in combination with a daily bout of high-intensity canter exercise. After these first five months, all foals were given access to physical activity and the group differences had disappeared after 11 months, showing that the tendon tissue was still capable of adaptation at this time. However, the results also showed a decrease in tendon thickness and strength after 11 months in the foals that were initially maintained at pasture, probably due to the fact that the amount of physical activity in the second part of the study was lower compared to the first part. This indicates that, in this study, pasture activity was most optimal for eliciting a response. The effect of exercise on top of pasture activity on tendon tissue has not yet been investigated. After the first year of life, the adaptability of tendon tissue to exercise stimuli decreases dramatically to the point where almost no adaptation and tissue recovery is possible.

Horses are born with a 'blank joint', which means that the biochemical composition of the cartilage is uniform. In response to mechanical loading, the biochemical heterogeneity of the cartilage increases to better resist the loads. Warmblood foals that were kept at pasture showed a significantly higher biochemical heterogeneity of the cartilage than foals kept in open stall systems. The foals that were deprived of physical activity showed significantly less functional adaption in the cartilage, which may have serious consequences for the injury susceptibility of this tissue. After the first 5 months, all foals were kept at pasture and some compensatory improvement was seen in the cartilage of the foals formerly kept in open stall systems. Another study found that an exercise regimen on top of normal pasture activity had an additional positive effect on the joint cartilage, priming the cartilage to better bear the training load later in life. After a year, the adaptability and repair capacity of the cartilage has decreased to extremely low levels as the turnover time of collagen in horse is estimated at ~150 years.

How current, valid and reliable is this research?

From the results of the above-mentioned studies, it is apparent that physical activity early in life is essential for the optimal development of the equine athlete. Furthermore, the evidence suggests that physical exercise combined with normal pasture activity even further stimulates the musculoskeletal system. The comparison between normal pasture activity and high-intensity physical exercise on top of box rest is valid since the studies investigating this difference made sure that both groups received the same amount of high-intensity exercise (i.e. equal amount of cantering). However, in the pasture groups the high-intensity exercise was superimposed on a basic exercise regimen consisting of rest and low-intensity exercise (walking and trotting). This should be kept in mind when interpreting the results of these studies. However, despite the promising results, the studies that have been performed to date are still limited and only tested a limited amount of horses.

How should we use this?

The evidence suggests that physical activity and exercise during the rearing phase are able to stimulate the musculoskeletal system, optimally preparing the tissues for a successful athletic career with few injuries. It is therefore recommended that a young horse has access to physical activity at all times, for example in a large pasture. In addition, performing physical exercise during the rearing phase would be worthwhile considering its additional value in the preparation of the equine athlete. The optimal exercise regimen still has to be properly determined, but evidence suggests that low-intensity exercise in combination with short bouts of high-intensity exercise is optimal. This means that physical exercise must always be superimposed on normal pasture activity as it can be harmful when imposed on box rest alone. In addition, caution is advised as inappropriate and excessive physical exercise can be deleterious for the young horse. At EquInnoLab, we specialise in adjusting exercise plans to specific goals, life phases and the age of the horse.

What do we want to know more?

The benefits of physical activity during the rearing phase are evident. However, the research on the effect of additional physical exercise in the young horse is still limited. Although a stimulating effect seems to be present, more research is needed. Furthermore, if a beneficial effect of physical exercise is found to be present, future studies must determine the optimal distribution between low- and high-intensity physical exercise. In addition, the optimal dose and intensity of imposed physical exercise at different ages must be established. In this light, it is also important to study what exercise intensity is too excessive to avoid overload and life-long damage of the musculoskeletal tissues.


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